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Simple Example of Managed Extensibility Framework (MEF) in Silverlight
As you may have heard, we recently shipped MEF support of Silverlight in our CodePlex drop.. I wanted to give you a very simple introduction to MEF and how to use it in Silverlight. This example will show how to use lose coupling, dependency injection and delay loading of components. This is also an update to my Simple MEF example from a few months ago as just about everything here applies to WPF, WinForms and ASP.NET apps as well.
The demo requires (all 100% free):
1. VS2008 SP1
2. Silverlight 3 RTM
3. MEF’s July CodePlex drop
Also, download the full demo files
Let’s start with the very cool Silverlight Navigation Application
image
Setting up The Problem
This is a very simple example just to show how the technology works. Let’s say you want to put some string a TextBlock… As a bigger example, think of pages in an application or components in a portal, etc, etc.
Replace the contents of the ContentStackPanel in Home.xaml with a ListBox and set up some binding.
<StackPanel x:Name="ContentStackPanel">
<TextBlock Text="{Binding Mode=OneWay}"></TextBlock>
</StackPanel>
Then in codebehind we set up the datacontext:
public Home()
{
InitializeComponent();
LayoutRoot.DataContext = "Hello World!";
}
Of course, we get exactly what we’d expect….
Now there are some problems with this.. For example, the value of the string is tied up in the constructor, i might want to have that factored out so that it can be accessed separately.
1: public partial class Home : Page
2: {
3: public Home()
4: {
5: InitializeComponent();
6:
7: var message = new SimpleHello();
8: LayoutRoot.DataContext = message;
9: }
10: }
11:
12: [Export("Message")]
13: public class SimpleHello
14: {
15: public override string ToString()
16: {
17: return "Hello World!";
18: }
19: }
This will certainly do the trick at some level, but there is still something odd about lines 7 and 8… I am creating an instance of exactly this type which locks the functionality down to one instance. That means the type has to be in my codebase. What if I’d like to enable some separation? maybe get that Message from a variety of sources based on config, the user preference, the role of the user, etc.
Enter MEF
Let’s see how MEF can help.. First we need to add a reference to the MEF assembly (System.ComponentModel.Composition.dll) that is found in the MEF_Preview_6\bin\SL3 folder of the latest bits. I copied it into the bin directly of the project (which is hidden by default) and added a reference to it from there.
As Jason Olson says: MEF is as easy as 1, 2, 3…Export It, Import It, and Compose It…
So the first step is Export it.. so let’s export the SimpleHello type… this is easy enough to do by simply putting an Export attribute on the type and specifying the contract name.
1: [Export("Message")]
2: public class SimpleHello
3: {
Next step is to Import it, so let’s import some type to databind to.
[Import("Message")]
public object Message;
public Home()
{
InitializeComponent();
LayoutRoot.DataContext = Message;
}
That looks very clean… I simply say what I have (an export) and what i need (an import). But who does the wiring up? Well, we have one more step… Compose It:
Add the following lines to the constructor…
1: var catalog = new PackageCatalog();
2: catalog.AddPackage(Package.Current);
3: var container = new CompositionContainer(catalog);
4: container.ComposeParts(this);
Line 1 shows off our new PackageCatalog that just new for this Silverlight release. Catalogs generally tell MEF where to look to find stuff to put in the container. Line 2 adds the current XAP to the catalog.. later we will show how to add XAPs that are asynchronously downloaded via a different catalog.
Line 3 creates a Composition container-- this is effectively the container that all the different parts will be wired up together.
Line 4 composes… that is wires up all the imports and exports. After this line, all imports are fully satisfied or an exception is raised.
Running the app has it work exactly the same way, but now loosely coupled.
image
That is great, but using strings such as “Message” to identify contracts is fraught with problems. It turns out the CLR already has a pretty well developed way to deal with contracts and that is CLR Types. So it is recommended in most cases that you use CLR Types to indicate the contracts. To do this, let’s define an IMessage interface in a separate assembly so that it can be shared by different assemblies contributing components.
Add a new project, and call it ContractsClassLibrary
image
Add a single type.. IMessage… right now it has no members, it is effectively a marker interface. We could of course define some methods if they are needed.
namespace ContractsClassLibrary
{
[InheritedExport]
public interface IMessage
{
}
}
Notice we put the new InheritedExport attribute on this type… this means that any type that implements this interface will automatically be exported as type IMessage. This keeps the “MEF goo” out of your public object model.
Now, add a reference to ContractsClassLibrary from the MyMEFApp project and let’s look at how that SimpleHello type changes..
public class SimpleHello : IMessage
{
public override string ToString()
{
return "Hello World";
}
}
Notice, no MEF goo on it…
Fun and the app and you get the exact same results… but this time our contract is a CLR type rather than a string.
image
Now, let’s say I had more than one message to show.. No problem, i just add another instance to the container with the same export.
public class SimpleHola: IMessage
{
public override string ToString()
{
return "Hola";
}
}
Hit F5 and… Wham.. I get an error..
image
(Side note: if you get a less helpful message that says “navigation failed”, in MainPage.cs change the ErrorWindow to show the inner exception…)
private void ContentFrame_NavigationFailed(object sender, NavigationFailedEventArgs e)
{
e.Handled = true;
ChildWindow errorWin = new ErrorWindow(e.Exception.InnerException);
errorWin.Show();
}
As these things go, it is actually a pretty helpful error message:
The composition remains unchanged. The changes were rejected because of the
following error(s): The composition produced a single composition error.
The root cause is provided below. Review the CompositionException.Errors
property for more detailed information.
1) More than one exports were found that match the constraint
'((exportDefinition.ContractName = "ContractsClassLibrary.IMessage")
&& (exportDefinition.Metadata.ContainsKey("ExportTypeIdentity") &&
"ContractsClassLibrary.IMessage".Equals(exportDefinition.Metadata.get_Item("ExportTypeIdentity"))))'
.
Resulting in: Cannot set import 'MyMEFApp.Home.Messages (ContractName="ContractsClassLibrary.IMessage")' on part 'MyMEFApp.Home'.
Element: MyMEFApp.Home.Messages (ContractName="ContractsClassLibrary.IMessage") --> MyMEFApp.Home
at System.ComponentModel.Composition.CompositionResult.ThrowOnErrors(AtomicComposition atomicComposition)
at System.ComponentModel.Composition.Hosting.ComposablePartExportProvider.Compose(CompositionBatch batch)
at System.ComponentModel.Composition.Hosting.CompositionContainer.Compose(CompositionBatch batch)
at System.ComponentModel.Composition.AttributedModelServices.ComposeParts(CompositionContainer container, Object[] attributedParts)
at MyMEFApp.Home..ctor()
Basically it is saying that we were only expecting one message, and the container has more than one..
The easy solution here is to accommodate more than one Message..
[ImportMany]
public ObservableCollection<IMessage> Messages { get; set; }
and let’s change the UI control to a ListBox…
<ListBox ItemsSource="{Binding Mode=OneWay}"></ListBox>
Run it and we get both messages..
image
Another thing that is really cool about MEF is that we do full composition, that is even the parts in the container can have dependencies that are satisfied and runtime. For example, let’s take out SimpleHello class and say we wanted to externalize providing the actual text… we could do that easily enough.
1: public class SimpleHello : IMessage
2: {
3: [Import("Text")]
4: public string Text { get; set; }
5: public override string ToString()
6: {
7: return Text;
8: }
9: }
10:
11: public class TextProvider
12: {
13: [Export("Text")]
14: public string Text { get { return "Hello World!"; } }
15: }
We simply export the text from a provider class and import it into our SimpleHello class. (I am using the string contract name here just for simplicity, you could of course define an IText interface and use it exactly as we showed above.) Notice the by product of this pattern is that each class gets very focused and specialized in what it deals with. SimpleHello now is just about overriding ToString() and doesn’t care a thing about where it gets the message from. That means we could easily add another way to offer text for binding… How about as a button?
1: public class ButtoHello: Button, IMessage
2: {
3: [Import("Text")]
4: public string Text {
5: get {
6: return this.Content.ToString();
7: }
8: set
9: {
10: this.Content = value;
11: }
12: }
13: }
This is time, we use the same text, but rather then ToString(), we use it to set the content property of the button. This is a great example of a single instance exporting and importing dependencies.
image
OK – that is cool… but all this is in the same project.. what if we wanted a bit more separation? Well, the first step we could do is put some parts into a separate assembly, but deployed with the same XAP.
image
We will need to reference our ContractsClassLibrary project.
Then just add a couple of classes that export “Message”
public class Class1 : CheckBox, IMessage
{
public Class1()
{
this.Content = "From Assembly: ma kore?";
}
}
public class Class2: IMessage
{
[Import("Text")]
public string Text { get; set; }
public override string ToString()
{
return "From Assembly:" + Text;
}
}
Notice, we can again, import the text optionally if we’d like. This shares the exact same instance with any other type that imports it.
Then we add a reference to this library from the main Silverlight app… this puts the library in the XAP
image
Now, run it and we get some more items..
image
Now the final step is to load the parts, asynchronously from the server.. This allows you to have components that are downloaded ONLY when they are needed. It also enables much faster startup time where the core of the app can start up and features can light up as they are downloaded. This is a much better experience than a long “loading…” screen.
To do this, we need to create a library that is packaged as a XAP.. the best way to do that is to create a new Silverlight Application, and just delete the MainPage.xaml…
image
Then we want to host this in the same web site.. .Notice you could host it in a different website, but you would then need to use the Silverlight networking stack’s mechanism to get at non-originating servers.
image
You can go ahead and delete MainPage.Xaml and App.Xaml so we don’t get confused and add a Class1.cs file.
image
We will also need to add a reference to our ContractsClassLibrary project
Then just define a class an Export it..
public class Class1: IMessage
{
public override string ToString()
{
return "From Dynamically Loaded: ciào";
}
}
You could optionally import the same Text property if you’d like as well..
public class Class2 : ComboBox, IMessage
{
[Import("Text")]
public string Text
{
set
{
this.ItemsSource = value.ToCharArray();
}
}
}
Now we need to go back into the main app and tell it where to dynamically load this from.
1: public Home()
2: {
3: InitializeComponent();
4:
5: var catalog = new PackageCatalog();
6: catalog.AddPackage(Package.Current);
7:
8: Package.DownloadPackageAsync(
9: new Uri("DynamicallyLoadClassLibrary.xap", UriKind.Relative),
10: (sender, pkg) => catalog.AddPackage(pkg)
11: );
12:
13:
14: var container = new CompositionContainer(catalog);
15: container.ComposeParts(this);
16:
17:
18: LayoutRoot.DataContext = Messages;
19: }
As you can see, we inserted lines 8-11 to deal with this new dynamically loaded XAP. Here we are simply doing delay loading, but you could wait and load on some user action (clicking a button for example) or you could load based on who is logged in, what functionality they prefer, what role they are in, etc.
In line 9, we create a URI pointing to our XAP.. in this case from the same server, but it could be from any server assuming the right policy files are in place.
In line 10, we are handling the callback.. remember this is an async call. When the call completes, we simply add any parts discovered to the catalog.
Now, the async nature of this is interesting, because effectively we are changing a programs dependencies mid-flight. Not always a good idea, so it is something you need to opt into.
[ImportMany("Message",AllowRecomposition=true)]
public ObservableCollection<object> Messages { get; set; }
And now we run and it looks great!
image
So, what have we seen? You can use MEF to improve the quality and structure of your code so that classes have a single set of responsibilities. MEF also makes it easy to optionally download components on demand.
Check out the completed example
We’d love to hear what you think!
Comments (13)
1. Michel Bergeron says:
Can you talk about difference between MEF and Prism for Silverlight ? There is a lot of differents kinds of frameworks for Silverlight now and it is difficult to know the differences and when to use one or another….
Thank you
Michel
2. Hi,
I think I need to delve into MEF a little bit more as I cannot understand the concept you are telling.
You can be little bit clear .
Thanks,
Thani
3. ElemarJr says:
Congratulations. Very nice introduction about how to use MEF with Silverlight!
4. Hi Michael
Prism is a set of guidance coming out of patterns & practices for building composite UI applications, that is applications that are deployed in separate modules where each module represents a different sub-system such as Order Entry, Reporting, Accounting, etc. Prism also includes a set of additional services such as the RegionManager and EventAggregator which aid in building such apps.
MEF is a general composition technology in the framework that assembles components pulling their dependencies from a myrida of different sources, including XAP files for Silverlight apps. MEF is not UI specific and can be used both in UI and non-UI applications.
As far as whether to use Prism or MEF, long term I’d say you e that you can use both together, as the two technologies complement one another. We are working with patterns & practices, to provide better integration with Prism going forward.
Today, the answer is not as straight forward. In general I would give these guidelines.
1. If you are building a composite application, with multiple modules that internal teams will be maintaining, then I would recommend using Prism.
2. If you building a Silverlight application that you want to allow third-parties to customoize and extend, through adding their own capabilities after the application has been deployed, then use MEF.
HTH
Glenn
5. Michel Bergeron says:
Thank you Glenn for the precisions.
Last question, does MEF is supported by Microsoft ?
Michel
6. bill simons says:
Can you use mef in a commercial application
or will that come later on??
7. Michel, Yes, MEF ships as part of the .NET Framework and is fully supported. The releases on CodePlex are previews of what is to come in the framework.
Bill, Yes you can, MEF ships on Codeplex under an MS-PL license thus you can use it without a "go-live" license.
8. Michel Bergeron says:
Thank you for the answer Glenn :))
9. bill simons says:
Thank you for your reply. I would love to use this a
asp.net mvc wcf restful web application. This is what
I have been searching for in doing very simple basic
paragraph development…..
10. bnaya says:
Seem great i will check it soon
11. niblumha says:
Nice post Brad!
A simple but handy tip mentioned in the video here (http://development-guides.silverbaylabs.org/Video/Silverlight-MEF): you can decrease the size of DynamicallyLoadedClassLibrary.xap by setting ‘CopyLocal’ for the System.ComponentModel.Composition reference to ‘false’.
>> And now we run and it looks great!
I like your code more than your UI design 😛
12. Kanary says:
Thank you Brad for you helpful posts!
I load xamls from different modules. When I click some button on one page I need to send additional information from this page to another and react to this event on the second page. How can I do it with MEF?
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| 0.881858 |
How to Find Critical Values with a Chi-Square Table
Chi-square distribution
A graph of a chi-square distribution, with the left tail shaded blue. C.K.Taylor
The use of statistical tables is a common topic in many statistics courses. Although software does calculations, the skill of reading tables is still an important one to have. We will see how to use a table of values for a chi-square distribution to determine a critical value. The table that we will use is located here, however other chi-square tables are laid out in ways that are very similar to this one.
Critical Value
The use of a chi-square table that we will examine is to determine a critical value. Critical values are important in both hypothesis tests and confidence intervals. For hypothesis tests, a critical value tells us the boundary of how extreme a test statistic we need to reject the null hypothesis. For confidence intervals, a critical value is one of the ingredients that goes into the calculation of a margin of error.
To determine a critical value, we need to know three things:
1. The number of degrees of freedom
2. The number and type of tails
3. The level of significance.
Degrees of Freedom
The first item of importance is the number of degrees of freedom. This number tells us which of the countably infinitely many chi-square distributions we are to use in our problem. The way that we determine this number depends upon the precise problem that we are using our chi-square distribution with. Three common examples follow.
In this table, the number of degrees of freedom corresponds to the row that we will use.
If the table that we are working with does not display the exact number of degrees of freedom our problem calls for, then there is a rule of thumb that we use. We round the number of degrees of freedom down to the highest tabled value. For example, suppose that we have 59 degrees of freedom. If our table only has lines for 50 and 60 degrees of freedom, then we use the line with 50 degrees of freedom.
Tails
The next thing that we need to consider is the number and type of tails being used. A chi-square distribution is skewed to the right, and so one-sided tests involving the right tail are commonly used. However, if we are calculating a two-sided confidence interval, then we would need to consider a two-tailed test with both a right and left tail in our chi-square distribution.
Level of Confidence
The final piece of information that we need to know is the level of confidence or significance. This is a probability that is typically denoted by alpha. We then must translate this probability (along with the information regarding our tails) into the correct column to use with our table. Many times this step depends upon how our table is constructed.
Example
For example, we will consider a goodness of fit test for a twelve-sided die. Our null hypothesis is that all sides are equally likely to be rolled, and so each side has a probability of 1/12 of being rolled. Since there are 12 outcomes, there are 12 -1 = 11 degrees of freedom. This means that we will use the row marked 11 for our calculations.
A goodness of fit test is a one-tailed test. The tail that we use for this is the right tail. Suppose that the level of significance is 0.05 = 5%. This is the probability in the right tail of the distribution. Our table is set up for probability in the left tail. So the left of our critical value should be 1 – 0.05 = 0.95. This means that we use the column corresponding to 0.95 and row 11 to give a critical value of 19.675.
If the chi-square statistic that we calculate from our data is greater than or equal to19.675, then we reject the null hypothesis at 5% significance. If our chi-square statistic is less than 19.675, then we fail to reject the null hypothesis.
Format
mla apa chicago
Your Citation
Taylor, Courtney. "How to Find Critical Values with a Chi-Square Table." ThoughtCo, Aug. 26, 2020, thoughtco.com/critical-values-with-a-chi-square-table-3126426. Taylor, Courtney. (2020, August 26). How to Find Critical Values with a Chi-Square Table. Retrieved from https://www.thoughtco.com/critical-values-with-a-chi-square-table-3126426 Taylor, Courtney. "How to Find Critical Values with a Chi-Square Table." ThoughtCo. https://www.thoughtco.com/critical-values-with-a-chi-square-table-3126426 (accessed January 23, 2021).
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__label__pos
| 0.857753 |
NxN矩阵的数列排数的问题，要求使用C语言的程序的代码的编写思想的方式来实现怎么做
Problem Description
Sheldon's board has been changed! Someone drew a N * N grid on his board. Although he has no idea who's the bad guy, but as a math geek, he decide to play a game on it. He will fill in the grids with integer 1 to N*N. And the sum of each row, each column and each diagonal are distinct (2N+2 distinct integers in total).
Input
The first line contains a single positive integer T( T <= 200 ), indicates the number of test cases.
For each test case: an integer N (3 <= N <= 200).
Output
For each test case: output the case number as shown and an N * N matrix.
The output must contains N+1 lines, or you maybe get wrong answer because of special judge.
The numbers in each line must be separated by a blankspace or a tab. leading and trailing blankspace will be ignored.
Sample Input
2
3
4
Sample Output
Case #1:
7 5 2
1 4 8
3 6 9
Case #2:
2 8 15 1
5 12 10 16
6 9 4 14
3 7 11 13
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抄袭、复制答案，以达到刷声望分或其他目的的行为，在CSDN问答是严格禁止的,一经发现立刻封号。是时候展现真正的技术了！
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__label__pos
| 0.55081 |
Overwriting arrays java
Plurality US presidential electionrun-off visitors, sequential run-off texts Australia, Ireland, Princeton blur committeesCondorcet.
Overwriting arrays java no particular is found with the key value, a negative number will be able. Here is what the output would recall like: Static Methods If a wide defines a static method with the same thing as a static method in the stage, then the method in the subclass outsiders the one in the superclass.
Shipmates could look in a Nice class of your own: Plain is is how blistering a multidimensional looks in Java: Automatically is how you find the different value in an array.
The colon to remove the element from, and the difference of the element to write. If you write that, use a teacher for-loop as shown earlier. A right in a different package can only allow the non-final methods declared bicycle or protected. Here is an argument of Java arrays: Assume that n is a fallacy of 2.
Arrays I will answer a few of the implications found in this class in the anti sections. But I will show it to you previously. The first day is the type to copy. Static leaves in interfaces are never inherited. The epic method in Animal The instance method in Cat As participant, the version of the hidden static fluent that gets invoked is the one in the story, and the version of the quoted instance method that students invoked is the one in the answer.
A method declared final cannot be said.
Java Arrays
Remember, in order to use holland. Variables can be assigned values in the following way: During the Mergesort process the composition in the collection are able into two collections.
In the perspective above we simply call the Time. At this point the bad written to the pressure the String trudge of the array looks like this: I pat computers, programming and solving no everyday. One style works for arrays of all finishing types, as well as many of strings. Divide the previous list into two sublists of about economic the size Sort each of the two sublists Precise the two sorted sublists back into one missed list Merge Sort Example In below comes, we have implemented merge sap algorithm in expressive way to short it more understandable.
Here is how you find the only value in an array. Xander Pot John To compare the Problem objects in the source first by their name, and if that is the same, then by my employee id, the compare guinea would look like this: Here is an idea of how to convert an essay of int to a Topic using Arrays.
For genius, an array of int is a narrative of variables of the key int. Consider the theory about computer-controlled cars that can now fly.
Let us now see how it does to sort the Employee objects by my employee id instead. The fantasy method in this continued creates an instance of Cat and circles testClassMethod on the class and testInstanceMethod on the phenomenon. Instance variables non-static connects are unique to each instance of a hard.
A specific element in an hour is accessed by its index. Hey thanks for replying so quick. Its there cos the input file looks something like this.
so i want to keep the same person object until it hits a blank line the the second loop will terminate begin with the first loop get the next line with data and create a new object. so basically that liens there to get the next line so that the second loops conditional will be false if the line is blank.
Anyways the problem I'm having has to deal with arrays, what I'm trying to do basically is use the String[] arrays to populate my form and display it on screen and then have the getForm() function return a String[] with the title of the form and the info in text[i].
Win a copy of Learning Java by Building Android Games this week in the Android forum! Overwriting an array.
Similar Threads
Stephane Bonett. Greenhorn but when the elements are primitives, that will be all right. Why overwrite the arrays? Why not simply clone it twice, and you will then have three copies. By the way: for a element array the.
Java matricies are arrays of 1-D array references, which is a tad unusual. If you aren't yet working with an IDE with a debugger I'd suggest you get one, Netbeans and Eclipse are both available online. Jun 27, · Re: overwriting entire array in java?
Jun 27, AM (in response to ) Do you think you would get a quicker response from posting on the forum or trying to compile that code and see if it executed? Java is a simple, object oriented, high performance language. It is distributed, portable, multi-threaded, and interpreted—mainly intended for the development of object oriented, network based.
Overwriting arrays java
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Overwriting arrays problem | Oracle Community
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| 0.768092 |
Hilfe
0
How to trigger update/compute field after install new module?
Avatar
Zbik
Any suggestion is welcome :)
Avatar
Verwerfen
1 Antwort
0
Best Answer
you can use def init(self): method inside the model and put your code over there.
init method execute install and update time.
5 Kommentare
Avatar
Verwerfen
Avatar
Zbik
-
Thanks for the hint but it is not a solution. I need an update after the installation of any module, including the standard one.
ok, Did you mean record update form the API? then you can use ir.cron.
you need to describe more what exactly you want.
Avatar
Zbik
-
I did not give a specific example because it is complicated. It is associated with GDPR and needs an online response (exclude cron). I will give a similar, simple example:
A field that counts the number of other fields in the entire system.
Avatar
Piotr Cierkosz
-
module for internal auditing/monitoring?
Avatar
Zbik
-
Yes, the module is to help in managing activities related to GDPR.
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| 0.8537 |
Opened 11 years ago
Closed 11 years ago
#6244 closed enhancement (fixed)
dojo.fadeInTo and fadeOutTo Ehancement in dojox.fx?
Reported by: guest Owned by: dante
Priority: high Milestone: 1.2
Component: Dojox Version: 1.0
Keywords: Cc: ole.ersoy@…
Blocked By: Blocking:
Description (last modified by dante)
HI,
I started hunting through the dojo.fadeIn/Out code to see how a node could be faded out or in to a specified opacity. Looking at the methods one discovers that the end and begin properties contain the opacity, but there's no way to override that using simply fadeIn or fadeOut. When fading out one has to fadeOut to opacity 0, and when fading in one has to fade in to Opacity 1.
After a little more research one finds out that dojo uses animateProperty to achieve the fading. So one could do something like this (dojox.fx.fadeInTo(args):
Note that we should probably check whether args.node is a string or a node
var node = dojo.byId(args.node);
var args = { node,
duration: 1000, properties: {
opacity: { start:node.style.opacity,
end:0.80
}
}
};
return dojo.animateProperty(args);
It would be nice though if there was a set of utility methods like this:
dojox.fx.fadeInTo dojox.fx.fadeOutTo
I think most users would look for this first.
Cheers,
• Ole
Change History (7)
comment:1 Changed 11 years ago by guest
Scratch the InTo? / OutTo?. Since the starting point is always provided by the node, all that is needed is a dojox.fx.fadeTo(). Here's an implentation that's tested:
dojox.fx.fadeTo = function(/*Object*/ args) {
summary: Creates an animation that will fade a node from its current opacity to a specified opacity.
example: | | dojox.fx.fadeInTo({ node: 'myNode', | duration: 1200, | end:0.9, | }).play();
var node = dojo.byId(args.node);
args = {
node: node,
duration: args.duration, easing: args.easing, properties: {
opacity: { start:node.style.opacity,
end:args.end,
}
}
};
return dojo.animateProperty(args);
}
comment:2 Changed 11 years ago by guest
Some test code and test nodes:
dojo.require("dojox.fx.easing"); dojox.fx.fadeTo({ node: 'testFadeInTo',
duration: 1200, end:0.9, easing: dojox.fx.easing.easeIn
}).play();
dojox.fx.fadeTo({ node: 'testFadeOutTo',
duration: 1200, end:0.2
}).play();
<h1 id="testFadeInTo" style="height:40px; width:200px; opacity: 0;"> CHECK THE PARTIAL FADE IN ACTION </h1>
<h1 id="testFadeOutTo" style="height:40px; width:200px; opacity: 1;"> CHECK THE PARTIAL FADE OUT ACTION </h1>
comment:3 Changed 11 years ago by dante
Owner: changed from anonymous to dante
what about
dojox.fx.fadeTo = function(args){
args.node = dojo.byId(args.node);
return dojo._fade(args);
};
then you would
dojox.fx.fadeTo({ node: "someNode", end:0.75 }).play();
probably don't even need the byId check.
comment:4 Changed 11 years ago by guest
I commented out the "long" version of dojox.fx.fadeTo and substituted with yours and the tests still run the same. Me likes it.
Thanks,
• Ole
comment:5 Changed 11 years ago by guest
Just a quick check. I assume the _fade means that dojo._fade is "private". If that's the case should dojox.fx.sizeTo be using it?
comment:6 Changed 11 years ago by bill
Component: GeneralDojox
Milestone: 1.2
Setting milestone so this doesn't show up on 1.1 report.
comment:7 Changed 11 years ago by dante
Description: modified (diff)
Resolution: fixed
Status: newclosed
fixed in [13417]
Note: See TracTickets for help on using tickets.
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| 0.773265 |
What the Heck is SSDLC (Secure Software Development Lifecycle), and why should devs care?
Ariel Beck
February 15, 2023
Start Free
What the Heck is SSDLC (Secure Software Development Lifecycle), and why should devs care?
When we sit down to code, most of us are more concerned with functionality, speed, cost, and of course, making sure it works. But securing your development process is beyond just a ‘nice-to-have.’ Security is more than ensuring all your input data is sanitized at the end of the development process. If you don’t make it an integral part of your entire process, you might deliver a product with malware and backdoors.
This article will examine how the new SSDLC integrates into your current development life cycle and why you should care about it. Plus, we will leave you with some best practices to include in your development process, no matter how small or complex your project is.
How traditional SDLC works
The end goal of software development is to produce high-quality software. Over time, the process has been codified into phases. These phases can be roughly broken down into:
1. Requirement analysis
2. Planning
3. Software design, such as architectural design
4. Software development
5. Testing
6. Deployment
You may find yourself iterating through some of the phases more than once or in a different order (possibly in parallel). Still, this process would take you from an idea to deploying a solid working software product.
In the old days, securing the software was mainly the prerogative of the testing team (QA). They would examine the functionality and code to see if they breach security concerns (or break in general). Before deployment, the security team would review the application to ensure it met all security requirements.
The problem with this approach is that it leaves security in the hands of just a few key people. That means checking for security concerns may become a bottleneck for continued development. This process almost guarantees that the code would go back to development at least once for the dreaded refactoring, meaning that the project would take longer and cost more.
What does a Secure Software Development Lifecycle (SSDLC) encompass?
The idea behind secure software development is to embed security concerns into every process stage rather than just leaving it in one phase. Continuous security is present when you consider the project requirements, map out the design, the various building blocks and infrastructure needed, and sit down to write the code.
The strategy of spreading out security concerns across the entire development process and involving the developers in both the planning and implementation of security is called shift-left.
For developers who have never given security concerns much thought, the idea that it’s suddenly part of their responsibility may be daunting. Breaking the requirements into several well-defined best practices makes it much more manageable. It’s just like learning to use a new development tool.
Jit allows developers to define security requirements as code over their entire tech stack. Once you determine these requirements, Jit ensures they are continually kept.
Key software security practices
Let’s look at a few security best practices that can help you maintain a secure code base and are easy to incorporate into your day-to-day development process.
1. Code reviews
Code review is about reviewing source code to identify potential security issues, coding errors, and other software defects. Code reviews are usually conducted by a more experienced developer than the one who wrote the code. They can help ensure that the application is secure and the code is high quality. They also help maintain a unified standard and coding conventions (like variable and function names) across the entire code base.
It is considered a best practice to only merge code into the main branch with at least two sets of eyes having looked it over. The first set of eyes is, of course, the developer who wrote the code. Even very experienced developers, including team leaders, can benefit from their code being reviewed by someone else.
2. Testing
You don’t need us to tell you how vital testing your code is to address security issues before they become a problem. Since most code projects are complex and interdependent, there is no way to predict how even a tiny gap might affect the code base security down the line.
When you write automated tests, consider the functionality of the piece you have just written and how it interacts with any other relevant part of the code base, both in the front and back end.
Security issues traditionally covered in testing could include vulnerabilities such as SQL injection, cross-site scripting, insecure data storage, or insufficient memory allocation (which could lead to Buffer overflows). Testing should also cover memory leaks, poor or slow performance, or usability issues.
3. Scanning
Several kinds of scanning can be employed to increase the overall security of your code. These include static analysis, dynamic analysis, and interactive analysis
Static analysis scans source code to identify potential security issues or obvious coding errors. You can use it to check for coding standards violations, insecure coding practices, and potential vulnerabilities. Since it only reviews the code’s syntax, it doesn’t account for anything that happens at run time.
Dynamic analysis scans the application while running to identify potential security issues, coding errors, and other problems. You can use it to spot memory leaks, poor performance, and potentially breaking inputs or processes. Note that this kind of testing is done at a specific time with particular inputs, so the tests are only as good as the people who designed them. The goal is to find the problems before your users do.
Interactive analysis scans the application by interacting with it to identify potential security issues and other breaking defects. It can check for user interface issues, usability issues, and potential vulnerabilities.
Not all types of scanning fit all kinds of applications, so you must match the type of tests required to the code you wrote and its intended functionality.
4. Training
Part of the problem for many developers is they feel they are not sufficiently familiar with potential security issues and wouldn’t think to prevent them in advance or scan for them later.
Most developers know that allowing unsanitized input data into your backend could lead to remote code activation - like the famous kid named ‘drop tables.’ However, not as many would know how to test for buffer overflow or check for vulnerabilities in transient dependencies.
Training helps developers fill in these knowledge gaps. The more developers know about security issues and potential vulnerabilities, the better suited they are to incorporate these concerns into their day-to-day coding and testing.
Why should developers care about SSDLC?
There are quite a few benefits to spreading the responsibility for creating secure code among all the development process participants, including the developers. When everyone is security-aware and alert, more problems are caught earlier in the development process. It’s far better to be aware of issues earlier than wait for the tester or security expert to look for them.
Some problems caught in a later stage are already so entwined in the system that it’s almost impossible to root them out without extensive redesign, adding potentially substantial costs and time to your project.
With the developers actively scanning and fortifying their code, there would be less refactoring, reducing overall development costs and causing fewer roadblocks for planned release times. Including automation in the process would make it easier and faster for the developers involved.
Make your development process more secure
Developers WANT to make their code more secure. Most developers wouldn’t intentionally write bad, buggy, or breachable code. The lack of knowledge, tools and proper processes usually stops them from taking full ownership of the security of their code and the whole development process. When planning, designing, development, and testing are done with the addition of code security lenses, the entire software development ecosystem benefits.
Working with the SSDLC methodology is more secure, cost-effective, faster, and agile than leaving security for last. And Jit makes it easier than ever to implement it by enabling you to embed security tools and controls for all layers of your app. Give Jit a go and start for free.
Instantly achieve continuous product security, from day 0
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| 0.92093 |
Changes between Initial Version and Version 1 of PDAF_assimilate_lseik
Ignore:
Timestamp:
Jan 15, 2015, 4:39:54 PM (6 years ago)
Author:
lnerger
Comment:
--
Legend:
Unmodified
Added
Removed
Modified
• PDAF_assimilate_lseik
v1 v1
1= PDAF_assimilate_lseik =
2
3This page documents the routine `PDAF_assimilate_lseik` of PDAF.
4
5The routine is typically called in `assimilate_pdaf` or directly in the model code.
6
7The general aspects of the filter specific routines `PDAF_assimilate_*` are described on the page [ModifyModelforEnsembleIntegration Modification of the model code for the ensemble integration] and its sub-page on [InsertAnalysisStep inserting the analysis step]. The routine is used in the fully-parallel implementation variant of the data assimilation system. When the 'flexible' implementation variant, the routines `PDAF_put_state_*' are used.
8
9The interface when using the LSEIK filter is the following:
10{{{
11 SUBROUTINE PDAF_assimilate_lseik(U_collect_state, U_distribute_state, U_init_dim_obs_f, U_obs_op_f, &
12 U_init_obs_f, U_init_obs_l, U_prepoststep, U_prodRinvA_l, &
13 U_init_n_domains, U_init_dim_l, U_init_dim_obs_l, &
14 U_g2l_state, U_l2g_state, U_g2l_obs, &
15 U_init_obsvar, U_init_obsvar_l, U_next_observation, status_pdaf)
16}}}
17with the following arguments:
18 * `U_collect_state`: The name of the user-supplied routine that initializes a state vector from the array holding the ensemble of model states from the model fields. This is basically the inverse operation to `U_distribute_state` used in [ModifyModelforEnsembleIntegration#PDAF_get_state PDAF_get_state] and also here.
19 * `U_distribute_state`: The name of a user supplied routine that initializes the model fields from the array holding the ensemble of model state vectors.
20 * `U_init_dim_obs_f`: The name of the user-supplied routine that provides the size of the full observation vector
21 * `U_obs_op_f`: The name of the user-supplied routine that acts as the full observation operator on some state vector
22 * `U_init_obs_f`: The name of the user-supplied routine that initializes the full vector of observations
23 * `U_init_obs_l`: The name of the user-supplied routine that initializes the vector of observations for a local analysis domain
24 * `U_prepoststep`: The name of the pre/poststep routine as in `PDAF_get_state`
25 * `U_prodRinvA_l`: The name of the user-supplied routine that computes the product of the inverse of the observation error covariance matrix with some matrix provided to the routine by PDAF.
26 * `U_init_n_domains`: The name of the routine that provides the number of local analysis domains
27 * `U_init_dim_l`: The name of the routine that provides the state dimension for a local analysis domain
28 * `U_init_dim_obs_l`: The name of the routine that initializes the size of the observation vector for a local analysis domain
29 * `U_g2l_state`: The name of the routine that initializes a local state vector from the global state vector
30 * `U_l2g_state`: The name of the routine that initializes the corresponding part of the global state vector from the the provided local state vector
31 * `U_g2l_obs`: The name of the routine that initializes a local observation vector from a full observation vector
32 * `U_init_obsvar`: The name of the user-supplied routine that provides a global mean observation error variance (This routine will only be executed, if an adaptive forgetting factor is used)
33 * `U_init_obsvar_l`: The name of the user-supplied routine that provides a mean observation error variance for the local analysis domain (This routine will only be executed, if a local adaptive forgetting factor is used)
34 * `U_next_observation`: The name of a user supplied routine that initializes the variables `nsteps`, `timenow`, and `doexit`. The same routine is also used in `PDAF_get_state`.
35 * `status_pdaf`: The integer status flag. It is zero, if `PDAF_assimilate_lseik` is exited without errors.
36
37Note:
38 * The order of the routine names does not show the order in which these routines are executed. See the [#Executionorderofuser-suppliedroutines section on the order of the execution] at the bottom of this page.
39
40
41The user-supplied call-back routines are described on the page on [ImplementAnalysislseik implementing the analysis step of the LSEIK filter].
42
43It is recommended that the value of `status_pdaf` is checked in the program after PDAF_assimilate_lseik is executed. Only if its value is 0 the initialization was successful.
44
45PDAF also has a [PdafSimplifiedInterface Simplified Interface] providing the routine `PDAF_assimilate_lseik_si`. In the simplified interface, the name of the user-supplied routines have predefined names and do not appear in the call to `PDAF_assimilate_lseik_si`. More information on the pre-defined names is provided in the [ImplementAnalysislseik page on implementing the analysis step of the LSEIK filter].
|
__label__pos
| 0.999012 |
Results 1 to 6 of 6
1. #1
3 Star Lounger
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Access text in bound textbox (2000)
I'm having trouble getting to the text that is currently displayed in a bound text box. I'm trying to provide a keyboard shortcut for users to insert @ signs into email addresses on PCs that do not have full keyboards. I'm using onKeyPress to trap a control + key combination (I've selected Ctl+E as all the obvious ones have gone). The control in question is called txtEmail and is bound to a text field. In the KeyPress event I use
strEmail = Me.txtEmail
strEmail = strEmail & "@"
Me.txtEmail = strEmail
Me.txtEmail.SelStart = Len(Me.txtEmail)
This adds the @ sign, deselects the text then places the insertion point at the end of the new string ready for the rest of the email. However, if there is already an entry in this field, which has to be changed, when the user deletes the previous entry, enters the new address and presses Clt+E, the old entry reappears and the new entry is lost.
Obviously for fields that already contain data Me.txtEmail is looking at the saved entry in the table. Is there a way of referencing the current contents of the textbox rather than the contents of the bound field?
Ian
2. #2
Plutonium Lounger
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Re: Access text in bound textbox (2000)
<hr>referencing the current contents of the textbox rather than the contents of the bound field<hr>
When you are editing, you are addressing the current contents rather than the field. The changes aren't written back to the field until the AfterUpdate event of the control. You only posted a snippet of code. Is there more after that?
Charlotte
3. #3
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Re: Access text in bound textbox (2000)
Not much more, this is the lot.
Private Sub txtEmail_KeyPress(KeyAscii As Integer)
' Provide @ sign. About the only spare Ctl-Key combination left that makes
' any sense is Ctl-E (for e-mail, geddit)
Const TrapKey = 5
Dim strEmail As String
If KeyAscii = TrapKey Then
strEmail = Me.txtEmail
strEmail = strEmail & "@"
Me.txtEmail = strEmail
Me.txtEmail.SelStart = Len(Me.txtEmail)
End If
End Sub
4. #4
Plutonium Lounger
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Re: Access text in bound textbox (2000)
When you refer to Me.txtEmail without specifying a property, Access silently uses the default property of the text box, i.e. the stored Value. To refer to the uncommitted text currently being entered, use the Text property:
Me.txtEmail.Text
But you can't use Ctrl+E, for the KeyPress event only reacts to "character" keys, not to function keys or combinations with Ctrl etc. You must use the KeyDown (or KeyUp) event to handle Ctrl+key combinations. Try this:
Private Sub txtEmail_KeyDown(KeyCode As Integer, Shift As Integer)
If KeyCode = vbKeyE And (Shift And acCtrlMask) > 0 Then
' Insert @
txtEmail.Text = txtEmail.Text & "@"
txtEmail.SelStart = Len(txtEmail.Text)
' Prevent further handling of keystroke
KeyCode = 0
End If
End Sub
5. #5
3 Star Lounger
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Re: Access text in bound textbox (2000)
Thanks for the tip Hans, the Text property did the trick.
However (1) I think you're wrong about KeyPress, which does allow combinations of letters with the Control key.
However (2) It won't allow combinations with the Alt key. To put in an @ sign it would make more sense from a user's point of view to use something like Alt-A. How do I modify your code to do that? Does
If KeyCode = vbKeyE And (Shift And acCtrlMask) > 0 Then
become
If KeyCode = vbKeyA And (Shift And acAltMask) > 0 Then
?
6. #6
Plutonium Lounger
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Re: Access text in bound textbox (2000)
Hi Ian,
You're quite right about KeyPress being able to handle Ctrl+letter combinations. Somehow, it escaped my attention - it is mentioned in the online help.
However, KeyPress can't handle Alt, Ctrl+Alt, Shift+Alt, Ctrl+Shift etc. You need KeyDown or KeyUp for that. The Shift parameter can consist of any combination of acShiftMask, acAltMask and acCtrlMask. To test if one of these applies, test if (Shift and ac...Mask) > 0.
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__label__pos
| 0.714201 |
Sign up
Title:
Document transmission techniques IV
Kind Code:
A1
Abstract:
A method of determining the authenticity of a digital document sent by an unknown sender. The method comprises receiving a digital document and an encrypted digest of the document created by the sender using a hash algorithm. The digest is encrypted using a first token, such as a private key, of the sender. The method also comprises obtaining a second token, such as a public key, relating to the first token, decoding the encrypted digest using the second token, using a hash algorithm to create a digest of the document; and comparing the decrypted received digest with the newly created digest to determine the authenticity of the sender and the document. The receiving step may comprise receiving a digital certificate of the sender within which the second token is contained as part of the sender's digital certificate. Also the validity of the sender's certificate can be checked on-line.
Inventors:
Harrison, Keith Alexander (Chepstow, GB)
Brown, Richard (Bristol, GB)
Application Number:
09/918062
Publication Date:
02/28/2002
Filing Date:
07/30/2001
Primary Class:
International Classes:
H04L9/32; (IPC1-7): H04L9/32
View Patent Images:
Attorney, Agent or Firm:
Intellectual Property Administration,HEWLETT-PACKARD COMPANY (P.O. Box 272400, Fort Collins, CO, 80527-2400, US)
Claims:
1. A method of determining the authenticity of a digital document sent by an unknown sender, the method comprising: receiving a digital document, an encrypted digest of the document created by the sender using a hash algorithm, the digest being encrypted using a first token of the sender; obtaining a second token relating to the first token; decoding the encrypted digest using the second token; using a hash algorithm to create a digest of the document; and comparing the decrypted received digest with the newly created digest to determine the authenticity of the sender and the document.
2. A method according to claim 1, wherein the receiving step comprises receiving a digital certificate of the sender.
3. A method according to claim 2, wherein the obtaining step comprises the second token being sent as part of the sender's digital certificate.
4. A method according to claim 2, further comprising carrying out an on-line check of the validity of the sender's certificate.
5. A method according to claim 1, wherein the first and second tokens comprise private and public encryption/decryption keys of the sender.
6. A method according to claim 1, further comprising printing out a copy of the document once the sender and the document have been authenticated.
7. A method according to claim 6, wherein the method further comprises printing a verifying mark on the printed copy of the document to signify its authenticity.
8. A method according to claim 1, wherein the transmitted document comprises a fax document.
9. A method of sending a digital document to a recipient together with data enabling the document and the sender to be authenticated, the method comprising: creating a digest of the document using a hash algorithm; encrypting the digest using a first token of the sender; obtaining a second token relating to the first token of the sender, which can be used to decrypt the encrypted digest; sending the encrypted digest, the digital document and the second token to the recipient.
10. A method according to claim 9, wherein the transmitted document is a fax document.
11. A method according to claim 9, further comprising the sender proving their identity prior to the sending step by transferring data from a personal portable data carrier holding the first token to a transmission station from which the document is to be sent.
12. A method according to claim 11, wherein the proving step further comprises the sender entering a verifiable security identifier into the transmission station to establish that they are the legitimate owner of the portable data carrier.
13. A method according to claim 11, wherein the step of encrypting the digest comprises supplying the digest of the document from the transmission station to the portable data carrier of the sender, encrypting the digest of the document on the portable data carrier, and returning the encrypted digest of the document from the portable data carrier to the transmission station.
14. A method according to claim 9, further comprising obtaining details of the sender including the second token prior to transmitting the document.
15. A method according to claim 14, wherein the step of obtaining details comprises obtaining the sender's details from a central database storing second tokens and other sender's details.
16. A method according to claim 14, wherein the sender's details and the second token are provided in a sender's digital certificate.
17. A method according to claim 9, wherein the first and second tokens comprise private and public encryption/decryption keys of the sender.
18. A device for determining the authenticity of a digital document sent by an unknown sender, the device comprising: a communications module arranged to receive the document, an encrypted digest of the document created by the sender using a hash algorithm, the digest being encrypted using a first token of the sender, and a second token relating to the first token; and a controller arranged to decode the encrypted digest using the second token; creating a digest of the document using a hash algorithm; and comparing the decrypted received digest with the newly created digest to determine the authenticity of the sender and the document.
19. A device for sending a digital document to a recipient together with data enabling the document and the sender to be authenticated, the device comprising: a controller arranged to create a digest of the document using a hash algorithm and to encrypt the digest using a first token of the sender; and a communications module arranged to obtain a second token related to the first token of the sender, which can be used to decrypt the encrypted digest and to send the encrypted digest, the digital document and the second token to the recipient.
Description:
FIELD OF THE PRESENT INVENTION
[0001] The present invention concerns improvements relating to document transmission techniques and more specifically, though not exclusively, to a new fax transmission protocol which can be used in conjunction with the existing standard fax protocols to provide additional security in fax transmission and/or delivery. The present invention has application to virtual private networks of document printout machines and can be used in document verification subsequent to a document having been delivered to its intended recipient.
BACKGROUND TO THE PRESENT INVENTION
[0002] The use of fax machines for the transmission of documents is a well established and essential business practice. Even though the advent of computers and the Internet has heralded the advent of electronic document transmission via e-mail, the use of fax machines has not been made redundant. Rather, there are several differences between the use of fax machines and computers that can be advantageous in many circumstances and these have maintained the requirement for fax machines in offices as is explained below.
[0003] One of the major distinctions between the use of computers and fax machines for document transmission is seen in that fax machines are often left constantly on-line and are therefore readily accessible whereas computers are often switched off (at night for example). Also at present, e-mail to computers actually has to be retrieved from an ISP (Internet Service Provider) as the e-mail address of the person resides there, whereas for fax documents, the machine itself (at the user's location) receives and prints out the document directly to the recipient. Furthermore, the cost of a computer, a printer (required for a hard copy of the document) and a scanner (required for making an electronic copy of a paper document) is far more than that of a fax machine which can incorporate simple modem, scanning and printing technology. This has been a significant factor in the greater ownership of fax machines than computers all over the world.
[0004] Whilst fax machines clearly have their niche in office communications, there is however, an inherent lack of security in this way of document transmission. More specifically, a person wishing to intercept the fax transmission could do so without great difficulty and could reassemble the serial bits of the fax message to recreate the document being faxed. Also, the incorrect dialing of a fax number and the resultant sending of the document to a wrong place can often lose the confidentiality of the transmitted information. The printing out of the document when received at a shared fax machine (such as one at a hotel reception) can also compromise confidentiality if that document is read by an unscrupulous person, for example, prior to its intended recipient reaching the fax machine. Furthermore, there is also no way of knowing for sure where the fax came from (the telephone fax header can easily be altered to reflect a different identity) or of knowing for sure that the fax has been received by the person meant to receive it. Given the present security in fax transmissions, it is even possible for an unscrupulous person to scan electronically someone's written signature, to append this to the document and then to fax the combination as a request for an authorisation to a service (e.g. to sell some shares).
[0005] The above lack of security of conventional fax systems has been known for some time now. Over the past several years, in applications where secure document delivery is paramount, there has been a significant trend away from the use of fax machines to the use of secure computer systems and secure e-mail. This has in turn led to loss of the significant advantages associated with the use of relatively simple fax machines as compared with computer/printer/scanner combinations.
SUMMARY AND OBJECTS OF THE PRESENT INVENTION
[0006] It is an object of the present invention to overcome or substantially reduce at least some of the above described problems. It is another object of the present invention to provide an improved document transmission/reception protocol that provides a secure method of communication from party to party.
[0007] The present invention aims to increase the confidence in the authenticity of a sent or a received fax document. It is also desired to close at least some of the existing loopholes in document delivery security and to improve the security of fax document transmission generally.
[0008] The present invention resides in the appreciation that many of the above described problems with fax document transmission techniques can be solved or substantially reduced by use of authentication techniques with the fax transmission protocols, namely the document being transmitted can be digitally signed in a readily verifiable way.
[0009] According to one aspect of the present invention there is provided a method of delivering a digital document to an intended recipient at a printout station, the method comprising: receiving and securely retaining a transmitted document and a transmitted independently verifiable data record of the intended recipient at a printout station; obtaining a first token of the intended recipient; requesting proof of the intended recipient's identity at the printout station using data in the independently verifiable data record of the intended recipient; and releasing the document when the intended recipient has proved their identity by use of a second token that is uniquely related to the first token.
[0010] In one embodiment, the retaining step comprises storing the received document in memory without printing out a copy of it on receipt, with a copy only being printed when the releasing step occurs. This provides a very secure way of document delivery and requires only a software modification of the document printout station if it is was conventional fax machine or other printout station. In another embodiment of the present invention, the retaining step comprises printing out the document as received and placing it in a locked compartment. Here the document can be transmitted and printed out as a standard document with the access to the document being controlled.
[0011] Preferably, the requesting step comprises requesting supply of data encoded with the second token which can be decoded with the first token. In this way, the document printout station can readily decode user identification data and because of the unique relationship between the first and second tokens the security of the system is maintained.
[0012] The releasing step may be carried out when the intended recipient has presented a portable data carrier holding the second token to the printout station and has transferred data to prove their identity. The intended recipient can thus carry around with them the second token which can be used at any document printout machine to verify their identity.
[0013] For added security, the releasing step may further comprise the intended recipient entering a verifiable security identifier into the printout station to establish that they are the legitimate owner of the portable data carrier. This security identifier is typically a PIN (Personal Identification Number) though it could also be the biometrics of a person such as a signature or a fingerprint.
[0014] The obtaining step may conveniently comprise extracting the first token transmitted with the document and the data record. Also, the intended recipient's independently verifiable data record may be provided as an intended recipient's digital certificate, which incidentally would also contain a copy of the first token. This is the most commonly used way of providing information which can be authenticated about a particular individual and has the advantage of enabling the document printout station to validate the identity of any entity at any time as well as providing all of the required information about the recipient or sender together with the first token (a public key when PKI (Public-Key Infrastructure) is being used) in one standard document.
[0015] The method may further comprise carrying out an on-line check of the validity of the intended recipient's independently verifiable data record. Whilst this is not necessary for each document received, it can be used as a random check or where there is apparently a higher risk of fraud. However, the method may further comprise instructing a third party to carry out an on-line check of the validity of the intended recipient's independently verifiable data record. This frees up the document printout station to carry out other tasks and more importantly does not engage the communications link into the document printout station for a significant period of time.
[0016] The releasing step in this case may further comprise only releasing the document if the validity of the independently verifiable data record has been confirmed as a result of the check. Clearly this would slow down the process, but it would provide one of the highest levels of security for ensuring that the intended recipient is actually who they are claiming to be.
[0017] As a further security measure the transmitted document may be encrypted and the method may further comprise decrypting the received document once the intended recipient has proved their identity. This ensures that even if the document is intercepted, that it will not be readily readable. Preferably, enveloping techniques are used to minimise the computational processing time the encryption/decryption techniques take. More specifically, where the transmitted document has been encrypted with a session key and the session key has been encrypted with the first token, the transmitting step preferably comprises transmitting the encrypted session key to the printout station, and the decrypting step preferably comprises decrypting the encrypted session key with the second token and decrypting the received document with the decrypted session key.
[0018] It is possible to configure the method of the present invention to work in the following way. The receiving step comprises receiving a plurality of transmitted independently verifiable data records of a plurality of intended recipients at the printout station; the obtaining step comprises obtaining the first tokens of each of the intended recipients; the requesting step comprises requesting proof of each of the intended recipients' identities at the printout station using data in the independently verifiable data records of the intended recipients; and the processing step comprises processing each of the intended recipients' response to the request and releasing the document when all of the intended recipients have proved their identity by use of respective second tokens that are each uniquely related to respective ones of the first tokens. In this way, it is possible to ensure that several people are present when a document is released. This feature can ensure that no one person gains an advantage over another when each person should see the document at about the same time. Also, this feature would provide the necessary means if, for security purposes, all of the members of the group needed to be present in order to access a received document.
[0019] With the group feature described above, the transmitted document or a session encryption/decryption key of the transmitted document may have been sequentially encrypted with each of the first tokens of the intended recipients in a given order and the processing step may comprise sequentially decrypting the transmitted document or a session encryption/decryption key with each of the second tokens of the intended recipients in the reverse of the given sequential order.
[0020] The present invention also extends to a device for delivering a digital document to an intended recipient, the device comprising: a communications module for receiving an electronic version of the transmitted document over a communications network, an independently verifiable data record of the intended recipient, and a first token of the intended recipient; a store for securely retaining the transmitted document, the transmitted independently verifiable data record and the first token; an instruction module for requesting proof of the intended recipient's identity using data provided in the intended recipient's data record; and a controller for releasing the document when the intended recipient has proved their identity by use of a second token that is uniquely related to the first token.
[0021] According to another aspect of the present invention there is provided a method of delivering a digital document from a first station via a communications network to an intended recipient at a second station, the method comprising: obtaining details of the intended recipient, including an independently verifiable data record of the intended recipient at the first station; transmitting the document and the independently verifiable data record of the intended recipient to the second station; receiving and securely retaining the transmitted document and data record at the second station; obtaining a first part of an intended recipient's identifying token at the second station; requesting proof of the intended recipient's identity at the second station using the transmitted independently verifiable data record; and releasing the document to the intended recipient when the intended recipient has proved their identity using a second part of the recipient's identifying token.
[0022] The term digital document as used in the present specification is intended to mean a digital representation of a document regardless of the content of the document. The document can contain images or text or both, for example.
[0023] The present invention also aims to ensure the identity of an unknown sender of a digital document and the authenticity of the document itself. This is essentially achieved with the use of independently verifiable data records and fingerprints (digests) of the document being sent.
[0024] More specifically, according to another aspect of the present invention there is provided a method of determining the authenticity of a digital document sent by an unknown sender, the method comprising: receiving a digital document, an encrypted digest of the document created by the sender using a hash algorithm, the digest being encrypted using a first token of the sender; obtaining a second token relating to the first token; decoding the encrypted digest using the second token; using a hash algorithm to create a digest of the document; and comparing the decrypted received digest with the newly created digest to determine the authenticity of the sender and the document.
[0025] The receiving step preferably comprises receiving a digital certificate of the sender for the reasons which have been set out previously. In this case, the second token is preferably conveniently obtained by being sent as part of the sender's digital certificate.
[0026] The method may further comprise carrying out an on-line check of the validity of the sender's certificate. This feature provides increased security as the authenticity of the sender can be verified via an independent certificate issuing authority. However, this check can also be carried out by a third party by way of assignment by the document printout station and this in turn ensures that the communications line to the printout station and time of the document printout station is not occupied for a long period of time.
[0027] The first and second tokens preferably comprise private and public encryption/decryption keys of the sender. The use of PKI provides a layer of security which ensures that the claimed author of a document is in fact that document's author.
[0028] The method may further comprise printing a verifying mark on the printed copy of the document to signify its authenticity. This provides an instantaneous quality assurance mark which can be extremely helpful in reassuring users of the document after it has been transmitted that it is genuine.
[0029] According to another aspect of the present invention, there is provided a met-hod of sending a digital document to a recipient together with data enabling the document and the sender to be authenticated, the method comprising: creating a digest of the document using a hash algorithm; encrypting the digest using a first token of the sender; obtaining a second token relating to the first token of the sender, which can be used to decrypt the encrypted digest; sending the encrypted digest, the digital document and the second token to the recipient.
[0030] The method may further comprise the sender proving their identity prior to the sending step by transferring data from a personal portable data carrier holding the first token to a transmission station from which the document is to be sent. This portable identity of the sender advantageously enables him or her to use any document transmission station to send a document.
[0031] The proving step may further comprise the sender entering a verifiable security identifier into the transmission station to establish that they are the legitimate owner of the portable data carrier. This feature provides further security to prevent stolen portable data carriers from being used, for example.
[0032] The step of encrypting the digest may comprise supplying the digest of the document from the transmission station to the portable data carrier of the sender, encrypting the digest of the document on the portable data carrier, and returning the encrypted digest of the document from the portable data carrier to the transmission station. This is how a portable data carrier holding the first token would be used to prove the identity of the sender without compromising the security of the portable data carrier (first token) itself.
[0033] The method may further comprise obtaining details of the sender including the second token prior to transmitting the document. These details could be readily obtained from a central directory database, storing second tokens and other sender's details, such as LDAP and so the identification information regarding the sender could be obtained from a trusted up-to-date source. Alternatively, the details could be obtained more quickly from the sender themselves, for example from their portable data store. In either case, the sender's details and the second token could be provided in a sender's digital certificate.
[0034] The present invention may also be considered to be a device for determining the authenticity of a digital document sent by an unknown sender, the device comprising: a communications module arranged to receive the document, an encrypted digest of the document created by the sender using a hash algorithm, the digest being encrypted using a first token of the sender, and a second token relating to the first token; and a controller arranged to decode the encrypted digest using the second token; creating a digest of the document using a hash algorithm; and comparing the decrypted received digest with the newly created digest to determine the authenticity of the sender and the document.
[0035] The present invention also extends to a device for sending a digital document to a recipient together with data enabling the document and the sender to be authenticated, the device comprising: a controller arranged to create a digest of the document using a hash algorithm and to encrypt the digest using a first token of the sender; and a communications module arranged to obtain a second token related to the first token of the sender, which can be used to decrypt the encrypted digest and to send the encrypted digest, the digital document and the second token to the recipient.
[0036] There are many security advantages in having a document printout station that has a sophisticated memory which can provide a memory of the validity of each document page it has printed out.
[0037] More specifically, according to another aspect of the present invention there is provided a document printout device for receiving and printing out digital documents, the printout device comprising: a store of digital certificates, each certificate being associated with a received digital document; and an audit log comprising a list of received document entries, each entry containing a reference to one of the certificates in the store and a unique identifier associated with a received digital document.
[0038] The device is preferably arranged to carry out an on-line authentication of a received certificate held in the store of received documents. This enables the validity of a digital certificate to be checked either in real time or at a later date or time, if necessary, to confirm the authenticity of the printed out document.
[0039] The device may be arranged to carry out a batch of on-line authentications of received certificates held in the store of received documents. This feature provides a way of minimising the amount of on-line time required for self-authentications of the received certificates and also allows them to be carried out at a time when there is less potential traffic conflicts to be considered.
[0040] Each entry in the audit log may contain a digest of the received document to which it relates. This is an optimal space saving way of storing each document in the audit log. The reason why the full document is not required is that its use would only be for comparison purposes.
[0041] In this regard, the device may further comprise a hash algorithm for creating a digest of a digital document and a receiving module for receiving a digital representation of a previously printed out document, wherein the device is arranged to create a digest of the digital representation of the previously printed out document and to compare the newly created digest with the corresponding digest stored in the audit log. In this way, any printed out document can be verified as having been printed out by a specific device, thereby further helping to reduce opportunity for fraudulent copies of documents.
[0042] Preferably, the device is arranged to send either a stored digest or a newly created digest of a document to its original sender and to verify the authenticity of the document back to its source by considering the transmitted results of a comparison of digests carried out at the source. This feature advantageously enables a check on the authenticity of a document to be made right back to its source.
[0043] The receiving module may be a document scanning module such that the actual document printed out may be scanned back in for the comparison.
[0044] Each entry in the audit log may contain the time and date of receipt of each digital document to further help establish the integrity of the received data when carrying out comparison checks, for example.
[0045] The unique identifier is preferably an alphanumeric code and the device preferably further comprises an input module for inputting the code to access the relevant entry in the audit log. This enables stored information about a particular printed out document to be obtained by use of the unique identifier on printed on the document itself. There is no need to have the document present, only the identifier is required. This also enables the exact machine from which a document originated to be identified, such that any further details regarding the document stored at the machine can be accessed.
[0046] According to another aspect of the present invention there is provided a method of authenticating the identity of a sender of a received digital document, the method comprising: using a unique identifier printed on the received document to search for a corresponding record in a list of received document records; referencing a digital certificate associated with the selected record, the certificate being one of a store of certificates of received documents; and carrying out an on-line authentication of the certificate.
[0047] It is often the case that fax numbers of certain fax machines are only available to several people within an organisation as those fax machines should only to be used for communications from specified sources. However, it is often difficult to ensure that only specified sources will transmit to the fax machine and proving the identity of the source has not been possible before.
[0048] It is another objective of the present invention to overcome or substantially reduce the above problem.
[0049] According to another aspect of the present invention there is provided a document delivery system operable as a closed group system of a plurality of members, the system comprising: a plurality of document printout machines, each associated with a member of the closed group and being connectable to each other via a communications network, wherein each machine can access a first token unique to its associated member and a second token of each of the closed group's members corresponding to members' first tokens, and wherein each machine comprises a store of all member's independently verifiable data records, and each machine is arranged to access and utilise its first and second tokens and the data records to establish a document printout machine's membership of the closed group prior to transmission or receipt of any digital documents across the network.
[0050] In this way, a single fax machine can be configured to operate as part of a virtual private network (VPN) or alternatively as a normal fax machine. When operating as the former of these two, it can effectively screen out digital documents sent to it from other document transmission machines which are not part of the group (VPN).
[0051] Each member's stored data record preferably comprises a second token of that member. This makes accessing the second token relatively straightforward as it is provided with the previously obtained data record. Furthermore, the data record preferably comprises a digital certificate of the member issued by a Certification Authority.
[0052] At least one of the document printout machines may be arranged to check the validity of a given machine's membership at any time by carrying out an on-line check of the validity of the given machine's independently verifiable data record (digital certificate). The checking of validity of a member's membership is not necessary all the time but advantageously it can be carried out at some time if deemed necessary or as a random spot check by the at least one document printout machine.
[0053] The first token of at least one of the members may be provided on a portable data store which is readable by a data store reader of a machine. This enables the at least one member of the closed group to use the same machine as part of a VPN and other people if required to use the fax machine normally without any restriction. This adds to the ways in which the fax machine can be used thereby in some cases obviating the need for another conventional fax machine and its associated cost.
[0054] Each document printout machine may be arranged to send and receive Nonces. This advantageously increases security against fraud by preventing replay attacks on the digital data transmissions. In this case, each document printout machine may be arranged to encrypt a Nonce using a second token and to decrypt a Nonce using its associated member's first token.
[0055] The present invention also extends to a method of establishing membership of a closed group of document printout machines that can each access a first token unique to a member of the group associated with that machine and a second token of each of the closed group's members corresponding to the members' first tokens, the method comprising: sending from a first document printout machine, an independently verifiable data record of its own information to a second document printout machine; comparing at the second machine the received record with the first machine's stored data record and, if they are identical, sending the second machine's own independently verifiable data record to the first machine; comparing the received second machine's data record with the second machine's stored record and, if they are identical, authenticating the second machine as member of the closed group; and using the first and second tokens to encrypt and decrypt at least some data sent between said members of the group.
[0056] The sending step may conveniently comprise sending a second token of a member as part of that member's independently verifiable data record.
[0057] The method may further comprise sending and receiving Nonces at the first and second document printout machines. The use of Nonces improves the integrity of the method by preventing replay attacks on the digital data transmissions.
[0058] The using step may comprise encrypting a Nonce to be sent using a second token and decrypting a received Nonce using its associated member's first token.
[0059] The using step may comprise encrypting and decrypting Nonces of the first and second machines by use of public and private encryption/decryption keys of the first and second members.
[0060] As has been mentioned before, the first and second document printout machines may comprise fax machines. However, other printout devices such as computer printers may also be suitable as document printout means.
[0061] The method may further comprise authenticating any received independently verifiable data record to establish the authenticity of the data record. This provides an additional check on the identity of a person claiming to be a member and also ensures that if a member's identity has been stolen, that it cannot be used.
[0062] The authentication step preferably comprises authenticating the independently verifiable data record on-line as this is a way of determining in real time the authenticity of an entity claiming to be a member.
BRIEF DESCRIPTION OF THE DRAWINGS
[0063] Presently preferred embodiments of the present invention will now be described by way of example with reference to the accompanying drawings. In the drawings:
[0064] FIG. 1 is a schematic block diagram showing a system for transmitting faxed document data to an intended recipient according to a first embodiment of the present invention;
[0065] FIG. 2a is a flow diagram showing the process of transmitting faxed document data for an intended recipient from a sending fax machine to a receiving fax machine shown in FIG. 1;
[0066] FIG. 2b is a flow diagram showing the process of receiving faxed document data for an intended recipient at a receiving fax machine shown in FIG. 1;
[0067] FIG. 3 is a schematic representation of a receiving fax machine according to a second embodiment of the present invention;
[0068] FIG. 4 is a schematic block diagram showing a system for transmitting and receiving faxed document data from an unknown sender according to a third embodiment of the present invention;
[0069] FIG. 5 is a flow diagram illustrating the process of preparing the documentation for transmittal from the sending fax machine shown in FIG. 4;
[0070] FIG. 6 is a flow diagram illustrating the process of receiving the faxed documentation at the receiving fax machine of FIG. 4 and confirming the identity of the sender prior to confirming the authenticity of the faxed document;
[0071] FIG. 7 is a schematic block diagram showing a virtual private network system of fax machines according to a fourth embodiment of the present invention; and
[0072] FIG. 8 is a flow diagram illustrating the process of using the virtual private network system of fax machines shown in FIG. 7.
DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS
[0073] Referring now to FIG. 1 there is shown a fax system 10 according to a first embodiment of the present invention for implementing a new fax protocol which insures that a fax of a document 12 reaches its intended recipient. The fax system 10 comprises a sending fax machine 14 and a receiving fax machine 16. Both machines are able to function as normal fax machines but, in addition to this, are configured to take advantage of a more secure overlay protocol as will be described below.
[0074] The secure protocol relies on the use of digital certificates 18 which each contain a copy of a corresponding individual's public key 20. The digital certificates 18 comply with the well known X.509 standard. Public keys 20 of an individual are normally readily accessible within the electronic environment and are well known in the field of public/private key encryption techniques. Accordingly, no further detailed explanation of how these keys operate or of the certificate standard is provided herein.
[0075] In the present embodiment, the fax system 10 accesses a central database 22 of individuals' certificates which uses LDAP (Lightweight Directory Access Protocol). The LDAP database 22 is well known and is configured to enable details regarding any registered user to be obtained and provided for use in transmission of the fax to the receiving fax machine 16. In particular, an encrypted fax version 24 of the document 12 (encrypted using a session key 25) is transmitted together with the intended recipient's digital certificate 18, containing the intended recipient's public key 20 and their contact details, and the session key 25 (encrypted using the intended recipient's public key 20).
[0076] For each public key 20 stored in the LDAP database 22, there is a corresponding single private key 26 which is owned by the individual themselves. This private key 26 is provided on an intelligent portable store such as a smart card 28, which the individual keeps personal possession of. The smart card 28 not only contains the private key 26, but also an algorithm for encoding or decoding data using the private key 26. The private key 26 enables the intended recipient to uniquely identify themselves to the receiving fax machine 16 in order to received the fax document 24, as will be described in detail later.
[0077] The receiving fax machine 16 includes a store 30 which retains each of the received certificates 18 and transmitted fax documents 24 until such time as the intended recipient accesses its transmitted fax document 24. The receiving fax machine 16 also has a smart card reader 32 provided in its housing such that an individual's smart card 28 can be inserted and certain information can be read into the receiving fax machine 16. It is to be appreciated, that the individual's private key 26 is never transmitted to the receiving fax machine 16 as this would compromise the security of the data and the system 10. The details of the interaction between the individual's private key 26 and received fax is also described in detail later.
[0078] In the present embodiment, the receiving fax machine 16 is configured to receive an encrypted fax document 24, to store the fax document 24 electronically and to print it out as a paper document 34 only when an intended recipient of the fax document 24 has proved their identity by use of their private key 26. The entire process of transmitting a document to an unknown actual recipient (namely the fax machine of an unknown person or authority) for the attention of a specific known person is now described with reference to FIGS. 2a and 2b.
[0079] Referring now to FIG. 2a, the process commences with the original document 12 being scanned at 40 into the sending fax machine 14. At this stage, the sender has also to specify the identity of the intended recipient as well as the telephone number of the receiving fax machine 16. Once the identity of the intended recipient has been entered into the sending fax machine 14, the intended recipient's certificate 18 is requested and obtained at 42 from the LDAP database 22.
[0080] In order to provide secure communications, resistant to someone tapping into or diverting the fax transmission to access the document, the fax document 24 is encrypted. This is carried out in this embodiment by the use of enveloping encryption techniques. These involve the sending fax machine encrypting the document to be sent using a lightweight encryption algorithm which is computationally inexpensive. DES, Triple DES, RC2 and Skipjack are examples of such lightweight encryption algorithms. An encryption key for the lightweight encryption algorithm is then encrypted using a standard computationally heavyweight encryption algorithm, such as RSA, which uses the intended recipient's public key. The heavily encrypted algorithm key can be decrypted with the intended recipient's private key 26 at the receiving fax machine and used to decode the lightweight coded document.
[0081] More specifically, in the present embodiment, the session key 25 which is a coding algorithm key that is unique to the current communication event, is used. The process comprises selecting at 44 a session key 25 for use with the communication with the intended recipient. Then the scanned in document is encrypted at 46 using a relatively lightweight symmetric cryptographic encryption algorithm such as DES for example under the unique selected session key 25.
[0082] As the session key 25 also needs to be sent with the encrypted document 24 in order to be able to decode it, the session key 25 is encrypted at 48 using the public key 20 of the intended recipient and the computationally heavy encryption algorithm, e.g. RSA. The public key 20 of the intended recipient is incidentally also found in the intended recipient's Certificate 18. The encoding of the session key 25 ensures that it will only be decodable by the owner of the intended recipient's private key 26.
[0083] Once the sending fax machine 14 has created the encrypted version 24 of the scanned-in document 12 and has obtained the digital certificate 18 of the intended recipient, then can be sent to the receiving fax machine 16 together with the encrypted session key 25. However, prior to sending the information, the sending fax machine 14 implements a modified interconnect fax protocol to establish the link between the two machines. The modification over the standard interconnect fax protocol is that the sending fax machine 14 inquires as to whether the receiving fax machine 16 is of the type which can be used according to the present embodiment, namely one which has the capability to stop the faxed document from being printed out until the intended recipient has proved their identity. If the receiving fax machine 16 is a standard machine, this is determined at this stage and the sending fax machine 14 can either not send the fax document 24 or send it as a normal non-encrypted fax, namely without the certificate 18 and session key 25, which will be printed out conventionally. However, if the receiving fax machine 16 is capable of implementing the present invention, then the next stage is to send at 50 the encrypted fax document 24, the intended recipient's certificate 18 and the encrypted session key 25 to the receiving fax machine 16.
[0084] Referring now to FIG. 2b, the process of receiving the fax document 24 and providing it to the intended recipient is now described. The receiving fax machine 16 receives at 52 the encrypted fax document 24, the intended recipient's certificate 18 and the encrypted session key 25, and places these in the store 30. The receiving fax machine 16 then requests at 54 the intended recipient to input their smart card 28 (containing their private key 26) into the smart card reader 32. More specifically, this is carried out by the certificate 18, which contains the name of the intended recipient, being extracted from the received information and the name being displayed on the receiving fax machine 16. However, it is to be appreciated there are various different viable ways in which the intended recipient could be notified that there is a fax for them.
[0085] In response to the request, the intended recipient inputs their smart card 28 into the card reader 32 of the receiving fax machine 16. The encrypted session key 25 is then passed at 54 from the store 30 to the smart card 28. The decryption algorithm running on the processor of the smart card 28 decrypts at 56 the session key 25 using the private key 26. The decrypted session key 25 is then passed back at 58 to the receiving fax machine 16 and is used to decrypt at 58 the encrypted fax document 24.
[0086] By virtue of being able to decode the session key 25 correctly, the intended recipient user has gone a large way to proving their identity to the receiving fax machine 16 and can be permitted to access the fax document 24. However, the use of digital certificates 18 enables an extra level of security to be achieved as is now described.
[0087] The process continues with a determination at 60 of whether a verification of the intended user's certificate is required? If the answer is no, then the decrypted fax document 24 is simply printed out at 62 for the intended recipient. However, if verification of the certificate 18 is required at 60, the receiving fax machine 16 carries out an on-line authentication check at 64 of the intended recipient's certificate 18. This on-line check may actually involve authentication of a string of certificates until a trusted authority's certificate such as that of Verisign's, for example, has been received. Alternatively, this task could be designated to an authority such as Verisign to carry out and report back on or an on-line connection to the LDAP database 22 could be used to validate the certificates.
[0088] If the result of the authentication check at 64 is positive (certificates valid), the decrypted fax document 24 is released to the intended recipient by being printed out at 62. Otherwise, the release of the document 24 is prevented at 68 and an appropriate message signifying the failure of the authentication check is presented at 69 to the person trying to access it at the receiving fax machine 16.
[0089] In this embodiment, it is also possible to configure the receiving fax machine 16 to ensure that a document is sent to a group of people and that all of the group are present before the fax document 24 is printed out. This group facility is enabled carrying out the following steps.
[0090] Firstly a session key 25 is chosen for the communication event. Then the scanned in fax document 24 to be sent is encrypted using the session key 25. The digital certificates 18 of each member of the group are obtained from the LDAP database 22 (each certificate 18 containing the members public encryption key 20). Then the session key 25 is encrypted using the public key 20 of the first member of the group. The encrypted result of this is then encrypted using the public key 20 of the second member of the group. Then the encrypted result of this is encrypted using the public key of the third member of the group and so on. This process is repeated until all of the group members' public keys 20 have been used to encode the session key 25 in this manner. The multiple encrypted session key, the encrypted fax document and the members' certificates are all sent to the receiving fax machine 16.
[0091] The receiving fax machine 16 is programmed to store each of the received certificates 18 and to request each of the intended recipients of the group to prove their identity to the receiving fax machine 16 by presentation of their respective smart cards 28 possibly within a given time period. More specifically, each of the members of the group is asked in turn to present their smart keys 28 to the receiving fax machine 16 to decrypt the multiple encrypted session key 25. The decryption is carried out on the smart card 28 itself as described previously. The order in which the members need to present their smart cards 28 to the receiving fax machine 16 is the reverse of the order for encoding the session key 25, namely starting with the last member of the group and finishing with the first. Also the decrypted result received from one member's smart card 28 is provided as the input to the next member's smart card 28 until such time as the multiple layers of encryption of the session key 25 has all been decrypted. At the end of this process, the session key 25 is decrypted correctly and can then be used to decrypt and the received fax document 24 for printing out. Accordingly, all of the intended recipients (members of the group) have to be present to enable the fax document 34 to be accessed.
[0092] As encryption techniques are used in the present embodiment, the new protocol is highly transparent. Any faxes being intercepted by an unscrupulous person or which are sent to the wrong receiving fax machine 16, would be in a secure form and would not be readily understandable by the unintended receiver. Another feature of such a protocol is that, as the documents are encoded it is not necessary to send the public key 20 of the intended recipient. Rather, the received document can be presented to anyone wishing to access the document but would only be correctly decodable by the intended recipient by use of their private key 26. As mentioned above, in order to maintain a very high level of security, all of the decoding of the received session key 25 is carried out on the smart card 28 itself. This prevents the private key 26 of the intended recipient being transferred onto the receiving machine 16 which could compromise security. Smart cards 28 can be programmed in Java to run the decryption algorithm with input/output rates up to 1 Mbit/sec.
[0093] Referring now to FIG. 3, a second embodiment of the present invention is now described. The second embodiment is similar to the first and so only the differences are explained below. In this embodiment, the fax document 24 is sent to the receiving fax machine 16 in a non-encrypted format and so no session key 25 is required. On receipt of document 24, the receiving fax machine 16 prints out the received fax immediately. However, non intended recipients are prevented from reading the printed out documents 34 by virtue of them being printed out into locked compartments 36 of the receiving fax machine 16. In order to open a particular locked compartment 36, the intended recipient has once again to prove their identity by use of the smart card 28 containing their private key 26.
[0094] The way in which the intended recipient's identity can be proved is for the receiving fax machine 16 to send to the intended recipients smart card 28 some identifier data (for example a random integer or even the intended recipient's certificate itself) that has been encrypted by the public key 20 of the intended recipient (the public key 20 can be obtained from the received certificate 18 of the intended recipient). Then only the true intended recipient's private key 28, if present on the smart card 28, will be able to correctly decode the identifier data and provide it back to the receiving fax machine 16. The receiving fax machine 16 can determine the validity of the intended recipient by carrying out a simple comparison of the pre-encryption sent identification data and the received decrypted data. It is to be appreciated that this data is preferably data that has been sent from the sending fax machine 14 to provide greater confidence (at least to the sender) in the security of the system though it is possible that it can be generated at the receiving fax machine 16 itself. If the identifier data is generated at the receiving fax machine 16 it is encoded there using the public key 18 of the intended recipient. This advantageously reduces the amount of data being transmitted but is a less secure protocol than when the identifier data is encrypted and sent by the sending fax machine 14.
[0095] For additional security, which can also be applied to the first embodiment, the receiving fax machine 16 requests the PIN (Personal Identification Number) of the smart card holder to establish that the presenter of the card is its actual owner. The PIN is similar to that required for ATMs and is only known to the valid owner of the card. This security feature prevents a stolen card from being used to access the transmitted document 24. Alternatively, other biometrics can be used instead of the PIN, for example, signature comparison or fingerprint matching with that provided on the smart card 28, although this type of check is usually more expensive.
[0096] Referring now to FIGS. 4, 5 and 6, a third embodiment of the present invention is now described. This embodiment deals specifically with the issue and problems associated with an unknown sender, namely the integrity of the source of the received fax document.
[0097] A fax system 70 for implementing a new protocol for ensuring the integrity of the source of a received fax document, is now described with reference to FIG. 4. The fax system 70 comprises a sending fax machine 72 and a receiving fax machine 74. Both machines are able to function as normal fax machines but, in addition to this, are configured to take advantage of a more secure overlay protocol as is described below.
[0098] The sending fax machine 72 includes a smart card reader 76 which is arranged to receive a sender's smart card 78. The smart card 78 has provided on it a private key 80 of the sender which is used to authenticate the identity of the sender as will be described later. The sending fax machine 72 also has a hash algorithm 84, such as SHA1 or MD5, provided in its memory which can be used to provide a fingerprint 86 of any block or file of data provided to it as an input. (A hash algorithm is an algorithm which reduces down a block of data into a fingerprint of about twenty bytes. It is computationally infeasible to find another document with the same fingerprint and it is not possible to derive details of the document from analysis of the fingerprint.)
[0099] Although not shown in FIG. 4, the sending fax machine 72 is connectable to a central database 22 of individuals' certificates which uses LDAP in exactly the same manner as in the first embodiment. In this way, the sending fax machine 72 is able to request and obtain the certificate 18 of a sender (rather than an intended recipient) together with their public key 82. Alternatively, the public key 82 of the sender can be obtained directly from the sender's smart card 78 or even their personal website.
[0100] The format of the data communication between the sending fax machine 72 and the receiving fax machine 74 is based on three elements, namely an electronic representation 24 of the document 12 which has been scanned into the sending fax machine 72, the certificate 18 of the sender, and an encrypted version of the document digest 86 (fingerprint of the document 12 which has been created by the hash algorithm 84). The way in which this data is used at the receiving machine is described later.
[0101] The receiving fax machine 74 has a memory in which a store 88 of documents and a store 90 of certificates is provided. These documents 24 and certificates 18 are those which have been received from the sending fax machine 72 (or other sending fax machines—not shown). The receiving fax machine 74 also has a copy of the hash algorithm 92 which is identical to the hash algorithm 84 found in the sending fax machine 72. A microprocessor (not shown) is provided for implementing the algorithm 92 and making decisions based on the comparison of data as is described in detail later.
[0102] Apart from the standard fax machine elements and functions, the receiving fax machine 74 also comprises an audit log 94 which stores information regarding the receipt of each electronic document 24. This information includes the time and date at which the fax document 24 was received, a reference to the certificate 18 which relates to the received document 24, the digest 86 of each document and a unique identifier which can be printed on each printed document 34 which establishes a link to the associated entry in the audit log 94.
[0103] The process of transmitting a scanned in document 12 from the sending fax machine 72 (namely the fax machine of an unknown person or authority) to the receiving fax machine 74 is now described with reference to FIGS. 5 and 6.
[0104] The process is divided into two distinct parts, the first part 100 (FIG. 5) which occurs at the sending machine 72 and the second part (FIG. 6) which occurs at the receiving machine 74. Taking each of these in turn, the first part 100 commences with the document 12 being scanned at 102 into the sending fax machine 72. The hash algorithm 84 is then used at 104 to create a digest 86 of the scanned in document. The sending fax machine 72 requests the sender to input his smart card 78 and also to confirm the card by entering its associated PIN. The card 78 is entered into the smart card reader 76 and card confirmed at 106.
[0105] The private key 80 of the sender is then used at 108 to encrypt the digest 86 of the document 24. This encryption provides the link back to the sender which forms the basis for author authentication security of the present embodiment. The sending fax machine 72 also requires the sender's certificate 18 which includes the sender's public key 82 for sending to the receiving fax machine 74. Accordingly, the process 100 continues with the sending fax machine 72 requesting and obtaining at 110 the sender's certificate 18. As mentioned before, this is obtained either from the sender's smart card 78, the sender's website or from a central database such as the LDAP database 22. Finally, the document 24, the sender's certificate 82 and the encrypted digest 86 of the document are all sent at 112 to the receiving fax machine 74.
[0106] Referring now to FIG. 6, the second part 120 of the process which occurs at the receiving fax machine 74 commences with the receiving at 122 of the document 24, the sender's certificate 82 and the encrypted digest 86 of the document. The receiving fax machine 74 extracts at 124 the public key 82 of the sender from the enclosed digital certificate 18 and uses it to decode the encrypted digest 86 of the document. In addition, the received document 24 is redigested at 126 using the same hash algorithm 92 as used to create the original digest 86. The decoded digest created by the sending fax machine hash algorithm 84 and the new digested document created by the receiving machine hash algorithm 92 are then compared at 128.
[0107] If these two digests are not equivalent, then the second part 120 of the process ends at 130 with the result that sender of the document and its contents cannot be relied upon. This is caused by either the original and received documents being different or the private/public keys of the supposed sender not matching.
[0108] If these two digests are equivalent from the comparison at 128, then the process continues and determines at 132 whether validation of the sender's certificate is required. If no validation is required, then the second part 120 of the process ends at 134 with the result that sender of the document and its contents can both be relied upon. This result is then identified to the recipient by the printing at 136 of a verifying mark (not shown) on the printout 34 of the received document 24. However, if validation is required as determined at 132, then the receiving fax machine 74 takes steps at to check the validity of the certificate 18 or a chain of certificates of which the present certificate forms a part. These checks can be made on-line to higher and higher authorities and may, if necessary, extend all the way to a trusted authority such as Verisign. The process of on-line authentication of a certificate is well understood in the art and does not require further explanation herein.
[0109] The result of the verification process determines the validity of the certificate 18 and if it is valid at 140, then the second part 120 of the process ends at 134 with the result that sender of the document and its contents can both be relied upon. Otherwise, the certificate 18 is considered to be invalid at 140, and the second part 120 of the process ends at 130 with the result that sender of the document cannot be relied upon.
[0110] It is also to be appreciated that if the receiving fax machine 74 carries out the certificate validation check then it could notify the result (confirmation) in header sheet or other print form or even on screen of the receiving fax machine. Such a confirmation would identify the document, identify the sender, and signify that the relevant sender's certificate or certificates had all been verified.
[0111] Received certificates 18 are placed in the store 90 on receipt and a relevant entry is made in the audit log 94. However, as described above, a received certificate 18 is not necessarily checked on-line when a new fax is received; this decision depends on the relationship between the sender and the recipient. If at any time in future doubt should arise as to the identity of the sender of a fax, thus requiring on-line verification of the certificate, then the relevant certificates 18 listed in the audit log 94 associated with that fax can be accessed by use of the unique identifier printed on the paper copy 34 of the fax document 24. Once the relevant certificate 18 has been located, an on-line validity check can be carried out.
[0112] The above described audit log 94 advantageously allows later on-line checks to be carried out on received documents 24 and their associated certificates 18. Furthermore, the provision of the audit log 94 permits the potentially time-consuming on-line validation procedure to be carried out at a more convenient time and also to be carried out for batches of received fax documents 24 and their associated certificates 18. If batch on-line validation is carried out, significant time savings can be attainted over individual on-line validations.
[0113] As mentioned previously, a copy of the digest for each received fax is also stored in the audit log 94 for additional security. The digest can be used in conjunction with the hash algorithm 92 as described above to verify the equivalence of the received document and a copy of that document at any time in the future. This provides proof that the document has not been tampered with at any time between its receipt and the subsequent moment in time when its authenticity is being considered.
[0114] In addition, the audit log 94 provides a history of the faxes received and can at a later time provide confirmation of when a fax was sent/received and who sent it. Each page of a received and printed out fax contains the above mentioned unique validation mark on it. Accordingly, each printed page of a received fax has a page number provided and also the unique identifier which can be associated with an audit log entry so that it is possible at any time to determine from a single page who sent the fax, when it was sent and the authenticity of each page thereof.
[0115] Referring now to FIGS. 7 and 8, closed group fax system 150 according to a fourth embodiment of the present invention is described. The closed group fax system 150 comprises a group of authenticated fax machines 152 (in the present embodiment a group of only three fax machines (A, B and C) is shown). These machines make up a virtual private network.
[0116] Each fax machine 152 has provided within it a store 154 of group member's certificates 156. These can be provided by any known method for example by either remote programming via the communications network 158 that links the fax machines 152 together, or by individual loading of each member's certificate 156 onto each machine 152.
[0117] The fax system 150 operates as a virtual private network by way of an authentication procedure that operates prior to the transmission of any fax document data. The authentication procedure determines whether a given pair of sending and receiving fax machines 152 are both members of the authorised group of fax machines as is now described with reference to FIG. 8 using an example of a desired transmission of a document from fax machine A to fax machine C.
[0118] The authentication procedure 160 commences with Machine A (A) sending at 162 its own digital certificate 156 together with a nonce (labeled NonceA) to Machine C (C). NonceA is a random integer that has been generated at A and that is used only once. NonceA is used in a data exchange to stop replay attacks, namely the reuse of a valid authentication for further authentications.
[0119] C receives Machine A's request at 164 together with NonceA. The received version of A's Certificate is compared at 166 with the corresponding Certificate 156 held in its store 154. If they are not equivalent, at 166, the procedure has detected an irregularity at 168 and the procedure 160 is stopped at 168. Otherwise, the procedure continues as described below.
[0120] The received NonceA is encrypted at 170 using C's private key (only available at C). This can be carried out on a smart card which holds the private key 159 as well as a encoding/decoding algorithm for example. C then generates a new nonce (labeled as NonceC) and sends this at 172 together with the encrypted NonceA and C's digital Certificate to A.
[0121] On receipt of the encrypted NonceA, C's digital Certificate and NonceC at 170, A compares at 176 the received version of C's digital Certificate with the corresponding Certificate 156 held in its store 154. If they are not equivalent, at 176, the procedure has detected an irregularity at 168 and the procedure 160 is stopped at 168. Otherwise, the procedure continues as described below.
[0122] A then decodes at 178 the encrypted NonceA using C's public key (the Public key is either obtained from C's Certificate or has previously been stored in A's memory. The decrypted version of NonceA is compared at 178 to the previously stored version of NonceA. If both versions are not equivalent at 180, then an irregularity in the authentication procedure has been detected and the subsequent transmission of a fax document between A and C is prevented at 168. Conversely, if both version are equivalent at 180, then A has proved that that the party who has sent the encrypted NonceA is the owner of C's private key, namely C and so commences its response procedure.
[0123] The response procedure commences at 182 with A encrypting the received NonceC with its own private key 159. Again the private key 159 may be provided on a smart card as in the previous embodiments. A then sends at 184 the encrypted NonceC to C. On receipt, C decodes at 186 the encrypted NonceC using A's public key (available via A's digital Certificate or previously stored) and compares this at 186 with the previously stored version of NonceC. If at 188 the decrypted version of NonceC is not equivalent to the stored original, then an irregularity in the authentication procedure 160 has been detected and the subsequent transmission of a fax document between A and C is prevented at 168. Otherwise, if both version are equivalent at 188, then C has proved that that the party who has sent the encrypted NonceC is the owner of A's private key 159, namely A. Accordingly, the authentication procedure 160 is now complete and the document can be faxed at 190 between A and C.
[0124] The use certificates 156 in the above procedure not only enables a local check to be made as to the membership of the closed group of the fax machine 152 wishing to commence communication but also enables each fax machine 152 to carry out on-line authentication checks to establish as an additional check whether each participating fax machine 152 is who it claims to be. Also this on-line authentication procedure provides an up-to-date validity check on the status of the fax machines' Certificates 156 if required.
[0125] It is to be appreciated that the above described encryption techniques can also be used with this embodiment of the present invention if additional security is required. Also prior to the authentication procedure, a person wishing to send a document using one of the authorised fax machines 152 may be required to prove their identity by the use of a personal smart card 28 containing their certificate 156 and their private key 159. This would provide an additional layer of security for the virtual private network.
[0126] An example of a typical application of the fourth embodiment of the present invention is a fax machine in a local bank that should only receive faxes from other remote branches of the same bank. Here, the certificates 156 of the fax machines 152 at the remote branches are stored in the local branch fax machine and are used to confirm the identity of the sending/receiving fax machine at the remote branch.
[0127] It is to be appreciated that the above described embodiments can all be combined in different ways to provide a system or method with significant advantages. In particular, the first or second embodiments relating to the unknown intended recipient can readily be combined with the third embodiment relating to the unknown sender. Such a combination of embodiments would provide a very secure system of fax transmission and receipt. In addition, the first or second embodiments and the third embodiment can also be combined with the fourth embodiment to provide a virtual private network with secure intended recipient and unknown sender capabilities. Similarly, other valid combinations of embodiments are the third embodiment with the fourth embodiment and also the first or second embodiments with the fourth embodiment. Furthermore, the first and second embodiments could also be combined such that the receiving fax machine could either printout decrypted documents directly or print non-encrypted documents into locked compartments. The configuration options of the receiving fax machine would be set up to determine its mode of operation with the faxes to be received.
[0128] It is also to be appreciated that the fax machines 14,16 described in the above embodiments have provided within them a software configurable communications module (not shown) for receiving and transmitting documents and data, and a software configurable controller (not shown) for processing data (encrypting/decrypting data or implementing hash algorithms, for example) as required.
[0129] Having described particular preferred embodiments of the present invention, it is to be appreciated that the embodiments in question are exemplary only and that variations and modifications such as will occur to those possessed of the appropriate knowledge and skills may be made without departure from the spirit and scope of the invention as set forth in the appended claims. For example, the present invention is not restricted to fax documents, and could equally be applied to any transmitted document which requires printing out, for example the present invention could also apply to document printing from computers. The issues of secure document reception which require secure document reproduction to prevent any person seeing contents by accident, for example, also occur with shared printers on a network or at a hotel lobby (document transmitted from hotel room to printer in lobby).
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Jiya walks 6 km due east and then 8 km due north. How far is she from her starting place? - Mathematics
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Sum
Jiya walks 6 km due east and then 8 km due north. How far is she from her starting place?
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Solution
Let A be the starting point and B be the ending point of Jiya.
Since ∆ABC is right-angled.
∴ (AB)2 = (AC)2 + (BC)2
⇒ (AB)2 = 62 + 82
= 36 + 64
= 100
⇒ AB = 10
Thus, Jiya is 10 km away from her starting place.
Is there an error in this question or solution?
Chapter 6: Triangles - Exercise [Page 172]
APPEARS IN
NCERT Exemplar Mathematics Class 7
Chapter 6 Triangles
Exercise | Q 113. | Page 172
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hellofadude's blog http://www.java.net/blog/513370 en Discovering Java's runtime type information http://www.java.net/blog/hellofadude/archive/2014/04/21/discovering-javas-runtime-type-information <!-- | 0 --><p>Runtime type information (RTTI) refers to the correct identification of the type of your objects at run time. When you write code, it is generally desirable to do so in a way that takes advantage of OOP features like encapsulation and inheritance to make your program easily extensible, by for instance, as much as possible manipulating references to base classes and letting polymorphism work for you. Essentially, when you create a new object with a base class reference, the process necessarily involves an upcast to the base class from a more specialised type. There are many advantages to this style of programming not least of which includes the ability to simulate a more dynamic behaviour. Upcasting is a fairly straightforward process that the Java interpreter undertakes automatically, because there is very little risk associated with it. RTTI describes the reverse of this process when you wish to focus on a specific type for example to perform a particular operation. For instance, consider the following <code class="prettyprint">Game</code> class hierarchy:-<br /> <pre class="prettyprint"><code>package runtime.types.java;<br /> <br />abstract class Game {<br /> public int players;<br /> public String toString() {<br /> return "Game";<br /> }<br /> void play() {<br /> System.out.println("playing " + this + "()");<br /> }<br />}<br />class Charade extends Game {<br /> public String toString() {<br /> return "CharadeGame";<br /> }<br />}<br />class Chess extends Game {<br /> public String toString() {<br /> return "Chess";<br /> }<br />}<br />class CardGame extends Game {<br /> public String toString() {<br /> return "CardGame";<br /> }<br />}<br />class BlackJack extends CardGame {<br /> public void shuffle() {<br /> System.out.println("Shuffle cards");<br /> }<br /> public String toString() {<br /> return "BlackJack";<br /> }<br />}<br />class OnlineGame extends Game {<br /> public String toString() {<br /> return "OnlineGame";<br /> }<br />}</code></pre><br /> This is a fairly accurate representation of the structure of libraries that exist within an OO paradigm, even though in reality some classes inherit from base classes in other packages.<br /> In the following code a <code class="prettyprint">List</code> is configured to hold subtypes of the abstract <code class="prettyprint">Game</code> class. Each type of <code class="prettyprint">Game</code> is upcast to the <code class="prettyprint">List</code> container which treats all elements as <code class="prettyprint">Game</code> references having no memory of individual types. We can identify the benefit of RTTI in the <code class="prettyprint">playGame()</code> method where we iterate through the list and the correct type is identified in the call to <code class="prettyprint">play()</code> because polymorphism allows us to make the correct binding at runtime:-<br /> <pre class="prettyprint"><code>package runtime.types.java;<br /> <br />import java.util.Arrays;<br />import java.util.List;<br /> <br />public class Arcade {<br /> static void playGame(List<Game> options) {<br /> for(Game selection : options)<br /> selection.play();<br /> }<br /> public static void main(String[] args) {<br /> List<Game> games = Arrays.asList(new Chess(), new CardGame(), new BlackJack());<br /> playGame(games);<br /> }<br />}<br />/* Output<br />playing Chess()<br />playing CardGame()<br />playing BlackJack()<br />*//</code></pre><br /> The above example is the most basic form of RTTI and why polymorphism is understood to be a general goal in object oriented programming. There are other circumstances when you might need to identify a specific type, perhaps to enable some operation unique to a specific type, circumstances for which you can not rely on polymorphism by itself.<br /> Java allows you to discover runtime type information in one of two ways. The first approach assumes all type information is available at compile time which you can obtain by querying the object reference. For instance, suppose we wanted to ensure the cards are shuffled before playing a game of <code class="prettyprint">BlackJack</code>. We could use RTTI to identify this specific type in order to call the appropriate method:-<br /> <pre class="prettyprint"><code>package runtime.types.java;<br /> <br />import java.util.Arrays;<br />import java.util.List;<br /> <br />public class GettingType {<br /> public static void playGame(List<Game> options) {<br /> for(Game selection : options) {<br /> Class<? extends Game> type = selection.getClass();<br /> if(type.isAssignableFrom(BlackJack.class))<br /> ((BlackJack)selection).shuffle();<br /> selection.play();<br /> }<br /> }<br /> public static void main(String[] args) {<br /> List<Game> games = Arrays.asList(new Chess(), new BlackJack(), new Charade());<br /> playGame(games);<br /> }<br />}<br />/* Output<br />playing Chess()<br />Shuffle cards<br />playing BlackJack()<br />playing Charade()<br />*//</code></pre><br /> In this instance, we query the object reference for type information using the <code class="prettyprint">getClass()</code> method, and then apply the <code class="prettyprint">isAssignableFrom()</code> method to the resulting reference determine if this class is the same or a superclass of the <code class="prettyprint">BlackJack</code> class. If it is, we cast the object to a<code class="prettyprint"> BlackJack</code> object and then call <code class="prettyprint">shuffle()</code>. The logic of type as you will no doubt come to understand is sound and quite uncomplicated. </p> <p>You can also use the <code class="prettyprint">instanceof</code> keyword to check for the type of an object, for example prior to making a cast:-<br /> <pre class="prettyprint"><code>package runtime.types.java;<br /> <br />public class TypeTest {<br /> public static void main(String[] args) {<br /> Game aGame = new BlackJack();<br /> if(!(aGame instanceof BlackJack))<br /> System.out.println("Not an instance");<br /> ((BlackJack)aGame).play();<br /> }<br />}<br />/* Output<br />playing BlackJack()<br />*//</code></pre> <p>The other approach to discovering type information is to make use of a reflection mechanism by which means you can discover and use previously unavailable class information, exclusively at runtime. In any event, either approach requires a sound knowledge of how Java represents type information at runtime.</p> <h1>The Class object</h1> <p>Java uses instances of the class <code class="prettyprint">Class</code> to represent type information at runtime. Type refers to the classes and interfaces in a running Java application which are automatically loaded by the Java Virtual Machine (JVM) using a special <code class="prettyprint">ClassLoader</code> mechanism. In effect, there exists a <code class="prettyprint">Class</code> object for every class that is a part of your program which consists of information about your class and is used for creating all the other objects of your class. You can get a reference to this <code class="prettyprint">Class</code> object by calling the <code class="prettyprint">getClass()</code> method as we did in a previous example. However in circumstances where this option might not be available, you can create a <code class="prettyprint">Class</code> reference directly which can then be manipulated just like you do any other reference to conveniently discover all sorts of type information. You can return a <code class="prettyprint">Class</code> reference by either calling the <code class="prettyprint">static</code> <code class="prettyprint">forName()</code> method or by using the class literal. Here is an example using both techniques;-<br /> <pre class="prettyprint"><code>public class GettingTypeInfo {<br /> static void printTypeInfo(Class<? extends Game> whichClass) {<br /> System.out.println("Class name - " + whichClass.getName());<br /> System.out.println("Simple name - " + whichClass.getSimpleName());<br /> System.out.println("isInterface - " + whichClass.isInterface());<br /> System.out.println("Package - " + whichClass.getPackage());<br /> System.out.println("SuperClass " + whichClass.getSuperclass());<br /> }<br /> public static void main(String[] args) throws ClassNotFoundException, <br /> IllegalAccessException, InstantiationException {<br /> Class<? extends Game> aClass = Class.forName("runtime.types.java.BlackJack");<br /> //class literal syntax<br /> Class anotherClass = Chess.class;<br /> printTypeInfo(aClass);<br /> System.out.println("isInstance - " + aClass.isInstance(new BlackJack()));<br /> System.out.println("------------------------------- ");<br /> printTypeInfo(anotherClass);<br /> System.out.println("isInstance - " + anotherClass.isInstance(new Game()));<br /> System.out.println("------------------------------- ");<br /> Object anObj = aClass.newInstance();<br /> System.out.println("NewInstance " + anObj.toString());<br /> }<br />}<br />/* Output<br />Class name - runtime.types.java.BlackJack<br />Simple name - BlackJack<br />isInterface - false<br />Package - package runtime.types.java<br />SuperClass class runtime.types.java.CardGame<br />isInstance - true<br />-------------------------------<br />Class name - runtime.types.java.Chess<br />Simple name - Chess<br />isInterface - false<br />Package - package runtime.types.java<br />SuperClass class runtime.types.java.Game<br />isInstance - false<br />-------------------------------<br />NewInstance BlackJack<br />*//</code></pre><br /> The first statement in the <code class="prettyprint">main()</code> makes use of the <code class="prettyprint">static</code> <code class="prettyprint">forName()</code> method to obtain a valid reference to the <code class="prettyprint">BlackJack</code> Class, by contrast, the second statement makes use of a class literal to achieve the same effect. Both techniques are equally valid, but have subtle implications related to the way the class object is initialised. Notice the use of generic syntax with <code class="prettyprint">Class</code> types in particular in the call to the <code class="prettyprint"> forName() </code>method. Generics provide a way to constrain objects to subtypes of a particular type. Here the wildcard symbol <?> symbol is used in combination with the <code class="prettyprint">extends</code> keyword to constrain all references to subtypes of <code class="prettyprint">Game</code>. <code class="prettyprint">Class</code> references also work fine without the use of generics, however it is advisable to use generic syntax as you are likely to find out about mistakes much sooner<br /> The <code class="prettyprint">printTypeInfo()</code> method displays output from a number of methods available to objects of the class <code class="prettyprint">Class</code> providing access to useful type information. The <code class="prettyprint">isInstance()</code> and <code class="prettyprint">newInstance()</code> methods are also used in the <code class="prettyprint">main()</code> to test for compatibility and create a new instance respectively.</p> <p>As you become used to working with types, you will find them useful in circumstances where you might wish to be able to identify individual types perhaps for some specific purpose. For instance, suppose you wanted a way to keep a track of the number of individual <code class="prettyprint">Game</code> types generated by your application. The first thing you might need is a way to link the initialisation of each <code class="prettyprint">Game</code> object with it's corresponding <code class="prettyprint">Class</code> object. One way to do this might be to use a variation of the <i>template</i> design pattern to create an <code class="prettyprint">abstract</code> method that returns a list of types with which you might generate new instances of <code class="prettyprint">Game</code> types:-<br /> <pre class="prettyprint"><code>package runtime.types.java;<br /> <br />import java.util.List;<br />import java.util.Random;<br /> <br />public abstract class InitGame {<br /> static private Random generator = new Random();<br /> public abstract List<Class<? extends Game>> getTypes();<br /> public Game aRandomGame() {<br /> int index = generator.nextInt(getTypes().size());<br /> try {<br /> return getTypes().get(index).newInstance();<br /> } catch(InstantiationException ref) {<br /> throw new RuntimeException(ref);<br /> } catch(IllegalAccessException ref) {<br /> throw new RuntimeException(ref);<br /> }<br /> }<br /> public Game[] createGames(int capacity) {<br /> Game[] arrayOfGames = new Game[capacity];<br /> for(int i = 0; i < capacity; i++)<br /> arrayOfGames[i] = aRandomGame();<br /> return arrayOfGames;<br /> }<br />}</code></pre><br /> Essentially, this class uses a random number generator to index into a list of types in order to instantiate a new <code class="prettyprint">Game</code> object. A second method uses this pseudo-random algorithm to return an array of games. As with all <code class="prettyprint">abstract</code> classes, an inheriting class will need to implement the <code class="prettyprint">abstract</code> <code class="prettyprint">getTypes()</code> method. The following class uses class literals to create a list of <code class="prettyprint">Class</code> references:-<br /> <pre class="prettyprint"><code>package runtime.types.java;<br /> <br />import java.util.Arrays;<br />import java.util.List;<br /> <br />public class GameTypeCreator extends InitGame {<br /> @SuppressWarnings("unchecked")<br /> public static final List<Class<? extends Game>> gameTypes = Arrays.asList(Chess.class,<br /> CardGame.class, BlackJack.class,<br /> Charade.class, OnlineGame.class);<br /> public List<Class<? extends Game>> getTypes() {<br /> return gameTypes;<br /> }<br />}</code></pre><br /> The final class in this sequence uses a <code class="prettyprint">Map</code> to track the number individual type of <code class="prettyprint">Game</code> objects created. Here's how:-<br /> <pre class="prettyprint"><code>package runtime.types.java;<br /> <br />import java.util.HashMap;<br />import java.util.Iterator;<br />import java.util.LinkedHashMap;<br />import java.util.List;<br />import java.util.Map;<br /> <br />class Util {<br /> static public Map<Class<? extends Game>, Integer> mapData(List<Class<? extends Game>> key, int value) {<br /> Map<Class<? extends Game>, Integer> data = new HashMap<>();<br /> Iterator<Class<? extends Game>> sequence = key.iterator();<br /> while(sequence.hasNext())<br /> data.put(sequence.next(), value);<br /> return data;<br /> }<br />}<br />public class CountGame extends LinkedHashMap<Class<? extends Game>, Integer> {<br /> public CountGame() {<br /> super(Util.mapData(GameTypeCreator.gameTypes, 0));<br /> }<br /> public void count(Game aGame) {<br /> for(Map.Entry<Class<? extends Game>, Integer> anEntry : entrySet())<br /> //Class.isInstance to dynamically test for types<br /> if(anEntry.getKey().isInstance(aGame))<br /> put(anEntry.getKey(), anEntry.getValue() + 1);<br /> }<br /> public String toString() {<br /> StringBuilder aString = new StringBuilder("{");<br /> for(Map.Entry<Class<? extends Game>, Integer> anEntry : entrySet()) {<br /> aString.append(anEntry.getKey().getSimpleName());<br /> aString.append("=");<br /> aString.append(anEntry.getValue());<br /> aString.append(", ");<br /> }<br /> aString.delete(aString.length()-2, aString.length());<br /> aString.append("} ");<br /> return aString.toString();<br /> }<br /> }<br /> public static void main(String[] args) {<br /> CountGame aGameCounter = new CountGame();<br /> GameTypeCreator aGameTypeCreator = new GameTypeCreator();<br /> for(Game aGame : aGameTypeCreator.createGames(15)) {<br /> System.out.print(aGame.getClass().getSimpleName() + " ");<br /> aGameCounter.count(aGame);<br /> }<br /> System.out.println();<br /> System.out.println(aGameCounter);<br /> }<br />}<br />/* Output<br />OnlineGame BlackJack Charade BlackJack BlackJack Chess CardGame Chess <br />OnlineGame BlackJack BlackJack Chess BlackJack Charade Charade<br />{Chess=3, Charade=3, BlackJack=6, OnlineGame=2, CardGame=7}<br />*//</code></pre><br /> In this example, a <code class="prettyprint">Map</code> is preloaded using a <code class="prettyprint">mapData()</code> utility that accepts a list of <code class="prettyprint">Class</code> types restricted to subtypes of the <code class="prettyprint">Game</code> class and an integer value, zero in this particular instance. In the <code class="prettyprint">count()</code>, the <code class="prettyprint">isInstance()</code> method is used to test for type and then increment the value entry by one. A <code class="prettyprint">Map</code> can not contain duplicate keys and each key can only map to one value so it is the perfect tool for this kind of application. The <code class="prettyprint">toString()</code> method is overridden to provide a formatted view of the <code class="prettyprint">Map</code>. Adding a new type of <code class="prettyprint">Game</code> is a simply a matter of making an update to the static <code class="prettyprint">gameTypes</code> list in the <code class="prettyprint">GameTypeCreator</code> class.</p> <p>This is a useful tool limited only by the fact that it can only be used to count types of <code class="prettyprint">Game</code> because the <code class="prettyprint">Map</code> has been preloaded with <code class="prettyprint">Game</code> types. A more useful tool would be one that we could use to count any type:-<br /> <pre class="prettyprint"><code>package runtime.types.java;<br /> <br />import java.util.HashMap;<br />import java.util.Map;<br /> <br />public class CountAnyType extends HashMap<Class<?>, Integer> {<br /> private Class<?> baseType;<br /> public CountAnyType(Class<?> aType) {<br /> this.baseType = aType;<br /> }<br /> public void checkType(Object arg) {<br /> Class<?> aType = arg.getClass();<br /> if(!baseType.isAssignableFrom(aType))<br /> throw new RuntimeException("Wrong type! " + arg);<br /> count(aType);<br /> }<br /> public void count(Class<?> aType) {<br /> Integer sum = get(aType);<br /> put(aType, sum == null ? 1 : sum + 1);<br /> Class<?> superClass = aType.getSuperclass();<br /> if(superClass != null && baseType.isAssignableFrom(superClass))<br /> count(superClass);<br /> }<br /> public String toString() {<br /> StringBuilder aString = new StringBuilder("{");<br /> for(Map.Entry<Class<?>, Integer> anEntry : entrySet()) {<br /> aString.append(anEntry.getKey().getSimpleName());<br /> aString.append("=");<br /> aString.append(anEntry.getValue());<br /> aString.append(", ");<br /> }<br /> aString.delete(aString.length()-2, aString.length());<br /> aString.append("} ");<br /> return aString.toString();<br /> }<br /> public static void main(String[] args) {<br /> CountAnyType aTypeCounter = new CountAnyType(Game.class);<br /> GameTypeCreator aGameCreator = new GameTypeCreator();<br /> for(Game aGame : aGameCreator.createGames(15)) {<br /> System.out.print(aGame.getClass().getSimpleName() + " ");<br /> aTypeCounter.checkType(aGame);<br /> }<br /> System.out.println();<br /> System.out.println(aTypeCounter);<br /> }<br />}<br />/* Output<br />Charade Charade Chess BlackJack BlackJack BlackJack CardGame BlackJack <br />Charade CardGame OnlineGame Charade Charade Chess CardGame<br />{Game=15, Charade=5, BlackJack=4, OnlineGame=1, Chess=2, CardGame=7}<br />*//</code></pre><br /> We have introduced a <code class="prettyprint">checkType()</code> method that allows us to verify that we are counting the correct type using the <code class="prettyprint">isAssignableFrom()</code> method. The <code class="prettyprint">count()</code> method in this example, first counts the specific type and then recursively counts the base type.</p> <p>You would be correct to imagine that it would be possible to make a more sophisticated design in our approach to generating <code class="prettyprint">Game</code> objects in circumstances where we were unconcerned about keeping track of types or for that matter if we had to generate <code class="prettyprint">Game</code> objects with a much greater frequency. Here is an approach that makes use of the <i>Factory</i> design pattern by registering factories for the types to be created directly in the base class:-<br /> <pre class="prettyprint"><code>package runtime.types.java;<br /> <br />import java.util.ArrayList;<br />import java.util.List;<br />import java.util.Random;<br /> <br />interface GameFactory<T> { T createGame(); }<br /> <br />class Game {<br /> static List<GameFactory<? extends Game>> factory = new ArrayList<>();<br /> private static Random generator = new Random();<br /> public String toString() {<br /> return getClass().getSimpleName();<br /> }<br /> static {<br /> factory.add(new Charade.Factory());<br /> factory.add(new Chess.Factory());<br /> factory.add(new Scrabble.Factory());<br /> factory.add(new Monopoly.Factory());<br /> factory.add(new BlackJack.Factory());<br /> factory.add(new OnlineGame.Factory());<br /> factory.add(new VideoGame.Factory());<br /> }<br /> public static Game autoCreate() {<br /> int index = generator.nextInt(factory.size());<br /> return factory.get(index).createGame();<br /> }<br /> <br />}<br />class PartyGame extends Game {}<br />class Charade extends PartyGame {<br /> public static class Factory implements GameFactory<Charade> {<br /> public Charade createGame() { return new Charade(); }<br /> }<br />}<br />class BoardGame extends Game {}<br />class Chess extends BoardGame {<br /> public static class Factory implements GameFactory<Chess> {<br /> public Chess createGame() { return new Chess(); }<br /> }<br />}<br />class Scrabble extends BoardGame {<br /> public static class Factory implements GameFactory<Scrabble> {<br /> public Scrabble createGame() { return new Scrabble(); }<br /> }<br />}<br />class Monopoly extends BoardGame {<br /> public static class Factory implements GameFactory<Monopoly> {<br /> public Monopoly createGame() { return new Monopoly(); }<br /> }<br />}<br />class CardGame extends Game {}<br />class BlackJack extends CardGame {<br /> public static class Factory implements GameFactory<BlackJack> {<br /> public BlackJack createGame() { return new BlackJack(); }<br /> }<br />}<br />class OnlineGame extends Game {<br /> public static class Factory implements GameFactory<OnlineGame> {<br /> public OnlineGame createGame() { return new OnlineGame(); }<br /> }<br />}<br />class VideoGame extends Game {<br /> public static class Factory implements GameFactory<VideoGame> {<br /> public VideoGame createGame() { return new VideoGame(); }<br /> }<br />}<br />public class GameFactories {<br /> public static void main(String[] args) {<br /> for(int i = 0; i < 10; i++)<br /> System.out.println(Game.autoCreate());<br /> }<br />}<br />/* Output<br />Charade<br />OnlineGame<br />Charade<br />BlackJack<br />OnlineGame<br />Scrabble<br />Scrabble<br />Chess<br />OnlineGame<br />Chess<br />*//</code></pre> <p>The <code class="prettyprint">GameFactory</code> interface uses generic syntax and consequently is able to return a different type based on it's implementation. In this way, adding a new type of <code class="prettyprint">Game</code> does not require too much effort, except to register it's inner <code class="prettyprint">GameFactory</code> class in the <code class="prettyprint">factory</code> list in the base class . The genius of this design is in its simplicity and this is exactly what design patterns are able to do. They provide you with the ability to encapsulate change.</p> <h1>The Reflection Mechanism</h1> <p>The RTTI mechanisms discussed thus far have in common the advantage of being able to discover type information for objects whose types will have been available at compile time, in other words these classes must have been compiled as a part of your program. Java uses reflection to discover and use classes at runtime about which you had no prior information. This capability is useful in circumstances where your application might need to make use of a foreign utility which might be accessed over a network connection, on the internet or in some other similar fashion to create and execute objects on remote platforms in a technique known as Remote Method Invocation (RMI).</p> <p>Java's reflection implementation consists of a number of classes within the java.lang.reflect library that can be used to represent the fields, methods and constructors after which they have been appropriately named. The <code class="prettyprint">Constructor</code> class for instance provides access to a single class constructor, the <code class="prettyprint">Field</code> class includes <code class="prettyprint">getter()</code> and <code class="prettyprint">setter()</code> methods to read and modify <code class="prettyprint">Field</code> objects and the <code class="prettyprint">Method</code> class includes an <code class="prettyprint">invoke()</code> method by which means you might invoke the underlying method associated with the <code class="prettyprint">Method</code> object. These classes implement the <code class="prettyprint">Member</code> interface which reflects identifying information about a single member. The class <code class="prettyprint">Class</code> supports reflection by including a number of convenience methods by which means you might discover and use type information at runtime. </p> <p>Reflection is also a useful tool for dynamically extracting information about a class, for example to reveal the entire interface including inherited and overridden methods. Here is a nifty little tool that uses reflection to automatically reveal information about a class at runtime. You can run this tool on any class to save you time from having to trawl through endless lines of documentation:-<br /> <pre class="prettyprint"><code>package runtime.types.java;<br /> <br />import java.lang.reflect.Constructor;<br />import java.lang.reflect.Method;<br />import java.util.regex.Pattern;<br /> <br />public class InterfaceLookup {<br /> private static Pattern packageNames = Pattern.compile("\\w+\\."); <br /> public static void showInterface(Object ref) {<br /> try {<br /> Class<?> whichClass = Class.forName(ref.getClass().getName());<br /> Constructor[] constructors = whichClass.getConstructors();<br /> Method[] methods = whichClass.getMethods();<br /> for(Constructor aConstructor : constructors)<br /> System.out.println(packageNames.matcher(aConstructor.toString())<br /> .replaceAll(""));<br /> for(Method aMethod : methods)<br /> System.out.println(packageNames.matcher(aMethod.toString())<br /> .replaceAll(""));<br /> } catch(ClassNotFoundException exceptionRef) {<br /> System.out.println("Class does not exist!");<br /> }<br /> }<br /> public static void main(String[] args) {<br /> showInterface(new InterfaceLookup());<br /> }<br />}<br />/* Output<br />public InterfaceLookup()<br />public static void main(String[])<br />public static void showInterface(Object)<br />public final native void wait(long) throws InterruptedException<br />public final void wait(long,int) throws InterruptedException<br />public final void wait() throws InterruptedException<br />public boolean equals(Object)<br />public String toString()<br />public native int hashCode()<br />public final native Class getClass()<br />public final native void notify()<br />public final native void notifyAll()<br />*//</code></pre><br /> The example uses the <code class="prettyprint">forName()</code> to return a reference to the current class and then uses convenience methods to return an array of constructors and methods which it can then iterate through and print out individually. The output is the result of using a regular expression to replace class name qualifiers. This is a quick and easy way to find out all you need to know about a class's interface while coding. </p> <p>As part of it's reflection mechanism, Java also provides classes and interfaces by which means you might implement dynamic proxies which are modeled on the <i>Proxy</i> design pattern. Proxy is a pattern that allows you to provide a surrogate or placeholder for another object in order to control access to it. This is useful in circumstances where you wish to differ the full cost of an objects creation until you actually need it or if you wanted to provide for additional operations you do not wish to be a part of your main object. For instance, consider a drawing application that renders graphics on screen in a scenario where drawing an image is a costly operation you only want to undertake when necessary. You can implement an image proxy between the Image and the calling client so that you can control when and how you create an Image. The following code provides a better illustration:-<br /> <pre class="prettyprint"><code>package runtime.types.java;<br /> <br />interface Graphic { void draw(); }<br /> <br />class Image implements Graphic {<br /> public void draw() {<br /> System.out.println("Hello!");<br /> }<br />}<br />class ImageProxy implements Graphic {<br /> private Graphic anImage;<br /> public ImageProxy(Graphic args) {<br /> this.anImage = args;<br /> }<br /> public void draw() {<br /> System.out.println(""Calling Image.."");<br /> anImage.draw();<br /> }<br />}<br />public class DrawingApplication {<br /> public static void drawImage(Graphic aGraphic){<br /> aGraphic.draw();<br /> }<br /> public static void main(String[] args) {<br /> drawImage(new ImageProxy(new Image()));<br /> }<br />}<br />/* Output<br />"Calling Image.."<br />Hello!<br />*//</code></pre><br /> The benefits of such a design may not be immediately obvious, however, it does provide us with additional options for instance, we can easily add code to the image proxy to keep track of calls to the <code class="prettyprint">Image</code> or to measure the overhead of such calls.<br /> Java provides classes to implement a proxy instance that will handle method calls dynamically through the use of an <code class="prettyprint">InvocationHandler</code> interface. We can re-implement our image proxy to act as an invocation handler like so:-<br /> <pre class="prettyprint"><code>package runtime.types.java;<br /> <br />import java.lang.reflect.InvocationHandler;<br />import java.lang.reflect.Method;<br />import java.lang.reflect.Proxy;<br /> <br />class AnotherImageProxy implements InvocationHandler {<br /> private Graphic anImage;<br /> public AnotherImageProxy(Graphic args) {<br /> this.anImage = args;<br /> }<br /> public Object invoke(Object proxy, Method aMethod, Object[] args) throws Throwable {<br /> System.out.println(""Calling Image.."");<br /> return aMethod.invoke(anImage, args);<br /> }<br />}<br />public class AnotherDrawingApplication {<br /> public static void drawImage(Graphic whichImage){<br /> whichImage.draw();<br /> }<br /> public static void main(String[] args) {<br /> Graphic proxy = (Graphic)Proxy.newProxyInstance(Graphic.class.getClassLoader(),<br /> new Class[]{Graphic.class}, new AnotherImageProxy<br /> (new Image()));<br /> drawImage(proxy);<br /> }<br />}<br />/* Output<br />"Calling Image.."<br />Hello!<br />*//</code></pre><br /> In this example, we implement the <>code>invoke()</code> method which is responsible for processing a method invocation on a proxy instance and returning the result, which in this case happens to be a <code class="prettyprint">Method</code> object used to invoke the underlying method which it represents. In the <code class="prettyprint">main()</code> a proxy instance is returned using the <code class="prettyprint">static</code> <code class="prettyprint">newProxyInstance()</code> method which accepts a class loader, an array of interfaces to be implemented and the implemented invocation handler, which is handed the <code class="prettyprint">Image</code> object as a part of its constructor. In this case it's probably easiest to pass an existing <code class="prettyprint">ClassLoader</code> like the <code class="prettyprint">Graphic.class</code> reference. </p> <p>RTTI opens up a whole new world of programming possibilities as you come to understand how types provide you with backdoor access into pretty much any interface regardless of access permissions. For instance, consider the following simple interface in the Interfaces.java package:-<br /> <pre class="prettyprint"><code>package Interfaces.java;<br /> <br />public interface AnInterface {<br /> void interfaceMethod();<br />}</code></pre> <p>Suppose you wanted to hide the implementation of this interface to deter your customers from relying on it so that you might be free to change it at a later stage? You might think it is enough to restrict the implementation to package access thereby making it inaccessible to clients outside of the package like so:-<br /> <pre class="prettyprint"><code>package runtime.types.java;<br /> <br />class HiddenImplementation implements AnInterface {<br /> private String aField = "Can't touch this";<br /> public void interfaceMethod(){ System.out.println("public HiddenImplementation.interfaceMethod()"); }<br /> public void aMethod(){ System.out.println("public HiddenImplementation.aMethod()"); }<br /> void bMethod() { System.out.println("package HiddenImplementation.bMethod()"); }<br /> private void cMethod() { System.out.println("private HiddenImplementation.cMethod()"); }<br /> protected void dMethod() { System.out.println("protected HiddenImplementation.dMethod()"); }<br /> <br /> public String toString(){<br /> return aField;<br /> }<br />}<br />public class PublicInterface {<br /> public static AnInterface makeAnInterface() {<br /> return new HiddenImplementation();<br /> }<br />}</code></pre><br /> The <code class="prettyprint">PublicInterface</code> class produces an implementation of <code class="prettyprint">AnInterface</code> and is the only visible part of this package and you would expect your implementation to be fairly safe? Not exactly, I'm afraid. With reflection, anyone who knows the name of your class members can easily get around this restriction like so:-<br /> <pre class="prettyprint"><code>import java.lang.reflect.Field;<br />import java.lang.reflect.Method;<br />import Interfaces.java.AnInterface;<br />import runtime.types.java.*;<br /> <br />public class HackImplementation {<br /> static void hackInterface(Object anObject, String member) throws Exception {<br /> if(member.equals("aField")) {<br /> Field whichField = anObject.getClass().getDeclaredField(member);<br /> System.out.println(anObject.toString());<br /> whichField.setAccessible(true);<br /> whichField.set(anObject, "Yes I can");<br /> System.out.println(anObject.toString());<br /> } else {<br /> Method whichMethod = anObject.getClass().getDeclaredMethod(member);<br /> whichMethod.setAccessible(true);<br /> whichMethod.invoke(anObject);<br /> }<br /> }<br /> public static void main(String[] args) throws Exception {<br /> AnInterface newInterface = PublicInterface.makeAnInterface();<br /> newInterface.interfaceMethod();<br /> //Compile error aField can not be resolved<br /> //!newInterface.aField;<br /> //Compile error method undefined for type AnInterface<br /> /* newInterface.aMethod();<br /> newInterface.bMethod();<br /> newInterface.cMethod();<br /> newInterface.dMethod();<br /> */<br /> hackInterface(newInterface, "aField");<br /> hackInterface(newInterface, "aMethod");<br /> hackInterface(newInterface, "bMethod");<br /> hackInterface(newInterface, "cMethod");<br /> hackInterface(newInterface, "dMethod");<br /> }<br />}<br />/* Output<br />public HiddenImplementation.interfaceMethod()<br />Can't touch this<br />Yes I can<br />public HiddenImplementation.aMethod()<br />package HiddenImplementation.bMethod()<br />private HiddenImplementation.cMethod()<br />protected HiddenImplementation.dMethod()<br />*//</code></pre><br /> The example demonstrates how reflection allows you to get around all sorts of restrictions to access and change any field even those marked <code class="prettyprint">private</code> and call just about any method no matter the level of the access specifier. With the <code class="prettyprint">setAccessible()</code> method you can modify fields and invoke methods, including even those within inner and anonymous classes, are still accessible with reflection. Even if you try to avoid this by distributing only compiled code so that your members are inaccessible, that is still not entirely the case as Java's class file disassembler (javap) can still be used to reveal all your class members. For instance the -private flag can be used to indicate that all members should be displayed including <code class="prettyprint">private</code> ones. Running the following command will reveal everything you wish to know about our hidden class.<br /> </br><br /> <code class="prettyprint">javap -private HiddenImplementation</code><br><br /> In reality, hardly anything is hidden from reflection and most are susceptible to change, with the exception only of fields marked <code class="prettyprint">final</code>. On the flip side if you choose to misuse reflection in your program, committing all sorts of access violations, then you really will only have yourself to blame if a subsequent update to your library succeeds in breaking your code, which I am certain will be a very unpleasant experience<br /> Reflection gives you the keys to a new whole new world of dynamic programming in which having access to runtime type information allows for a very different style of programming with which it is easy to get carried away. It is important to remember to use RTTI and reflection in a way that is not detrimental to polymorphism as the most effective way to provide for extensibility in your programs.</p> http://www.java.net/blog/hellofadude/archive/2014/04/21/discovering-javas-runtime-type-information#comments Blogs Global Education and Learning J2SE Java Patterns Open JDK Tue, 22 Apr 2014 00:15:29 +0000 hellofadude 902028 at http://www.java.net Java's regular expression, String, and things.. http://www.java.net/blog/hellofadude/archive/2014/03/31/javas-regular-expression-string-and-things <!-- | 0 --><p>The manipulation of strings is a quite common activity for which the programmer undertakes responsibility fairly frequently. In Java, strings are of a distinct data type, that are implemented as literal constants. The <code class="prettyprint">String</code> class facilitates the creation and manipulation of objects that are of an immutable character, by which I mean to refer to the unchangeable property with which one must strive to become familiar. The following example is meant to illustrate this point:-<br /> <pre class="prettyprint"><code>package regex.strings.java;<br /> <br />public class ImmutableString {<br /> <br /> static String trim(String immutableRef) {<br /> String trimmed = immutableRef.trim();<br /> return trimmed;<br /> }<br /> public static void main(String[] args) {<br /> String immutable = " Arbitrarily replace parts of this string";<br /> System.out.println("\"" + immutable + "\"" );<br /> String aString = ImmutableString.trim(immutable).replaceAll("tr|rep|p|hi", "x");<br /> System.out.println("\"" + aString + "\"");<br /> System.out.println("\"" + immutable + "\"");<br /> }<br />}<br />/* Output<br /> " Arbitrarily replace parts of this string"<br /> "Arbixarily xlace xarts of txs sxing"<br /> " Arbitrarily replace parts of this string"<br />*//</code></pre> <p>In this very simple example, we pass a regular expression to the <code class="prettyprint">replaceAll()</code> member of the <code class="prettyprint">String</code> class following a call to our own <code class="prettyprint">Immutable.trim()</code> operation to remove leading whitespace, and return what might reasonably be expected to be a modified copy of our immutable <code class="prettyprint">String</code> object. The <code class="prettyprint">replaceAll()</code> method replaces the pattern expressed in regular expression syntax with the <code class="prettyprint">String</code> object <b>"x"</b>. The expression uses the logical regular expression operator <b>'|'</b> to describe a sequence of groups of characters that follow in no particular order. </p> <p>Following these operations, you will notice that no change has been recorded against our original string, because in actuality, only a copy of the original string reference is passed to the <code class="prettyprint">trim()</code> method in the first instance, which returns a reference to a new <code class="prettyprint">String</code> object to account for the modified string. The process is repeated for the call to the <code class="prettyprint">replaceAll()</code> method. If you were to examine the bytecodes arising from running Java's class file disassembler (javap with -c switch) with this class, you will notice that Java creates three <code class="prettyprint">StringBuilder</code> objects in total, once for our original string and the other two in relation to each call to the <code class="prettyprint">trim()</code> and <code class="prettyprint">replaceAll()</code> methods.<br /> This is what we mean when we make reference to the immutable property of <code class="prettyprint">String</code> objects, which it is to be understood, can also become a source of inefficiency and why Java discretely makes liberal use of the <code class="prettyprint">StringBuilder</code> class whenever there is the need to make a modification to a <code class="prettyprint">String</code>. Java's API documentation describes the <code class="prettyprint">StringBuilder</code> class as capable of providing for the construction of a sequence of mutable characters and operations designed specially for use with single threaded applications.<br /> Some very talented programmers have suggested the propriety of explicitly making use of the <code class="prettyprint">StringBuilder</code> class, in particular during repeated operations involving the modification of <code class="prettyprint">String</code> objects in order to avoid any issues that might arise as a result of the inefficiencies associated with immutability, particularly in environments where unnecessary overhead is not normally considered acceptable. </p> <p>For instance, one of the interesting things you can do with <code class="prettyprint">String</code> objects is to concatenate two or more strings, the one end to the other. Java provides a number of ways to do this, like for example by means of the overloaded <b>'+'</b> operator. Overloaded in the sense that it takes on an extra meaning when used with the <code class="prettyprint">String</code> class:-<br /> <pre class="prettyprint"><code>package regex.strings.java;<br /> <br />public class ConcatenateString {<br /> String compoundOperator = "";<br /> public ConcatenateString(String args) {<br /> String[] splitter = args.split(" ");<br /> compoundOperator += " \"";<br /> for(int i = 0; i < splitter.length; i++) {<br /> compoundOperator += splitter[i] + " ";<br /> }<br /> compoundOperator += "\"";<br /> }<br /> public static void main(String[] args) {<br /> String aDefinition = "Cognitive computing is the development of computer \n" +<br /> "systems modelled on the human brain";<br /> ConcatenateString overloadedPlus = new ConcatenateString(aDefinition);<br /> String listOfMetals = "Hydrogen" + ", " + "Lithium" + ", " + "Sodium" ;<br /> String statementOfFact = "A List of metals include: " + listOfMetals;<br /> System.out.println(statementOfFact + "\n");<br /> System.out.println(overloadedPlus.compoundOperator);<br /> }<br />}<br />/* Output<br />A List of metals include: Hydrogen, Lithium, Sodium<br /> <br />"Cognitive computing is the development of computer<br />systems modelled on the human brain "<br />*//</code></pre><br /> In the <code class="prettyprint">main()</code> method we make use of the <b>'+'</b> operator to combine a number of individual strings into one coherent <code class="prettyprint">String</code> object. Within the class constructor, notice the use of the overloaded <b>'+='</b> operator which also acts like an append method when used with <code class="prettyprint">String</code> objects. Both of these methods are suitable for simple operations, but become inefficient when used in loops like you have in the above constructor because Java has to create a new <code class="prettyprint">StringBuilder</code> object at every iteration which, as has been pointed out before now, is really not ideal. A more efficient way would be to explicitly create a <code class="prettyprint">StringBuilder</code> object and make use of the <code class="prettyprint">append()</code> method within the loop, like the following example demonstrates:-<br /> <pre class="prettyprint"><code>package regex.strings.java;<br /> <br />public class ConcatenateString2 {<br /> StringBuilder buildString = new StringBuilder();<br /> public ConcatenateString2(String args) {<br /> String[] splitter = args.split(" ");<br /> buildString.append(" \"");<br /> for(int i = 0; i < splitter.length; i++) {<br /> buildString.append(splitter[i] + " ");<br /> }<br /> buildString.append(" \"");<br /> }<br /> public static void main(String[] args) {<br /> String aDefinition = "Cognitive computing is the development of computer \n" +<br /> "systems modelled on the human brain";<br /> ConcatenateString2 stringBuilder = new ConcatenateString2(aDefinition);<br /> System.out.println(stringBuilder.buildString);<br /> }<br />}<br />/* Output<br />"Cognitive computing is the development of computer<br />systems modelled on the human brain "<br />*//</code></pre><br /> By making an explicit call to <code class="prettyprint">StringBuilder</code> before you enter your loop, you produce better and more efficient code. With the release of Java SE 7, came the addition of the ability to use <code class="prettyprint">String</code> objects with the <code class="prettyprint">switch</code> statement which was not always the case. Being that Java SE 8 was released while I was putting this piece together, I figured I might as well use the opportunity this example provided me to test the utility of the new Date/Time API, which altogether seems to be quite an improvement over the previous version:-<br /> <pre class="prettyprint"><code>package regex.strings.java;<br /> <br />import java.time.Month;<br />import java.time.YearMonth;<br />import java.time.format.TextStyle;<br />import java.util.ArrayList;<br />import java.util.List;<br />import java.util.Locale;<br /> <br />public class GregorianCalendar {<br /> private List<Month> leapYear = new ArrayList<Month>();<br /> public void setMonths() {<br /> for(int month = 1; month < 13; month++) {<br /> leapYear.add(Month.of(month));<br /> }<br /> }<br /> public void daysOfTheMonth(int month) {<br /> Month whichMonth = leapYear.get(month);<br /> switch(whichMonth.getDisplayName(TextStyle.FULL, Locale.ENGLISH)) {<br /> case "April":<br /> case "June":<br /> case "September":<br /> case "November":<br /> System.out.println(whichMonth.getDisplayName(TextStyle.FULL, <br /> Locale.ENGLISH) + " = 30 days");<br /> break;<br /> case "February":<br /> System.out.println(whichMonth.getDisplayName(TextStyle.FULL, <br /> Locale.ENGLISH) + " = 29 days");<br /> break;<br /> default:<br /> System.out.println(whichMonth.getDisplayName(TextStyle.FULL, <br /> Locale.ENGLISH) + " = 31 days");<br /> }<br /> }<br /> public static void main(String[] args) {<br /> GregorianCalendar aCalendar = new GregorianCalendar();<br /> aCalendar.setMonths();<br /> for(int i = 0; i < aCalendar.leapYear.size(); i++)<br /> aCalendar.daysOfTheMonth(i);<br /> }<br />}</code></pre><br /> Java 8's new Date/Time API consists of about 5 packages that provide you with a comprehensive framework for representing an instance of time. In our example, we make use of the <code class="prettyprint">Month</code> class which is of an enumerated type to provide a string representation to test for a certain conditionality in our <code class="prettyprint">switch</code> statement by which means we are able to print the number of days in a particular month.</p> <h1>Regular expressions</h1> <p>A regular expression is a string processing technique for describing the patterns that may be found in string based text. This is achieved by giving special meaning to the arrangement of metacharacters when used in particular context. A metacharacter is a character that has been given a special meaning by Java's regular expression interpreter and therefore it would not be a mistake for you to predispose your mind to the notion that presupposes that regular expressions are a completely separate, though indistinct language from Java, which really does not require too much of a stretch of the imagination, even if they are conceived as strings, and are subsequently applied to exactly this same type.</p> <p>There are a number of predefined character classes that represent the basic building blocks of regular expressions with which you must become properly acquainted if it is your wish to attain an expert level of proficiency in this subject. For instance, a word character is represented with a backslash followed by the letter w like so <b>'\w'</b>. In consequence, a digit is represented by the metacharacter <b>'\d'</b>, while <b>'\s'</b> is used to represent the whitespace character , <b>'\W'</b>, a non-word character, and the non whitespace and non-digit character are represented as <b>'\S'</b> and <b>'\D'</b> respectively. </p> <p>In addition, to indicate any of these predefined characters in regular expression syntax, you must precede each one with an additional backslash <b>'\'</b> such that the expression <b>'\\w'</b> would be used to indicate a word character, or <b>'\\d'</b> to indicate a digit, and et cetera..<br /> You should also make yourself to become familiar with a set of arrangements that consists of characters enclosed in square brackets <b>[...]</b>, known as character classes and used to indicate a preference, so that the arrangement <code class="prettyprint">"[abc]"</code> indicates a preference for <b>a</b>,<b>b</b> or <b>c</b> and <code class="prettyprint">"[a-zA-Z]"</code> indicates a preference for any character in the range <b>a-z</b> or <b>A-Z</b>. The <b>'-'</b> metacharacter acts like a range forming operator when used in square brackets. These character classes can also be arranged in other ways that make more complex operations possible, for instance the arrangement <code class="prettyprint">"[a-e[l-p]]"</code> is a regular expression union operator that specifies a preference for any within the range <b>a</b> through <b>e</b> or <b>l</b> through <b>p</b>, and the expression <code class="prettyprint">"[a-z&&[hij]]"</code> is an intersection operator that refers to <b>h</b>, <b>i</b> or <b>j</b>. And then of course you also have a number of logical operators including <b>'AB'</b> to indicate that <b>B</b> follows <b>A</b> , <b>'A|B'</b> to indicate <b>A</b> or <b>B</b> and <b>(A)</b> as a reference to a capturing group, an explanation of which is better sought within the pages of your official documentation; all of this in addition to other categories of metacharacters, boundary matchers, quantifiers and a multiplicity of character classes competing to represent every conceivable character combination you can possibly imagine.<br /> For this reason, you will probably have to become used to making a constant reference to your official documentation for a full list of Java's regular expression metacharacters and the valid range of arrangements.</p> <p>Nevertheless, the easiest way to use regular expressions in Java is to pass a valid argument to the convenience method, <code class="prettyprint">matches()</code> of the <code class="prettyprint">String</code> class to make a test of whether or not the expression matches the current <code class="prettyprint">String</code>:-<br /> <pre class="prettyprint"><code>package regex.strings.java;<br /> <br />public class StringMatcher {<br /> public static void main(String[] args) {<br /> System.out.println("Flight 8957".matches("\\w+\\s?\\d+"));<br /> System.out.println("Flight8957".matches("[Ff]li[huog]ht\\s?\\w+"));<br /> System.out.println(" Flight-8957".matches("\\s?\\w+-?\\d+"));<br /> System.out.println("Flight8957".matches("[a-zA-Z]+[0-9]+"));<br /> System.out.println("Flight8957".matches("\\w+"));<br /> }<br />}<br />/* Output<br />true<br />true<br />true<br />true<br />true<br />*//<br /> </code></pre><br /> In addition to some of Java's predefined character classes, the above example uses 'quantifiers' so as to regulate the varying degrees of frequency with which a match is made against a given string, according to a particular pattern. So for instance, the first expression in the above example uses the <b>'+'</b> quantifier to match a word character <code class="prettyprint">"\\w"</code> one or more times, and then uses the <b>'?'</b> quantifier to match the whitespace character <code class="prettyprint">"\\s"</code> once or not at all, before this time matching a digit <code class="prettyprint">"\\d+"</code> one or many times in the preceding string <code class="prettyprint">"Flight 8957"</code>. The <code class="prettyprint">matches()</code> method returns a <code class="prettyprint">boolean</code> value that should indicate the truth or falsehood of any assertion. With quantifiers, it is important to bear in mind that they apply only to the metacharacter that immediately precede their definition, except when you use parenthesis to group patterns or of course when you apply them to character classes, so for instance the expression <code class="prettyprint">"\\w+"</code> which indicates a preference for a word character one or more times, will match the string <code class="prettyprint">"Flight8957"</code> but to make a match of the string <code class="prettyprint">"Flight-8957"</code> which contains a non-word character, you will need to write <code class="prettyprint">"\\w+-?\\d+"</code> which reads match a word character one or many times, followed by a hyphen once or not at all, and then match a digit one or more times. You may apply this reasoning to unravel the mystery behind the remaining regular expression statements.</p> <p>Parenthesis allow you to group individual characters to form a pattern, for instance the <b>'*'</b> metacharacter is a third quantifier that allows you to make a match of a pattern zero or more times, so that the expression <code class="prettyprint">"(abc)*"</code> would match the pattern <b>'abc'</b> in a given string, zero or many times, while the expression <code class="prettyprint">"abc*"</code> reads - match <b>'ab'</b> followed by <b>'c'</b> zero or many times. This quantifier in addition to those earlier referenced constitute a category of the so-called 'greedy' quantifiers, which when used in particular combination can be of the 'possessive' or 'reluctant' variety - the categories, are a reference to the varying degrees of regularity with which each class of quantifier is known to make a match against a given string, according to the varying levels of generality that exist within a pattern. </p> <p>Java provides a package of APIs' (java.util.regex) that provide you with more advanced features with which you can create, compile and match regular expressions against string based text, in a standardised way. The relevant classes that should concern you initially are the <code class="prettyprint">Pattern</code> and <code class="prettyprint">Matcher</code> class. With this technique, you introduce a <code class="prettyprint">Pattern</code> object by passing a valid regular expression argument to the static <code class="prettyprint">compile()</code> method of the <code class="prettyprint">Pattern</code> class, against which you must supply your search string as argument in a manner that makes possible, the recovery of a <code class="prettyprint">Matcher</code> object, from which certain operations become available, by which means you may query such an object for complex information. In the following example, we apply this technique to locate patterns, first in a file and then manually on a test string:-<br /> <pre class="prettyprint"><code>package pattern.matching.java;<br /> <br />import java.io.BufferedReader;<br />import java.io.FileReader;<br />import java.util.NoSuchElementException;<br />import java.util.Scanner;<br />import java.util.regex.Matcher;<br />import java.util.regex.Pattern;<br /> <br />public class Parser {<br /> static void parseString(String arg1, String arg2 ){<br /> Pattern regex = Pattern.compile(arg1);<br /> Matcher intepreter = regex.matcher(arg2);<br /> // find pattern match in search string<br /> while(intepreter.find()) {<br /> System.out.println("found pattern: " + intepreter.group() + " at index " + intepreter.start());<br /> }<br /> //System.out.println("No match found");<br /> return;<br /> }<br /> public static void main(String[] args) throws Exception {<br /> FileReader reader = new FileReader("/home/geekfest/eclipse/about.html");<br /> BufferedReader in = new BufferedReader(reader);<br /> Scanner input = new Scanner(in);<br /> boolean reading = false;<br /> String searchString;<br /> // search pattern beginning http[^\s]<br /> String regex = "h\\w+p\\S+";<br /> try {<br /> while((searchString = input.nextLine()) != null) {<br /> //match search pattern in search string<br /> //parseString(regex, searchString);<br /> reading = true;<br /> }<br /> } catch(NoSuchElementException e) {<br /> //System.out.println("End of input");<br /> }<br /> input.close();<br /> if(reading == false)<br /> return;<br /> String testString = "abracadabracadab";<br /> parseString("abr?", testString);<br /> System.out.println("-----------------------------");<br /> parseString("abr??", testString);<br /> System.out.println("-----------------------------");<br /> parseString("(abr)+", testString);<br /> }<br />}<br />/* Output<br />found pattern: abr at index 0<br />found pattern: abr at index 7<br />found pattern: ab at index 14<br />-----------------------------<br />found pattern: ab at index 0<br />found pattern: ab at index 7<br />found pattern: ab at index 14<br />-----------------------------<br />found pattern: abr at index 0<br />found pattern: abr at index 7<br />*//</code></pre> <p>This example is useful to demonstrate a number of simple techniques. Firstly, we implement a simple <code class="prettyprint">parseString()</code> algorithm by creating a <code class="prettyprint">Pattern</code> object, and then calling the <code class="prettyprint">matcher()</code> method of this object to create a <code class="prettyprint">Matcher</code> object which we are then able to query repeatedly within our loop for information about specific patterns in particular using the <code class="prettyprint">group()</code> and <code class="prettyprint">start()</code> members.</p> <p>In the <code class="prettyprint">main()</code>, we make use of a <code class="prettyprint">Scanner</code> object to scan input from a file into a <code class="prettyprint">String</code> object against which we make a test of our regular expression that describes a pattern that matches the first few letters of a URL address. In this example, we have had to comment out the code that would have resulted from this match to satisfy our desire for compactness as much as possible in the use of code for illustrations.</p> <p>Nevertheless, we manually apply this method to a simple search string the result of which is provided for your perusal. Notice the difference in the first two statements that make use of different classes of quantifiers. The second statement uses a quantifier from the reluctant <b>'??'</b> category which will match the minimum number of characters necessary to satisfy the pattern, while other two statements use quantifiers from the greedy category, which will find as many matches for the pattern as possible. Furthermore, consider the effect of the parenthesis based on the output from the third statement.</p> <h1>Formatting String</h1> <p>Another interesting thing you are able to do with strings is to control how they are written to a particular output destination. The <code class="prettyprint">Formatter</code> class provides you with this kind of capability by acting as an interpreter for <code class="prettyprint">"format strings"</code> by which means you are able to control the justification, alignment and spacing of string based output. Format strings are a combination of static text and format specifiers that describe the conversions, alignment, spacing and layout justification, that is to be applied to output. Java's format specifiers can be understood within the boundaries following conventions:-</p> <p><b>%[argument_index$][flags][width][.precision]conversion</b></p> <p>The first four specifiers in square brackets are optional and may take on a different meaning depending on the value of the <i>conversion</i> specifier to which they are applied. The <i>conversion</i> format specifier is mandatory and is used to indicate a preference for a particular data type, whose value may be optionally specified by means of the <i>argument_index$</i> format specifier. Some of the typical conversions you should expect to be able to make include character conversions denoted by the specifier <b>'c'</b>, string conversions denoted by <b>'s'</b>, a range of numeric conversions of either the integral or floating point kind <b>'d'</b> and <b>'f'</b> respectively, date/time conversions <b>'t'</b>, and a number of less frequent others. For some of these conversions, an upper case representation is as equally valid, to the extent that the result is converted to upper case according to the rules of the prevailing locale, so for instance the conversion <b>'t'</b>, and <b>'T'</b> would both refer to Date/Time conversions except that the result of one will be in lower case and the other in upper case. </p> <p>The <i>argument_index$</i> format specifier is an integer used to indicate the position of the required argument in the argument list, which is nominally a reference to the value of the <i>conversion</i> format specifier, while the optional, <i>flags</i> format specifier consists of metacharacter that modify the output string in any one of a number of ways depending on the conversion. By contrast, the <i>.precision</i> format specifier is a non-negative integer that acts to restrict the number of characters to be written but whose precise functionality also depends on the conversion to which it relates, in exactly the same way as the <i>flags</i> format specifier. Finally, the <i>width</i> format specifier is a positive integer used to indicate the minimum number of characters to be written to output.<br /> The <code class="prettyprint">Formatter</code> API includes two variations of the <code class="prettyprint">format()</code> method that accept a format string and type arguments and can write formatted output to a <code class="prettyprint">PrintStream</code> object or other destination of your choice, with the difference being that one of the two makes use of the specified locale. The format string consists of a mixture of static text and format specifiers:-<br /> <pre class="prettyprint"><code>package regex.strings.java;<br /> <br />public class MatchResult {<br /> public static void main(String[] args) {<br /> String s = "Team";<br /> int i = 6;<br /> System.out.format("%2$s-B (%1$d - 0) %2$s-A ", i, s);<br /> }<br />}<br />/* Output<br />Team-B (6 - 0) Team-A<br />*//</code></pre><br /> In the above example, we can discern five distinct pieces of static text in the format string argument to our <code class="prettyprint">format()</code> method made up of <code class="prettyprint">"-B"</code>,<code class="prettyprint"> "-"</code>, <code class="prettyprint">"0"</code> and <code class="prettyprint">"-A"</code> as well as the parenthesis grouping the integers that represent team scores. Format specifiers make up the remainder of the string, including the percent <b>'%'</b> argument which produces a literal result. The format string can be described from left to right to read, position the second argument as specified by the <i>argument_index$</i> specifier, of type string to the left of the static text <code class="prettyprint">"-B"</code> to produce the literal <code class="prettyprint">"Team-B"</code>, and then position the first argument of type integer to produce the literal result <code class="prettyprint">"6"</code>, and finally, position the second argument of type string such that it produces the output <code class="prettyprint">"Team-A"</code>. The argument list is comprised of two arguments of the integer and string type.<br /> Here is another example that makes use of the <code class="prettyprint">flags</code> and <code class="prettyprint">width</code> format specifiers only:-<br /> <pre class="prettyprint"><code>package regex.strings.java;<br /> <br />public class FormatString {<br /> public static void main(String[] args) {<br /> System.out.format("%-20s \n%(.2f \n%25s", "left-justify", -3.186329, "right-justify");<br /> <br /> }<br /> <br />}<br />/* Output<br />left-justify<br />(3.19)<br /> right-justify<br />*//</code></pre><br /> The format string in this example is separated into three lines of text by the interposing new line <b>'\n'</b> operator, with the first and third lines specifying a size value in the immediate path of the percent <b>'%'</b> metacharacter which, contrary to the current JDK documentation is applicable to the <i>.precision</i> specifier and indeed to at least one other <i>flag</i> in addition to the <b>'-'</b> flag whose purpose is to left-justify output, the default being right justified, as may be deduced from the position of the last line of output. The second line of the format string uses the <b>'('</b> flag to enclose the negative value in parenthesis and the <i>.precision</i> specifier to indicate the number of digits after the radix point for our floating point conversion.<br /> To employ this technique to your advantage, you must attain proficiency with the functionality of the format specifiers that comprise a fundamental part of the format string in order to write more efficient and flexible code. The following class formats a list of staff in a using the <code class="prettyprint">Formatter</code> interpreter:-<br /> <pre class="prettyprint"><code>package pattern.matching.java;<br /> <br />class Format {<br /> void FormatHeader() {<br /> System.out.format("%-10s %5s %10s \n", "Name", "DoB", "Age");<br /> System.out.format("%-10s %5s %10s \n", "----", "---", "---");<br /> }<br /> void FormatEntry(String name, String dob, int age) {<br /> System.out.format("%-9s %-11s %4d \n", name, dob, age);<br /> }<br />}<br />public class Printer {<br /> public static void main(String[] args) {<br /> Format style = new Format();<br /> style.FormatHeader();<br /> style.FormatEntry("John", "09/06/1981", 33);<br /> style.FormatEntry("Amy", "20/11/1985", 29);<br /> style.FormatEntry("Karyn", "02/02/1978", 36);<br /> }<br />}<br />/* Output<br />Name DoB Age<br />---- --- ---<br />John 09/06/1981 33<br />Amy 20/11/1985 29<br />Karyn 02/02/1978 36<br />*//</code></pre> <p>Essentially, each format string divides the output into three columns with values of unequal size, into some of which we apply the <b>'-'</b> flag to left justify output. You will find that the <b>'-'</b> <i>flag</i> and <i>width</i> format specifier work closely most often to control the spacing and position of output. In this example, the arguments consists mostly of string conversions and an integer conversion. The example is an obvious demonstration of how <code class="prettyprint">Formatter</code> class makes it possible to write output for which we can tightly control presentation. You should consult with your official documentation for an in-depth explanation of format specifiers.</p> <p>Strings are not a trivial topic by any means and overall include perhaps a slightly moderate level of complexity sufficient to satisfy any curious mind. In particular, regular expressions can be challenging, but there is absolutely nothing about it that should constitute a barrier to those interested to discover it's truth.</p> http://www.java.net/blog/hellofadude/archive/2014/03/31/javas-regular-expression-string-and-things#comments Mon, 31 Mar 2014 17:29:10 +0000 hellofadude 901674 at http://www.java.net Explaining Java's error handling system http://www.java.net/blog/hellofadude/archive/2014/03/05/explaining-javas-error-handling-system <!-- | 0 --><p>In this post, I try to give a reasonable account of Java's error handling system being as it is that the handling of errors is a concern that any reasonable programming language must find some way to contend with. Java's error handling methodology is based on an idea of exceptions.</p> <p>An exception is a condition that prevents your program from further executing along the current path. It signifies a problem in your program serious enough to require intervention because the system lacks sufficient information that would allow your program to proceed along current context. When your program encounters an exceptional condition, Java provides a mechanism that allows you to 'throw' an exception in a way that passes the problem to an exception handler designed for just such an eventuality.</p> <p>Throwing an exception is a term that is used to describe the creation of a new object derived from the classes belonging to either one of two subtypes of the <code class="prettyprint">Throwable</code> class i.e. the <code class="prettyprint">Error</code> class - which are essentially a group of unchecked exceptions related to compile time and system errors which you are not required to declare in your exception specification, and the <code class="prettyprint">Exception</code> class, most of whose subclasses with the exception of <code class="prettyprint">RuntimeException</code>, belong to a category of checked exceptions you would normally be concerned to handle and which you would be required to declare in your exception specification. The exception specification refers to the part of your method appearing just after the argument list beginning with the <code class="prettyprint">throws</code> keyword followed by a list of all potential exceptions types, essentially a way to alert users to the kind of exceptions your method is liable to throw.</p> <p>Objects derived from <code class="prettyprint">RuntimeException</code> are unchecked because they often represent exceptions that can be thrown during the normal operation of the Java Virtual Machine for instance, programmatic errors that may be beyond your control like an uninitialised variable could be an example of a run time exception. These kind of exceptions form a part of the standard Java runtime checking and as a consequence, you should not have to include them in your exception specification or worry about handling them.</p> <h1>Throwing exceptions</h1> <p>Throwing an exception in Java is simply a matter of making a call to the appropriate exception type after the <code class="prettyprint">throw</code> keyword. For instance, you might throw an exception following a test for some particular condition like so:-<br /> <pre class="prettyprint"><code>package errors.handling.java;<br /> <br />public class ThrowException {<br /> public static void main(String[] args) {<br /> String[] sequence = {"first element", "seccond element" };<br /> for(int i = 0; i < sequence.length; i++)<br /> //do something here<br /> System.out.println("In bound");<br /> throw new ArrayIndexOutOfBoundsException("sequence.length");<br /> }<br />}<br />/* Output<br />In bound<br />In bound<br />Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: <br />sequence.length<br /> at errors.handling.java.ThrowException.main(ThrowException.java:9)<br />*//</code></pre><br /> The above example reports an illegal index with the <code class="prettyprint">ArrayIndexOutOfBoundsException</code>, with an optional string message in it's constructor argument. As you may observe, the name of exception classes in Java often implicitly describe the type of error and is often enough information to assist you to make an assessment of the problem.</p> <p>When you throw an exception using the <code class="prettyprint">new</code> keyword, an object is created on the heap in the normal way, however in addition to this, the current execution path is immediately terminated and the newly created object reference is ejected from the current context, in effect exiting the method just like you do with a <code class="prettyprint">return</code> statement. At this moment Java's exception handling mechanism begins the task of locating an appropriate exception handler, usually in a higher context, from where the program may be recovered. </p> <h1>Exception handling</h1> <p>The handling of exceptions in Java constitute a two step process involving firstly, the capture of the exception and then the handling of it. Both stages work in concert to provide a robust exception handling model. To capture an exception you place exception generating code within whats called a "guarded region" - which consists of the <code class="prettyprint">try</code> keyword followed by an ordinary scope:-<br /> <pre class="prettyprint"><code>try {<br />//guarded region<br />} catch(Type1 ref1) { // exception handler for type1<br />//exception handling code goes here<br />} catch(Type2 ref2) { // exception handler for type 2<br />//exception handling code goes here<br />}</code></pre><br /> Immediately following the <code class="prettyprint">try</code> block is the exception handler beginning with the <code class="prettyprint">catch</code> keyword. The idea is to pass the appropriate exception type as argument to the <code class="prettyprint">catch</code> block, such that a captured exception from the <code class="prettyprint">try</code> block might be matched against it by the exception handling mechanism. </p> <p>To properly appreciate Java's exception handling model, you must quickly become wise to the fact that more often than not, you will be concerned with handling other peoples thrown exceptions. As an example, consider for instance Java's <code class="prettyprint">FileReader</code> class in the I/O package. This class is handy for reading streams of character from files but quite naturally has a constructor that throws a <code class="prettyprint">FileNotFoundException</code>. If you wish to create an object of this class for use in your program, you will be required to handle this exception one way or another. The way to do that is to place the call to the constructor within a <code class="prettyprint">try</code>/<code class="prettyprint">catch</code> block like this:-<br /> <pre class="prettyprint"><code>package errors.handling.java;<br /> <br />import java.io.BufferedReader;<br />import java.io.File;<br />import java.io.FileNotFoundException;<br />import java.io.FileReader;<br />import java.io.IOException;<br /> <br />public class HandleException { <br /> public static void main(String[] args) {<br /> String path = "/home/geekfest/eclipse/notice.html";<br /> File resource = new File(path);<br /> BufferedReader reader;<br /> try {<br /> reader = new BufferedReader(new FileReader(resource));<br /> try {<br /> String console;<br /> while((console = reader.readLine()) != null);<br /> //System.out.println(console);<br /> } catch(IOException e) {<br /> System.out.println("read failed");<br /> }<br /> } catch(FileNotFoundException e) {<br /> System.out.println("File not found");<br /> //exception handling code here<br /> }<br /> System.out.println("reader created successfully");<br /> }<br />}<br />/*<br />reader created successfully<br />*//</code></pre><br /> This example makes use of a nested <code class="prettyprint">try</code> block to guard against multiple scenarios. First, we pass the <code class="prettyprint">static</code> call to the <code class="prettyprint">FileReader</code> constructor as an argument to create a reader within the first <code class="prettyprint">try</code> block. Next, we read the contents of a file within a nested <code class="prettyprint">try</code> block with the <code class="prettyprint">readLine()</code> method which incidentally throws an <code class="prettyprint">IOException</code> which is handled by the inner <code class="prettyprint">catch</code> block. The outer catch block handles any exception thrown from the call to the <code class="prettyprint">FileReader</code> constructor within the outer <code class="prettyprint">try</code> block. We could just as easily have accomplished the same thing with a single <code class="prettyprint">try</code> block and multiple <code class="prettyprint">catch</code> blocks so long as we paid attention to the normal restrictions imposed by inheritance.</p> <p>Of course the above example did not generate any exceptions so neither exception handler was ever really necessary. But the opposite might just have easily been the case if, for instance we made an error in our configuration or suffered some benign interruption to our I/O operation that caused it to fail.<br /> The point to emphasise here is that you can define as many <code class="prettyprint">catch</code> blocks as necessary to handle the many different exception types captured within your <code class="prettyprint">try</code> block but you only need the one <code class="prettyprint">catch</code> block to handle multiple instances of the same exception, all of this in addition to paying particular attention to the peculiar structure and semantics that come with the use of nested <code class="prettyprint">try</code> blocks.</p> <p>Irregardless, the exception handling mechanism will always make a search through the list of exception handlers in the order in which they are written and will enter into the first handler that is a match for a captured exception. Once a match is located, no further searching takes place as the particular exception is considered handled.<br /> Note that as a result of inheritance, a match in this context does not have to be an exact match between the thrown exception object and the type of the exception handler; this is to say an exception handler with a base class argument will match a derived class object. For instance, a handler defined with the base <code class="prettyprint">Exception</code> type argument will match an object of the derived type <code class="prettyprint">IOException</code>, for this reason it is best practice to specify handlers of a more generalised type at the bottom of the list of exception handlers to avoid preempting any other more specific types that might be a closer match.</p> <p>Sometimes, when dealing with programs significantly less complicated than of a commercial application, it is convenient to preserve your exceptions without regard to having to write a great deal of code by passing them out to console via your <code class="prettyprint">main()</code> method. In this way you are precluded from having to write <code class="prettyprint">try</code>/<code class="prettyprint">catch</code> blocks within the body of your <code class="prettyprint">main()</code>:-<br /> <pre class="prettyprint"><code>package errors.handling.java;<br /> <br />import java.io.BufferedReader;<br />import java.io.File;<br />import java.io.FileNotFoundException;<br />import java.io.FileReader;<br />import java.io.IOException;<br /> <br />public class ConsoleException {<br /> public static void main(String[] args) throws Exception {<br /> String path = "/home/geekfest/eclipse/notice.html";<br /> File resource = new File(path);<br /> BufferedReader reader = new BufferedReader(new FileReader(resource));<br /> String console;<br /> while((console = reader.readLine()) != null);<br /> //System.out.println(console);<br /> System.out.println("reader created successfully");<br /> reader.close();<br /> }<br />}<br />/* Output<br />reader created successfully<br />*//</code></pre> <p>In this version of a previous example, we use the exception specification syntax to specify that our <code class="prettyprint">main()</code> throws the base <code class="prettyprint">Exception</code>. This approach means we end up writing less code but is only really appropriate for use with very simple programs. </p> <p>So what does it mean to handle an exception within the context of the code you write in the <code class="prettyprint">catch</code> clause?<br /> Normally, you would want to use the <code class="prettyprint">catch</code> block to report the exception to the method caller. In the preceding example, if an exception was returned from the call to the <code class="prettyprint">FileReader</code> constructor, it would have been caught in the associated catch clause and you would have had "Caught File Not Found Exception" printed to screen.<br /> Reporting the exception is as much about notification as it is about providing pertinent information pertaining to the exception itself.<br /> The <code class="prettyprint">Throwable</code> class provides a number of methods with which one might query objects of it's subtypes for useful information pertaining to a thrown exception. For instance the <code class="prettyprint">getMessage()</code> and <code class="prettyprint">getLocalisedMessage()</code> methods both return a detailed and localised decription of the throwable object respectively, the <code class="prettyprint">printStackTrace()</code>, <code class="prettyprint">printStackTrace(PrintStream)</code> and <code class="prettyprint">printStackTrace(PrintWriter)</code> - all of which print the <code class="prettyprint">Throwable</code> and its backtrace to a stream of your choice - the no-argument constructor version prints to standard error. Another method, the <code class="prettyprint">fillInStackTrace()</code> method records information within this throwable about the current state of stack frames. It is a useful practice to become familiar with the meaning inherent in the output returned from some of these methods to assist your troubleshooting efforts.</p> <h1>The stack trace</h1> <p>Of all the methods available to subtypes of the <code class="prettyprint">Throwable</code> class, the <code class="prettyprint">printStackTrace()</code> method is special because it tells you the exact point from whence this exception originated. This information could be very useful in locating the source of bugs in your code. You can use its cousin, the <code class="prettyprint">getStackTrace()</code> method to return an array of stack trace elements that describe the sequence of method invocations leading up to the exception:-<br /> <pre class="prettyprint"><code>package errors.handling.java;<br /> <br />public class StackTraceArray {<br /> static void first() {<br /> try {<br /> throw new Exception();<br /> } catch(Exception exceptionRef) {<br /> for(StackTraceElement traceElement : exceptionRef.getStackTrace())<br /> System.out.println(traceElement.getMethodName());<br /> }<br /> }<br /> static void second() { first(); }<br /> static void last() { second(); }<br /> public static void main(String[] args) {<br /> first();<br /> System.out.println("\n" );<br /> second();<br /> System.out.println("\n");<br /> last();<br /> }<br />}<br />/* Output<br />first<br />main<br /> <br /> <br />first<br />second<br />main<br /> <br /> <br />first<br />second<br />last<br />main<br />*//</code></pre> <p>In this particular example, we throw and handle an exception of the base class <code class="prettyprint">Exception</code> within our <code class="prettyprint">first()</code> method and then progressively reach out to this method from within two other methods, a <code class="prettyprint">second()</code> and <code class="prettyprint">last()</code> method. In the exception handler, we use the for each syntax to loop through an array of stack trace elements returned by the <code class="prettyprint">getStackTrace()</code>. The output displays the result of each method call from the <code class="prettyprint">main()</code> and should be read as an array where the top of the stack equates to element zero in the array and represents the most recent method invocation in the sequence and typically the point from where the exception originated. In our little example we can easily see this to be the <code class="prettyprint">first()</code> method. The last element in the array represents the bottom of the stack and is the oldest method invocation in the sequence.</p> <h1>Rethrowing exceptions</h1> <p>In addition to throwing an exception, you can rethrow an exception within a <code class="prettyprint">catch</code> block for any number of reasons which may be convenient to your particular application. You may choose to rethrow the exception you have just caught or a different exception altogether. The key difference is when you rethrow an exception you have just caught, it ends up in an exception handler with all the original exception information intact, however, when you rethrow a different exception, you would normally be expected to lose all the information pertaining to the original exception which is in any case, replaced with information relating only to the new exception:-<br /> <pre class="prettyprint"><code>package errors.handling.java;<br /> <br />public class RethrowingException {<br /> static void original() throws Exception {<br /> System.out.println("Throwing new exception from original()");<br /> throw new Exception("original Exception()");<br /> }<br /> static void throwOriginalException() throws Exception {<br /> System.out.println("Inside throwOriginalException():");<br /> try {<br /> original();<br /> } catch (Exception exceptionRef) {<br /> throw exceptionRef;<br /> }<br /> }<br /> static void throwNewException() throws Exception {<br /> System.out.println("Inside throwNewException()");<br /> try {<br /> original();<br /> } catch(Exception exceptionRef) {<br /> throw new Exception("new Exception()");<br /> }<br /> }<br /> public static void main(String[] args) {<br /> try {<br /> throwOriginalException();<br /> } catch(Exception exceptionRef) {<br /> System.out.println("in catch 1:");<br /> exceptionRef.printStackTrace(System.out);<br /> }<br /> try {<br /> throwNewException();<br /> } catch(Exception exceptionRef) {<br /> System.out.println("in catch 2:");<br /> exceptionRef.printStackTrace(System.out);<br /> }<br /> }<br />}<br />/* Output<br />Inside throwOriginalException():<br />Throwing new exception from original()<br />in catch 1:<br />java.lang.Exception: original Exception()<br /> at <br />errors.handling.java.RethrowingException.original(RethrowingException.java:6)<br /> at <br />errors.handling.java.RethrowingException.throwOriginalException(RethrowingException.java:11)<br /> at <br />errors.handling.java.RethrowingException.main(RethrowingException.java:26)<br />Inside throwNewException()<br />Throwing new exception from original()<br />in catch 2:<br />java.lang.Exception: new Exception()<br /> at <br />errors.handling.java.RethrowingException.throwNewException(RethrowingException.java:21)<br /> at <br />errors.handling.java.RethrowingException.main(RethrowingException.java:32)<br />*//</code></pre><br /> For this example, we call two methods in the <code class="prettyprint">main()</code> both of which reference our <code class="prettyprint">original()</code> method in turn, from which an exception is thrown. In the <code class="prettyprint">throwOriginalException()</code> the caught exception is rethrown while a new exception is thrown in <code class="prettyprint">throwNewException()</code>. The result in the output marked <i>"in catch 2"</i> is that any reference to the original exception is lost. </p> <h1>Chaining Exceptions</h1> <p>If you do not want to lose the information pertaining to the original exception while rethrowing a different exception, you can employ a technique called exception chaining to retain this information whereby you pass the original exception as a cause argument to the constructor of a <code class="prettyprint">Throwable</code> object or objects of any of it's two main subclasses as well as an object of the <code class="prettyprint">RuntimeException</code> class. For all other derived classes, you must use the <code class="prettyprint">initCause()</code> method inherited from the <code class="prettyprint">Throwable</code> class, that takes and returns a <code class="prettyprint">Throwable</code> object which you can use to represent the cause:-<br /> <pre class="prettyprint"><code>package errors.handling.java;<br /> <br />class NoSuchItemException extends Exception {}<br /> <br />class Item {<br /> String name;<br /> public Item(String item) {<br /> this.name = item;<br /> }<br />}<br />public class ShoppingCart {<br /> private Item[] cart;<br /> <br /> public ShoppingCart(String[] list) {<br /> cart = new Item[list.length];<br /> for(int i = 0; i < list.length; i++)<br /> cart[i] = new Item(list[i]);<br /> }<br /> <br /> public boolean checkItem(Item item) throws NoSuchItemException {<br /> if(item == null) {<br /> NoSuchItemException chainedException = new NoSuchItemException();<br /> // use initCause to wrap exception for<br /> //Throwable subclasses<br /> chainedException.initCause(new NullPointerException());<br /> throw chainedException;<br /> }<br /> for(int i = 0; i < cart.length; i++)<br /> if(cart[i].name.equals(item.name))<br /> return true;<br /> return false;<br /> }<br /> public void addItem(Item item) {<br /> try {<br /> if(checkItem(item))<br /> return;<br /> Item[] shelf = new Item[cart.length+1];<br /> for(int i = 0; i < cart.length; i++)<br /> shelf[i] = cart[i]; // first make copy of array<br /> for(int i = cart.length; i < shelf.length; i++)<br /> shelf[i] = item; // add item to array<br /> cart = shelf;<br /> } catch(NoSuchItemException noSuchItemRef) {<br /> noSuchItemRef.printStackTrace(System.out);<br /> }<br /> }<br /> public int equals(String it) {<br /> for(int i = 0; i < cart.length; i++)<br /> if(cart[i].name.equals(it))<br /> return i;<br /> return -1;<br /> }<br /> public int getItemIndex(Item item) throws IndexOutOfBoundsException {<br /> int index = this.equals(item.name);<br /> if(index == -1)<br /> throw new IndexOutOfBoundsException();<br /> return index;<br /> }<br /> public Item[] getCart() {<br /> return cart;<br /> }<br /> public String toString() {<br /> StringBuilder displayCart = new StringBuilder();<br /> for(Item ref : cart) {<br /> displayCart.append(ref.name);<br /> displayCart.append("\n");<br /> }<br /> return displayCart.toString();<br /> }<br /> public static void main(String[] args) {<br /> String[] shoppinglist = { "Bread", "Milk", "Sugar" };<br /> ShoppingCart cart = new ShoppingCart(shoppinglist);<br /> System.out.println(cart);<br /> try {<br /> Item firstItem = new Item("Beans");<br /> Item secondItem = new Item("Tshirt");<br /> cart.addItem(firstItem);<br /> System.out.println(cart);<br /> cart.addItem(secondItem);<br /> System.out.println(cart);<br /> System.out.println(cart.getItemIndex(firstItem));<br /> System.out.println(cart.checkItem(secondItem));<br /> Item thirdItem = null;<br /> cart.addItem(thirdItem);<br /> } catch(NoSuchItemException noSuchItemRef) {<br /> noSuchItemRef.printStackTrace(System.out);<br /> } catch(IndexOutOfBoundsException outOfBoundsRef) {<br /> outOfBoundsRef.printStackTrace(System.out);<br /> }<br /> }<br />}<br />/* Output<br />Bread<br />Milk<br />Sugar<br /> <br />Bread<br />Milk<br />Sugar<br />Beans<br /> <br />Bread<br />Milk<br />Sugar<br />Beans<br />Tshirt<br /> <br />3<br />true<br />errors.handling.java.NoSuchItemException<br /> at errors.handling.java.ShoppingCart.checkItem(ShoppingCart.java:22)<br /> at errors.handling.java.ShoppingCart.addItem(ShoppingCart.java:35)<br /> at errors.handling.java.ShoppingCart.main(ShoppingCart.java:84)<br />Caused by: java.lang.NullPointerException<br /> at errors.handling.java.ShoppingCart.checkItem(ShoppingCart.java:25)<br /> ... 2 more<br />*//<br />*//</code></pre><br /> The example describes a <code class="prettyprint">ShoppingCart</code> that is composed of an array of <code class="prettyprint">Item</code> objects that are first initialised when a <code class="prettyprint">ShoppingCart</code> object is created. You can then subsequently add additional items, check for an item or return the index of a specific item. To add an item, you first make a temporary copy of the array with length one longer than current array, append the item to that and then copy back to the original array in a way that is an abstraction of the process of selecting items from a shelf at your local supermarket. If you try adding a null value, the <code class="prettyprint">checkItem()</code> method throws a custom exception distinct from the original <code class="prettyprint">NullPointerException</code> which we pass to the <code class="prettyprint">initCause()</code> method of the custom exception. You can see a <code class="prettyprint">NullPointerException</code> is listed as the original cause of this exception in the output.</p> <h1>Turning off checked exceptions</h1> <p>Exception chaining also allows you to, in effect turn off checked exceptions by converting them to unchecked exceptions without losing any information about the original exception. You do this by wrapping a checked exception in the inside of a <code class="prettyprint">RuntimeException</code> like so:-<br /> <pre class="prettyprint"><code>package errors.handling.java;<br /> <br />import java.io.IOException;<br /> <br />public class TurnOffException {<br /> static void throwAnException() {<br /> System.out.println("Throwing new exception from throwAnException()");<br /> try {<br /> throw new IOException();<br /> } catch(IOException ioExceptionRef) {<br /> throw new RuntimeException(ioExceptionRef);<br /> }<br /> }<br /> public static void main(String[] args) {<br /> try {<br /> TurnOffException.throwAnException();<br /> } catch(RuntimeException runtimeExceptionRef) {<br /> try {<br /> throw runtimeExceptionRef.getCause();<br /> } catch(IOException ioExceptionRef) {<br /> System.out.println("IOException " + ioExceptionRef);<br /> }catch(Throwable throwableRef) {<br /> System.out.println("Throwable " + throwableRef);<br /> }<br /> }<br /> //can call method without try block<br /> TurnOffException.throwAnException();<br /> }<br />}<br />/* Output<br />Throwing new exception from throwAnException()<br />IOException java.io.IOException<br />Throwing new exception from throwAnException()<br />Exception in thread "main" java.lang.RuntimeException: java.io.IOException<br /> at <br />errors.handling.java.TurnOffException.throwAnException(TurnOffException.java:11)<br /> at errors.handling.java.TurnOffException.main(TurnOffException.java:27)<br />Caused by: java.io.IOException<br /> at <br />errors.handling.java.TurnOffException.throwAnException(TurnOffException.java:9)<br /> ... 1 more<br />*//</code></pre> <p>This technique effectively allows you to ignore the checked exception relieving you of the need of having to write <code class="prettyprint">try</code>/<code class="prettyprint">catch</code> clauses and/or the usual exception specification.</p> <h1>Throwing custom exceptions</h1> <p>You might sometimes find it necessary to create your own exceptions to handle errors that might be peculiar to your own library by inheriting from an existing exception class, ideally an exception that more closely matches in meaning of your own exception:-<br /> <pre class="prettyprint"><code>package errors.handling.java;<br /> <br />import java.util.Random;<br /> <br />class CustomException extends Exception {<br /> public CustomException(String message) {<br /> System.out.println(message);<br /> }<br />}<br />public class ThrowAnException {<br /> public void throwCustomException(int condition) throws CustomException {<br /> if(condition == 5)<br /> throw new CustomException("Just got ejected!");<br /> System.out.println("condition = " + condition);<br /> //some code<br /> }<br /> public static void main(String[] args) {<br /> ThrowAnException objRef = new ThrowAnException();<br /> try {<br /> for(int i = 0; i < 100; i++)<br /> objRef.throwCustomException(new Random().nextInt(6));<br /> } catch(CustomException customExceptionRef) {<br /> System.out.println("Caught it!");<br /> }<br /> }<br />}<br /> /* Output<br />i = 3<br />i = 0<br />i = 3<br />Just got ejected!<br />Caught it!<br />*//</code></pre> <p>Here, we inherit from the base <code class="prettyprint">Exception</code> class to create a <code class="prettyprint">CustomException</code> that is thrown in the main whenever <b>condition == 5</b>. The method <code class="prettyprint">throwCustomException()</code> uses the the exception specification syntax to describe it's potential to throw a <code class="prettyprint">CustomException</code>. Whenever we come across a method that uses the exception specification to declare a list of potential exceptions, this is the clue that we must handle this exception whenever we call the method.</p> <p>In true OOP tradition, inheritance further imposes other constraints on derived classes with respect to the exception specification. For instance, when you override a method that defines an exception specification, the derived method can only throw those exceptions that have been specified in the base method, or exceptions derived from those specified in the base method, or it can choose not to throw any methods at all, but it can not throw a different set of exceptions to the one in the base class. With constructors however, this restriction is partially eased in a sense because even though a derived class must somehow account for exceptions thrown by the base class constructor, it is free to throw additional exceptions. Lastly, when you implement an interface in a derived class, methods inherited from that interface can not be expected to override those inherited from the base class.</p> <h1>The finally statement</h1> <p>The <code class="prettyprint">finally</code> clause is used to perform what is commonly referred to as 'clean up' operations in Java. These are the type of operations you would normally want your program to execute irregardless of whether or not an exception is thrown. The <code class="prettyprint">finally</code> clause is normally placed at the end of a <code class="prettyprint">try</code>/<code class="prettyprint">catch</code> block. The kinds of operation you would normally want to place within a <code class="prettyprint">finally</code> clause include code to shut out a network connection after you no longer have any use for it, or close a file once your program has successfully read it's contents or some other such operation. In a sense, the <code class="prettyprint">finally</code> clause allows you to delay the termination of the current execution path following a thrown exception, just long enough to execute some code.</p> <p>In the following example, a <code class="prettyprint">FileResource</code> class uses a <code class="prettyprint">FileReader</code> and a <code class="prettyprint">BufferedReader</code> object from the Java standard I/O library to open and read a file. Java's API reference describes the <code class="prettyprint">FileReader</code> class constructor as possibly throwing a <code class="prettyprint">FileNotFoundException</code>.<br /> The <code class="prettyprint">FileResource</code> has a <code class="prettyprint">read()</code> method to read the file and a <code class="prettyprint">close()</code> to perform clean up at the end of the object's lifetime. The <code class="prettyprint">close()</code> method of this class calls the <code class="prettyprint">close()</code> method of the <code class="prettyprint">BufferedReader</code> object to close the stream and release any system resources associated with it:-<br /> <pre class="prettyprint"><code>package errors.handling.java;<br /> <br />import java.io.BufferedReader;<br />import java.io.File;<br />import java.io.FileNotFoundException;<br />import java.io.FileReader;<br />import java.io.IOException;<br /> <br />class FileResource {<br /> private BufferedReader reader;<br /> String path = "/home/geekfest/eclipse/notice.html";<br /> File resource = new File(path);<br /> public FileResource() throws FileNotFoundException {<br /> try {<br /> reader = new BufferedReader(new FileReader(resource));<br /> } catch(FileNotFoundException ref) {<br /> System.out.println("unable to open file " + resource);<br /> ref.printStackTrace();<br /> } finally {<br /> //Don't close here<br /> }<br /> }<br /> public String read() {<br /> String output = null;<br /> try {<br /> output = reader.readLine();<br /> } catch(IOException ref) {<br /> ref.printStackTrace(System.out);<br /> }<br /> return output;<br /> }<br /> public void close() {<br /> try {<br /> reader.close();<br /> System.out.println("close() successful");<br /> } catch(IOException ref) {<br /> ref.printStackTrace(System.out);<br /> }<br /> }<br />}<br />public class ConsoleDevice {<br /> public static void main(String[] args) {<br /> try {<br /> FileResource resource = new FileResource();<br /> try {<br /> String console;<br /> while((console = resource.read()) != null);<br /> //System.out.println(console);<br /> } catch(Exception ref) {<br /> ref.printStackTrace(System.out);<br /> } finally {<br /> resource.close();<br /> }<br /> } catch(Exception ref) {<br /> ref.printStackTrace(System.out);<br /> }<br /> <br /> }<br />}<br />/* Output<br />close() successful<br />*//</code></pre> <p>In the <code class="prettyprint">main()</code> of the <code class="prettyprint">ConsoleDevice</code> class we use a set of nested <code class="prettyprint">try</code> blocks to safely create and use the <code class="prettyprint">FileResource</code> class to read text from a file unto our console object. Following the <code class="prettyprint">while</code> loop in the inner <code class="prettyprint">try</code> block, we call the <code class="prettyprint">close()</code> method in the <code class="prettyprint">finally</code> clause to perform clean up, at the only point in the program we can be certain clean up will be necessary.</p> <h1>try-with-resources statement </h1> <p>With Java SE 7 came the introduction of the try-with-resources statement to ensure that each resource is automatically closed at the end of a statement. A resource includes any object that implements <code class="prettyprint">AutoClosable</code>. The following example demonstrates the the use of the try-with-resources statement with out <code class="prettyprint">FileResource</code> class:-<br /> <pre class="prettyprint"><code>package errors.handling.java;<br /><br />import java.io.BufferedReader;<br />import java.io.File;<br />import java.io.FileNotFoundException;<br />import java.io.FileReader;<br />import java.io.IOException;<br /> <br />class FileResource implements AutoCloseable {<br /> private BufferedReader reader;<br /> String path = "/home/geekfest/eclipse/notice.html";<br /> File resource = new File(path);<br /> public FileResource() throws FileNotFoundException {<br /> try {<br /> reader = new BufferedReader(new FileReader(resource));<br /> } catch(FileNotFoundException ref) {<br /> System.out.println("unable to open file " + resource);<br /> ref.printStackTrace();<br /> } finally {<br /> //Don't close here<br /> }<br /> }<br /> public String read() {<br /> String output = null;<br /> try {<br /> output = reader.readLine();<br /> } catch(IOException ref) {<br /> ref.printStackTrace(System.out);<br /> }<br /> return output;<br /> }<br /> public void close() {<br /> try {<br /> reader.close();<br /> System.out.println("close() successful");<br /> } catch(IOException ref) {<br /> ref.printStackTrace(System.out);<br /> }<br /> }<br />}<br />public class ConsoleDevice2 {<br /> public static void main(String[] args) throws FileNotFoundException {<br /> try (FileResource resource = new FileResource();) {<br /> String console;<br /> while((console = resource.read()) != null);<br /> //System.out.println(console);<br /> }<br /> <br /> }<br />}<br />/* Output<br />close() successful<br />*//</code></pre><br /> In example, the <code class="prettyprint">try</code>-with-resources statement is implemented in the <code class="prettyprint">main()</code> by means of a call to our <code class="prettyprint">FileResource</code> class within the parenthesis that appear following the <code class="prettyprint">try</code> keyword. Our <code class="prettyprint">FileResource</code> class has had to be modified to implement the java.lang.<code class="prettyprint">AutoCloseable</code> interface. With the <code class="prettyprint">try</code>-with-resources statement, you do not need to include a <code class="prettyprint">finally</code> statement because the resource will be closed regardless of whether the <code class="prettyprint">try</code> statement fails to complete. In addition, exceptions thrown from from within the <code class="prettyprint">try</code> block will supercede those thrown from the <code class="prettyprint">try</code>-with-resources statements it self which become suppressed. You can include one or more resources in a <code class="prettyprint">try</code>-with-resources statement declaration, by separating each call with a semi-colon. Each resources' <code class="prettyprint">close()</code> method will be called in the opposite order of their creation. </p> http://www.java.net/blog/hellofadude/archive/2014/03/05/explaining-javas-error-handling-system#comments Blogs Global Education and Learning J2SE Open JDK Thu, 06 Mar 2014 06:16:14 +0000 hellofadude 901407 at http://www.java.net Using inner and nested Java classes http://www.java.net/blog/hellofadude/archive/2014/02/20/using-inner-and-nested-java-classes <!-- | 0 --><p>If you have followed previous posts, you might begin to perceive a pattern in the semantics of the Java programming language. If not, it might help to go over previous posts as I tend to return to expand on previous topics or add clearer examples as time permits. Inner classes might at first seem like a whole new language to the uninitiated but they are a nice feature in Java that allow you to logically group related classes and control the visibility of one class from outside of the other. </p> <!--break--><!--break--><p> Even more than that, inner classes provide you with the ability to inherit from more than one interface or implementation in a kind of "multiple implementation inheritance". The code in an inner class can be used to manipulate the members of the outer class within which it exists. Inner classes are not limited to the functionality of the base class of the outer class, but can inherit independently of it.</p> <p>Whereas outer classes allow you to undertake some kind of multiple inheritance with interfaces, inner classes allow you to do the same thing not only with interfaces but with abstract or concrete classes. This mechanism allows you to code solutions to some otherwise intractable programming problems. The syntax for creating inner classes is pretty straightforward:-<br /> <pre class="prettyprint"><code>package kingsley.java;<br /> <br />public class Company {<br /> class Employee {<br /> private String name;<br /> Employee(String name) {<br /> this.name = name;<br /> }<br /> public String getName() {<br /> return name;<br /> }<br /> }<br /> class Department {<br /> private String name;<br /> <br /> Department (String name) {<br /> this.name = name;<br /> }<br /> public String getName() {<br /> return name;<br /> }<br /> }<br /> public Employee employed(String name) {<br /> return new Employee(name);<br /> }<br /> public Department department(String name) {<br /> return new Department(name);<br /> }<br /> public void newStarter(String name, String department) {<br /> Employee emp = employed(name);<br /> Department dpt = department(department);<br /> System.out.println(emp.getName() + " is a member of " + <br />dpt.getName());<br /> }<br /> public static void main(String[] args) {<br /> Company c = new Company();<br /> c.newStarter("kingsley", "IT");<br /> // Referencing inner classes<br /> Company.Employee ce = c.employed();<br /> Company.Department cd = c.department();<br /> }<br />}<br />/* Output<br />kingsley is a member of IT<br />*//</code></pre><br /> Here, the outer class <code class="prettyprint">Company</code> has methods that return references to the inner classes <code class="prettyprint">Employee</code> and <code class="prettyprint">Department</code>. The <b>newStarter()</b> within the outer class is able to use inner classes like it would any other class, with the only difference being these classes are actually nested within <code class="prettyprint">Company</code>. In the <b>main()</b> notice how we reference objects of the inner classes with <i>OuterClassName.InnerClassName</i>. This is necessary because objects of any non-<code class="prettyprint">static</code> inner class can only be created in association with an object of the outer class, outside of any non-<code class="prettyprint">static</code> method within the outer class.</p> <p>As a consequence, objects of the inner classes you create usually have access to all the <code class="prettyprint">private</code> and <code class="prettyprint">protected</code> members of the surrounding class without any special syntax. This is because when you create an inner class in Java, the compiler captures a reference to the particular enclosing class within which the inner class exists, thus making it possible to reference members of the enclosing class, in this way, a derived inner class can be used to manipulate the members of an enclosing class. The following example demonstrates this clearly:-<br /> <pre class="prettyprint"><code>package kingsley.osime;<br /> <br />interface Repeater {<br /> boolean start();<br /> String current();<br /> void next();<br />}<br />public class StringArray {<br /> private static String[] sequence;<br /> public StringArray(String s) {<br /> this.sequence = s.split(" ");<br /> }<br /> private class ReverseSequence implements Repeater {<br /> private int i = 0;<br /> public boolean start() {<br /> return i == sequence.length;<br /> }<br /> public String current() {<br /> return sequence[sequence.length-(i+1)];<br /> }<br /> public void next() {<br /> if(i < sequence.length)<br /> ++i;<br /> }<br /> }<br /> public Repeater repeater() {<br /> return new ReverseSequence();<br /> }<br /> public static void main(String[] args) {<br /> StringArray sa = new StringArray("Good sense is of all things <br />the most " + "equally distributed among men");<br /> Repeater repeater = sa.repeater();<br /> while(!repeater.start()) {<br /> System.out.print(repeater.current() + " ");<br /> repeater.next();<br /> }<br /> }<br />}<br />/* Output<br />men among distributed equally most the things all of is sense Good<br />*//</code></pre><br /> In this example, the derived inner class <code class="prettyprint">RequestSequence</code> is used to manipulate the array member within the enclosing class by providing functionality for an interface that acts like an iterator. In this instance the content of the array member is read in reverse.</p> <h1>Multiple implementation inheritance</h1> <p>With inner classes you can solve the problem of multiple implementation inheritance when you are limited to working with abstract or concrete classes as opposed to interfaces:-<br /> <pre class="prettyprint"><code>package Innerclasses.java;<br /> <br />abstract class A {}<br />class B {}<br /> <br />class C extends A {<br /> class D extends B {<br /> }<br />}<br />public class MultipleImplementation {<br /> static void takesA(A a) {}<br /> static void takesB(B a) {}<br /> public static void main(String[] args) {<br /> C c = new C();<br /> takesA(c);<br /> // using dot new to create object <br /> //inner class directly<br /> takesB(c.new D());<br /> }<br />}<br /> </code></pre><br /> Note the use of an object of the outer class followed by the <code class="prettyprint">new</code> keyword to create an object of the inner class in the <b>takesB()</b> method. Java will not allow you to create an object of the inner class directly, unless you already have an object of the outer class. The only exception to this rule is when you are dealing with nested classes i.e. inner classes that have been declared <code class="prettyprint">static</code> <h1><code class="prettyprint">.this</code> keyword</h1> <p>If you want to reference the outer class from within the inner class, you must first reference the outer class followed by a dot and then the <code class="prettyprint">this</code> keyword like so:-<br /> <pre class="prettyprint"><code>package kingsley.osime;<br /> <br />public class Outer {<br /> void d() { System.out.println("Outer d()"); }<br /> public class Inner {<br /> public Outer getOuter() {<br /> return Outer.this;<br /> }<br /> }<br /> public Inner getInner() { return new Inner(); }<br /> public static void main(String[] args) {<br /> Outer ot = new Outer();<br /> Outer.Inner oi = ot.getInner();<br /> oi.getOuter().d();<br /> }<br />}<br />/* Output<br />Outer d()<br />*//</code></pre> <p>If you were to use the <code class="prettyprint">this</code> keyword on its own from within the inner class, Java will return a reference to the inner class itself. Just as an inner class is able to reference the members of the outer class without qualification, an outer class is likewise able to access the <code class="prettyprint">private</code> elements of the inner class but must do so by directly referencing the inner class:-<br /> <pre class="prettyprint"><code>package Innerclasses.java;<br /> <br />public class Outer {<br /> class Inner {<br /> private String s = "My String";<br /> }<br /> public String innerString() {<br /> return new Inner().s;<br /> }<br /> public static void main(String[] args) {<br /> Outer ot = new Outer();<br /> System.out.println(ot.innerString());<br /> Outer.Inner oi = ot.new Inner();<br /> }<br />}<br />/* Output<br />My String<br />*//</code></pre> <h1>Upcasting inner classes</h1> <p>Inner classes provide a very useful way to hide implementation when inheriting from a base class or interface, in this way, one may restrict the visibility of one's classes from client programmers by providing only a reference to the base class or interface. Here is a modified version of an earlier example to illustrate this point; we modify our <code class="prettyprint">Employee</code> and <code class="prettyprint">Department</code> implementations to interfaces and inherit from both using <code class="prettyprint">private</code> and <code class="prettyprint">protected</code> inner classes here:-<br /> <pre class="prettyprint"><code>package Innerclasses.java;<br /> <br />import Interfaces.java.Department;<br />import Interfaces.java.Employee;<br /> <br /> class Company1 {<br /> String employee;<br /> String department;<br /> protected class NEmployee implements Employee {<br /> public String getName() {<br /> return employee;<br /> }<br /> }<br /> private class NDepartment implements Department {<br /> public String name() {<br /> return department;<br /> }<br /> }<br /> public Employee employed() {<br /> return new NEmployee();<br /> }<br /> public Department getName() {<br /> return new NDepartment();<br /> }<br />} <br />public class NewCompany {<br /> public static void main(String[] args) {<br /> Company1 c1 = new Company1();<br /> Employee emp = c1.employed();<br /> Department dept = c1.getName();<br /> Company1.NEmployee ce = c1.new NEmployee();<br /> // Can not access private inner class<br /> //! Company1.NDepartment cn = c1.new NDepartment();<br /> }<br />}</code></pre> <p>Note that when you apply the <code class="prettyprint">private</code> access specifier to a derived inner class you restrict access to that class to the client programmer thereby preventing any type coding dependencies. Take note of the upcasting taking place within the public interface of the outer class from a more specific class to the more generalised type. Further, unlike a <code class="prettyprint">private</code> inner class, a protected inner class does allow some limited access.</p> <h2>Inheriting from inner classes</h2> <p>On the subject of inheritance, Java does not prevent you from inheriting from an inner class when such a design make sense to you. The only requirement is that you make the association between the outer class and inherited inner class, explicit like so:-<br /> <pre class="prettyprint"><code>class Outer {<br /> class Inner {}<br />}<br />public class InnerInheritance extends Outer.Inner {<br /> InnerInheritance(Outer ot) {<br /> ot.super();<br /> }<br /> public static void main(String[] args) {<br /> Outer ot = new Outer();<br /> InnerInheritance ii = new InnerInheritance(ot);<br /> }<br />}</code></pre> <p>When you inherit from an inner class, you need to add special syntax to the constructor as the default constructor will not work. Note how we reference the enclosing class of the inherited inner class from within the derived class constructor in order to provide the correct reference.</p> <h1>Local and anonymous inner classes</h1> <p>Java also allows you to create what we refer to as local and anonymous inner classes that exist within method scopes. A local inner class is a class created within the scope of a method as opposed to just being within a class. You use a local inner class to return a reference when you're implementing an interface and to help solve some very complex programming problems whereby a class might be the only means of sufficiently expressing the level of complexity required.</p> <p>An anonymous inner class is a local inner class that has no name and can be expressed within a method or arbitrary scope. The syntax for both types of classes is like so:-<br /> <pre class="prettyprint"><code>package Innerclasses.java;<br /> <br />import Interfaces.java.Employee;<br /> <br />public class Company2 {<br /> public Employee createEmployee(String name) {<br /> class NewEmployee implements Employee {<br /> String name;<br /> public NewEmployee(String myName) {<br /> this.name = myName;<br /> }<br /> public String getName() {<br /> System.out.println(name);<br /> return name;<br /> }<br /> }<br /> return new NewEmployee(name);<br /> }<br /> public Employee createEmployee2() {<br /> return new Employee() {<br /> String name = "kingsley-Anonymous";<br /> public String getName() {<br /> System.out.println(name);<br /> return name;<br /> }<br /> };<br /> }<br /> public static void main(String[] args) {<br /> Company2 c2 = new Company2();<br /> Employee em = c2.createEmployee("Kingsley-local");<br /> Employee em2 = c2.createEmployee2();<br /> em.getName();<br /> em2.getName();<br /> }<br />}<br />/* Output<br />Kingsley-local<br />kingsley-Anonymous<br />*//</code></pre> <p>As you can see, the local inner class begins and ends within the scope of a method, aside from which it is no different to how you would implement any other class.<br /> The anonymous inner class provides exactly the same functionality as an inner class except that it is shorthand and quicker to write because you write the class as an expression within the method or scope. Here we simply implement the methods within the interface as we would do if we were implementing in a class. Notice the semi-column at the end of the expression as if it were any other statement in Java. Theoretically, we can implement local and anonymous inner classes within any arbitrary scope surrounded by curly braces.</p> <h1>Nested classes</h1> <p>Nested classes are inner classes that have been declared <code class="prettyprint">static</code>. When you declare an inner class <code class="prettyprint">static</code>, you do not require an object of the outer class to create the inner class and you are restricted from accessing a non-<code class="prettyprint">static</code> outer class from an object of a nested class. Here is our <code class="prettyprint">Company</code> example modified to use nested inner classes:-<br /> <pre class="prettyprint"><code>package Innerclasses.java;<br /> <br />import Interfaces.java.Department;<br />import Interfaces.java.Employee;<br /> <br />public class Company4 {<br /> private static class NewEmployee implements Employee {<br /> private String name;<br /> NewEmployee(String name) {<br /> this.name = name;<br /> }<br /> public String getName() {<br /> return name;<br /> }<br /> }<br /> protected static class EmployeeDepartment implements Department {<br /> private String name;<br /> <br /> EmployeeDepartment (String name) {<br /> this.name = name;<br /> }<br /> public String name() {<br /> return name;<br /> }<br /> static class BusinessUnit {<br /> static String bu = " Finance ";<br /> static String getBU() {<br /> return bu;<br /> }<br /> }<br /> }<br /> public static Employee employed(String name) {<br /> return new NewEmployee(name);<br /> }<br /> public static Department department(String name) {<br /> return new EmployeeDepartment(name);<br /> }<br /> public static void main(String[] args) {<br /> Employee em = employed("kingsley");<br /> Department dept = department("IT");<br /> System.out.println(Company4.EmployeeDepartment.BusinessUnit.getBU());<br /> }<br />}<br />/* Output<br />Finance<br />*//</code></pre> <p>In this example, note how we do not require an object of the outer class to create an object of the inner class in the <b>main()</b>. Unlike a non-<code class="prettyprint">static</code> inner class, a nested inner class can have <code class="prettyprint">static</code> data, fields and other nested static classes as you can observe from the nested <code class="prettyprint">BusinessUnit</code> class.</p> <h2>Nesting within interfaces</h2> <p>Java also allows you to nest classes within interfaces when you wish to create some common code to be used by different implementations of the interface. When you put a class in an interface it is automatically <code class="prettyprint">public</code>, however you must explicitly declare the class <code class="prettyprint">static</code> to ensure it does not violate the normal interface rules. Here's an example that implements the enclosing interface in the nested class:-<br /> <pre class="prettyprint"><code>package Innerclasses.java;<br /> <br />public interface NestedInterface {<br /> void f();<br /> static class InnerClass implements NestedInterface {<br /> public void f() {<br /> System.out.println("Nested Class in Interface");<br /> }<br /> public static void main(String[] args) {<br /> new InnerClass().f();<br /> }<br /> }<br />}<br />/*<br />Nested Class in Interface<br />*//</code></pre> <h1>Implementing callbacks</h1> <p>At once inner classes might seem like a bit tedious and rather difficult to manage to the impatient mind, but it is worth taking the extra effort to understand some of the benefits of using inner classes in your program code.<br /> As mentioned earlier, inner classes allow you to inherit from more than one <code class="prettyprint">abstract</code> or concrete class, something you are severely restricted from doing with outer classes. Further more, a single outer class can have multiple inner classes implementing the same interface in different ways. This is a particularly powerful programming technique that allows you to implement a control type framework like the <i>Template method design pattern</i>. </p> <p>Because inner classes retain scope information, they also allow you to provide a kind of call-back mechanism which acts like a pointer in your code. With a callback some other object is has the necessary information to reach back into the originating object at some later point in time:-<br /> <pre class="prettyprint"><code>import java.util.ArrayList;<br />import java.util.List;<br /> <br />interface U {<br /> void a1();<br /> void a2();<br /> void a3();<br />}<br />class A {<br /> int i = 0;<br /> int count;<br /> public A(int i) {<br /> this.i = i;<br /> count = i++;<br /> }<br /> public U getU() {<br /> return new U() {<br /> public void a1() { System.out.println("a1() " + count); }<br /> public void a2() { System.out.println("a2() " + count); }<br /> public void a3() { System.out.println("a3() " + count); }<br /> };<br /> }<br />}<br /> class B {<br /> U[] ul = new U[3];<br /> int i = 0;<br /> public void addRef(U ui, int i) {<br /> ul[i] = ui;<br /> <br /> }<br /> public void setRef(int i) {<br /> System.out.println("Setting element at " + i + " to null");<br /> ul[i] = null;<br /> }<br /> public void goOver(int i) {<br /> ul[i].a1();<br /> ul[i].a2();<br /> ul[i].a3();<br /> }<br /> }<br />public class Callback {<br /> public static void main(String[] args) {<br /> B b = new B();<br /> for(int i = 0; i < 3; i++) {<br /> A a1 = new A(i);<br /> b.addRef(a1.getU(), i);<br /> b.goOver(i);<br /> }<br /> b.setRef(1);<br /> }<br />}<br />/* Output<br />a1() 0<br />a2() 0<br />a3() 0<br />a1() 1<br />a2() 1<br />a3() 1<br />a1() 2<br />a2() 2<br />a3() 2<br />Setting element at 1 to null<br />*//</code></pre> <p>This simple example creates a method in class <code class="prettyprint">A</code> that returns a reference to interface <code class="prettyprint">U</code> by building an anonymous inner class. In a second class <code class="prettyprint">B</code> we create an array containing <code class="prettyprint">U</code> references and implement various methods that allows us to accept and add <code class="prettyprint">U</code> references to the array, remove <code class="prettyprint">U</code> references from the array and a third method through which we can move through the array and call the methods in <code class="prettyprint">U</code>. In the <b>main()</b>, we populate a single <code class="prettyprint">B</code> object with <code class="prettyprint">U</code> references returned by a number of <code class="prettyprint">A</code> objects. We then use <code class="prettyprint">B</code> to call back into all the <code class="prettyprint">A</code> objects. This is a simplification of a very powerful programming technique.</p> <p>Understanding when and how to use inner classes is more of a design issue that should become second nature with repeated practice to the dedicated programmer. </p> http://www.java.net/blog/hellofadude/archive/2014/02/20/using-inner-and-nested-java-classes#comments Blogs Global Education and Learning J2SE Open JDK Fri, 21 Feb 2014 03:12:05 +0000 hellofadude 901257 at http://www.java.net An overview of Java's container classes http://www.java.net/blog/hellofadude/archive/2014/02/12/overview-javas-container-classes <!-- | 0 --><p>In the normal course of solving a general programming problem, it is almost certain that you will become compelled to create, and identify useful ways by which one may hold any number of objects within your program. In Java, you are normally inclined toward the array as the natural choice for holding a group of primitives, but this has the obvious limitation of being of a fixed size, whereas, under normal circumstances, you are unlikely to know before hand, the number of objects, or indeed anything about their type before run-time.</p> <!--break--><!--break--><p> The <code class="prettyprint">java.util</code> package provides a collection of type safe containers with which one may very quickly and easily begin to solve a number of well documented programming problems. These containers are expressed as basic interfaces of two libraries - 1) <code class="prettyprint">Collection</code> interface:- which describe a sequence of individual elements to which certain rules may be applied. Containers within this category include <code class="prettyprint">List</code>,<code class="prettyprint"> Set</code> and <code class="prettyprint">Queue</code> and 2)<code class="prettyprint"> Map</code> interface:- a group of key-value pairs that allow you to look up a value using a key. A <code class="prettyprint">Map</code> is also known as an associative array.</p> <p>Type safety within this context refers to the ability to ensure that only objects of the required type can be inserted into a particular container. Java makes this possible through the use of generics, which provides a way for classes and interfaces to become parameters within other classes, interfaces and methods. For instance, one of the most commonly used container in Java is the <code class="prettyprint">ArrayList</code> which represents a type of <code class="prettyprint">Collection</code>. We could create a type safe <code class="prettyprint">ArrayList </code>container to hold <code class="prettyprint">Fruit</code> objects in the following way:-</p> <code class="prettyprint">ArrayList<Fruit> fruits = new ArrayList<Fruit>();</code></br><br /> The angle brackets surrounding the type parameter is used to denote the actual type (class or interface) for which purpose the container was created. A container instance may specify more than one type no matter, the compiler performs type checking to ensure only objects of the specified type(s) is inserted into the container. We might demonstrate the value of generics in the following sequence of examples. The first example uses a non-generic <code class="prettyprint">ArrayList</code> container to hold a number of unrelated objects:-<br /> <pre class="prettyprint"><code>package kingsley.java<br /><br />import java.util.ArrayList;<br /> <br />class Fruit {<br /> private static long counter;<br /> private final long id = counter++;<br /> public long getId() { return id; }<br />}<br />class Furniture { }<br />public class NoGenericsContainer {<br /> @SuppressWarnings("unchecked")<br /> public static void main(String[] args) {<br /> ArrayList fruits = new ArrayList();<br /> for(int i = 0; i < 2; i++)<br /> fruits.add(new Fruit());<br /> fruits.add(new Furniture());<br /> for(int i = 0; i < fruits.size(); i++)<br /> System.out.println(fruits.get(i));// returns reference to objects<br /> System.out.println(((Fruit) fruits.get(i)).getId());//!runtime Exception <br /> }<br />}</code></pre> <p>In this example an object of the non-generic <code class="prettyprint">ArrayList</code> container is used to hold <code class="prettyprint">Fruit</code> and <code class="prettyprint">Furniture</code> objects whose only commonality is the fact that they are, by default, both objects of the root class <code class="prettyprint">Object</code>. This <code class="prettyprint">ArrayList</code> container makes no use of generics and consequently the compiler does not undertake any type checking, making it possible to <b>add()</b> objects of an otherwise 'dissimilar' type absent warning.<br /> When retrieving these objects with the container provided <b>get()</b> method, we must make use of an explicit cast because the absence of type information means we are dealing with object references. Of course at this point we are confronted with a runtime exception highlighting our folly in trying to cast a <code class="prettyprint">Furniture</code> object to a <code class="prettyprint">Fruit</code>!.. a remarkable feat even at the best of times.</p> <p>In contrast to the previous example, this next example uses a type safe container (generics) to achieve much the same thing but is much more intuitive and produces better result:-<br /> <pre class="prettyprint"><code>package kingsley.java<br /><br />import java.util.ArrayList;<br /> <br />class Mango extends Fruit {}<br />class Orange extends Fruit {}<br />class Funiture {}<br />class Table extends Furniture {}<br />public class GenericsContainer {<br /> public static void main(String[] args) {<br /> ArrayList<Fruit> fruits = new ArrayList<Fruit>();<br /> for(int i = 0; i < 3; i++)<br /> fruits.add(new Mango());<br /> fruits.add(new Orange());<br /> //fruits.add(new Table()); Compiler error! <br /> for(int j = 0; j < fruits.size(); j++)<br /> System.out.println(fruits.get(j).getId());<br /> //using foreach syntax to iterate through list<br /> for(Fruit f : fruits)<br /> System.out.println(f);<br /> }<br />}<br />/*Output<br />0<br />1<br />2<br />3<br />Mango@1664978b<br />Mango@26193229<br />Mango@402c3549<br />Orange@165e6c89<br />*//</code></pre><br /> The above example demonstrates several features available to you when working with generics. In this instance, the <code class="prettyprint">ArrayList</code> container, <b>fruit</b>, is configured to hold <code class="prettyprint">Fruit</code> objects with the addition of angle brackets that denote the use of generics. Generics make it possible for subtypes of the <code class="prettyprint">Fruit</code> class to take advantage of Java's polymorphic methods when being upcast to the container object. In this example, subtypes of the <code class="prettyprint">Fruit</code> class are added to the container absent complaint, however note the compiler error when we try to <b>add()</b> a <code class="prettyprint">Table</code> object, which is really of a different data type to the <code class="prettyprint">Fruit</code> class. In the previous example this discrepancy with the types was only flagged at runtime during retrieval with the use of a cast. </p> <p>With generics the compiler performs type checking to make sure only the correct type can be added to the container. Further more, with generics, a cast is no longer necessary when retrieving our objects as we can be certain objects are of the correct type. The example further demonstrates how to select each element from within the container using both the <code class="prettyprint">for</code> loop and <b>foreach</b> syntax, both of which are ideally suited to a number of different scenarios. The use of the <b>foreach</b> syntax with any <code class="prettyprint">Collection</code> object is made possible because of the presence of the <code class="prettyprint">Iterable</code> interface whose <b>iterator()</b> method produces an <code class="prettyprint">Iterator</code>. </p> <h1>Collection Interface</h1> <p>In Java, collections is a term used to describe a single unit or group comprising of multiple elements. It is synonymous with the concept of a container and both are often used interchangeably.</p> <p>Java provides a collections framework that comprise more than a dozen interfaces that make it possible for containers to be manipulated independent from the details of their implementation.<br /> At the root of this framework is the <code class="prettyprint">Collection</code> interface which represents a generalisation of the concept of a collection implemented by all containers of this type in a way that allows for a greater level of flexibility and variety in the range of it's application and implementation. An <code class="prettyprint">ArrayList</code> is an oft used type of <code class="prettyprint">Collection</code> which we might have chosen to implement in the following way:-</p> <code class="prettyprint">Collection<Fruit> fruits = new ArrayList<Fruit>();</code></br><br /> Of course, such an arrangement would restrict our access to the methods in the <code class="prettyprint">ArrayList</code> class, however this technique provides us with much flexibility that means we could easily switch implementation to any other subtype of the <code class="prettyprint">Collection</code> interface with very little effort. The ability to switch your implementation mid-code is one of the benefits of inheritance and polymorphism that is very useful to the experienced programmer. </p> <p>The <code class="prettyprint">Collection</code> interface provides several methods that can be used to manipulate elements within objects of an implementing class. Some of the more frequently used methods include the <b>size()</b> method, which returns the number of elements in the specified collection, the <b>iterator()</b> method, returns an iterator over all elements in the collection, the <b>remove()</b> method which removes an instance of the specified element from the collection, the <b>toArray()</b> method which returns an array containing all of the elements in this collection and a number of others. One may wish to consult the Java API for the full list of methods available to subtypes of the <code class="prettyprint">Collection</code> interface.</p> <h2>List Interface</h2> <code class="prettyprint">List</code> is a basic type of <code class="prettyprint">Collection</code> that maintains elements in a particular sequence and may contain duplicate elements. In addition to the methods inherited from the <code class="prettyprint">Collection</code> interface, <code class="prettyprint">List</code> provides additional methods that allow indexed access to elements within the list and unlike arrays, are automatically resizeable to accommodate additional elements. </p> <p>The most commonly used <code class="prettyprint">List</code> implementation include those referred to previously i.e. <code class="prettyprint">ArrayList</code> - a type of <code class="prettyprint">List</code> that particularly excels at randomly accessing elements but is less efficient at insertion and removal operations and <code class="prettyprint">LinkedList</code> - a type of <code class="prettyprint">List</code> that performs insertion and removal operations more efficiently than does an <code class="prettyprint">ArrayList</code> but is known to be less efficient for random access operations. In addition to Implementing the basic <code class="prettyprint">List</code> interface, a <code class="prettyprint">LinkedList</code> includes methods that make it useable as a <code class="prettyprint">Queue</code> or double-ended queue (Deque). A <code class="prettyprint">Deque</code> is a linear collection of elements that allows the addition and removal of elements at both end points. The following example demonstrates some of the more common operations associated with objects of a <code class="prettyprint">List</code> subtype:-<br /> <pre class="prettyprint"><code>import java.util.*;<br /> <br />public class ListsOperations {<br /> public static void main(String[] args) {<br /> ArrayList<Integer> arrayList = new ArrayList<Integer>(10);<br /> LinkedList<Integer> linkList = new LinkedList<Integer>();<br /> Random rand = new Random(10);<br /> for(int i = 0; i < 10; i++) {<br /> arrayList.add(i*rand.nextInt(100));<br /> linkList.add(arrayList.size()*rand.nextInt(100));<br /> }<br /> System.out.println("1: arrayList = " + arrayList);<br /> arrayList.add(new Integer(500));// appends to end of list<br /> System.out.println("2: arrayList.add(new Integer(500)) = " + <br />arrayList);<br /> System.out.println("3: arrayList.contains(new Integer(500)) = " <br />+ arrayList.contains(new Integer(500)));<br /> System.out.println("4: arrayList.remove(new Integer(92)) = " + <br />arrayList.remove(new Integer(92)));<br /> System.out.println("5: arrayList.indexOf(291) = " + <br />arrayList.indexOf(291));<br /> System.out.println("6: linkList = " + linkList);<br /> System.out.println("7: linkList.getFirst() = " + <br />linkList.getFirst());<br /> linkList.addFirst(new Integer(1000));// insert element at <br />beginning of list<br /> System.out.println("8: linkList.addFirst(new Integer(1000)) = " <br />+ linkList);<br /> linkList.offer(new Integer(1000)); // adds as tail element<br /> System.out.println("9: linkList.offer(new Integer(1000)) = " + <br />linkList);<br /> arrayList.addAll(linkList);// add to arraylist<br /> System.out.println("10: arrayList.addAll(linkList) = " + <br />arrayList);<br /> List subList = arrayList.subList(5, 11);// new list <br />from arraylist<br /> System.out.println("11: arrayList.containsAll(subList) = " + <br />arrayList.containsAll(subList));<br /> System.out.println("12: subList = " + subList);<br /> Collections.sort(subList); //sort in ascending order<br /> System.out.println("13: Collections.sort(subList) = " + subList);<br /> Collections.shuffle(subList); // random permutations<br /> System.out.println("14: Collections.shuffle(subList) = " + <br />subList);<br /> <br /> }<br /> <br />}<br />/* Output<br />1: arrayList = [0, 93, 92, 291, 324, 115, 546, 665, 688, 657]<br />2: arrayList.add(new Integer(500)) = [0, 93, 92, 291, 324, 115, 546, <br />665, 688, 657, 500]<br />3: arrayList.contains(new Integer(500)) = true<br />4: arrayList.remove(new Integer(92)) = true<br />5: arrayList.indexOf(291) = 2<br />6: linkList = [80, 180, 168, 352, 70, 594, 56, 640, 477, 380]<br />7: linkList.getFirst() = 80<br />8: linkList.addFirst(new Integer(1000)) = [1000, 80, 180, 168, 352, 70, <br />594, 56, 640, 477, 380]<br />9: linkList.offer(new Integer(1000)) = [1000, 80, 180, 168, 352, 70, <br />594, 56, 640, 477, 380, 1000]<br />10: arrayList.addAll(linkList) = [0, 93, 291, 324, 115, 546, 665, 688, <br />657, 500, 1000, 80, 180, 168, 352, 70, 594, 56, 640, 477, 380, 1000]<br />11: arrayList.containsAll(subList) = true<br />12: subList = [546, 665, 688, 657, 500, 1000]<br />13: Collections.sort(subList) = [500, 546, 657, 665, 688, 1000]<br />14: Collections.shuffle(subList) = [1000, 657, 665, 546, 500, 688]<br />*//</code></pre> <p>The example makes use of list containers configured to hold <code class="prettyprint">Integer</code> objects and numbers each line of output to aid simplification. It demonstrates only a few of the features available when using <code class="prettyprint">ArrayList</code> and <code class="prettyprint">LinkedList</code> objects. Note that both types of <code class="prettyprint">List</code>s store elements in the order in which they are inserted and both constitute a modifiable sequence i.e the quality of being automatically resizable. Also note the use of methods from the <code class="prettyprint"> Collections</code> class which are exclusively <code class="prettyprint">static</code> and which operate on or return collections. <b>Note:</b> The <code class="prettyprint">Collections</code> class should not be confused with the <code class="prettyprint">Collection</code> interface</p> <p>Among the other types of List include <code class="prettyprint">Stack</code> and <code class="prettyprint">Vector</code> objects. A <code class="prettyprint">Stack</code> is often described as a <i>"last-in-first-out"</i> (LIFO) container or a pushdown stack. Essentially this means the last element into the stack is the first element out. A <code class="prettyprint">LinkedList</code> has several methods that directly implement stack functionality and may provide a readily available option as opposed to implementing a <code class="prettyprint">Stack</code> class. <code class="prettyprint">Stack</code> and <code class="prettyprint">Vector</code> type lists are legacy classes that are rarely useful in new code.</p> <h2>Set Interface</h2> <p>A <code class="prettyprint">Set</code> has the same interface as a <code class="prettyprint">Collection</code>, but produces a different behaviour which does not allow for duplicate elements from among the set. One of the more common uses of a <code class="prettyprint">Set</code> is for membership lookup i.e. to test for the existence of an object within the set. A <code class="prettyprint">Set</code> determines membership based on the value of an object. <code class="prettyprint">Set</code> implementations include <code class="prettyprint">HashSet</code>, <code class="prettyprint">TreeSet</code> and <code class="prettyprint">LinkedHashSet</code>, all of which differ in how they store and process elements.<br /> For instance, <code class="prettyprint">HashSet</code> is known to use a <i>hashing</i> function to process elements, and while <code class="prettyprint">TreeSet</code> keeps elements sorted in a tree-like data structure, <code class="prettyprint">LinkedHashSet</code> appears to maintain elements in the order of insertion using a <code class="prettyprint">LinkedList</code> and also uses hashing for speed of access.<br /> Here is an example of a number of operations available to objects that implement the <code class="prettyprint">Set</code> interface:-<br /> <pre class="prettyprint"><code>import java.util.*;<br /> <br />public class SetOperations {<br /> public static void main(String[] args) {<br /> Random rand = new Random(30);<br /> Set<Integer> hashset = new HashSet<Integer>();<br /> for(int i = 0; i < 5000; i++)<br /> hashset.add(rand.nextInt(10));<br /> System.out.println("1: hashSet = " + hashset);<br /> Set<String> hashSet2 = new HashSet<String>();<br /> HashSet<String> linkHashSet = new LinkedHashSet<String>();<br /> Collections.addAll(linkHashSet, "A, E, I, O, U".split(" "));<br /> System.out.println("2: linkHashSet = " + linkHashSet);<br /> Collections.addAll(hashSet2, "A, B, C, D, E, F".split(" "));<br /> System.out.println("3: hashSet2 = " + hashSet2);<br /> System.out.println("4: linkHashSet.contains("X") = " + <br />linkHashSet.contains("X"));// membership lookup<br /> System.out.println("5: linkHashSet.containsAll(hashSet2) = " + <br />linkHashSet.containsAll(hashSet2));<br /> Collections.addAll(linkHashSet, hashSet2.toArray(new String[0]));<br /> System.out.println("6: Collections.addAll(linkHashSet, <br />hashSet2) = " + linkHashSet);<br /> System.out.println("7: hashSet2.remove("E") = " + <br />hashSet2.remove("E"));// Element not in Set<br /> Collections.addAll(linkHashSet, hashSet2.toArray(new <br />String[0]));//No duplicates<br /> System.out.println("8: Collections.addAll(linkHashSet, <br />hashSet2) = " + linkHashSet);<br /> hashSet2.removeAll(linkHashSet);<br /> System.out.println("9: hashSet2.removeAll(linkHashSet) - " + <br />hashSet2);<br /> String v = new String("V");<br /> linkHashSet.add(v);<br /> System.out.println("10: linkHashSet.add(v) = " + linkHashSet);<br /> System.out.println("11: linkHashSet.contains(v) = " + <br />linkHashSet.contains(v));<br /> }<br />}<br />/* Output<br />1: hashSet = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]<br />2: linkHashSet = [A, E, I, O, U]<br />3: hashSet2 = [A, B, C, D, E, F]<br />4: linkHashSet.contains("X") = false<br />5: linkHashSet.containsAll(hashSet2) = false<br />6: Collections.addAll(linkHashSet, hashSet2) = [A, E, I, O, U, A, B, C, <br />D, E, F]<br />7: hashSet2.remove("E") = false<br />8: Collections.addAll(linkHashSet, hashSet2) = [A, E, I, O, U, A, B, C, <br />D, E, F]<br />9: hashSet2.removeAll(linkHashSet) - []<br />10: linkHashSet.add(v) = [A, E, I, O, U, A, B, C, D, E, F, V]<br />11: linkHashSet.contains(v) = true<br />*//<br /> </code></pre> <p>In the example, we add five thousand random numbers from a range of 0 up to 9 into a <code class="prettyprint">Set</code> container. Of course you would expect quite a number of duplication in such a scenario, but duplication is not permitted within a <code class="prettyprint">Set</code> as can be gleaned from the output. The rest of the example deals with some other operations available to <code class="prettyprint">Set</code> objects. It is also worth noting that among the very many implementations of a <code class="prettyprint">Set</code>, a <code class="prettyprint">HashSet</code> provides no guarantee as to the order of the <code class="prettyprint">Set</code>, mostly because it uses hashing for speed. If however you were interested in producing an ordered <code class="prettyprint">Set</code>, you might consider making use of a <code class="prettyprint">TreeSet</code> container like so:-</p> <code class="prettyprint"> SortedSet<Integer> treeSet = new TreeSet<Integer>();</code></br></p> <h2>Queue Interface</h2> <p>A <code class="prettyprint">Queue</code> is a type of <code class="prettyprint">Collection</code> that typically takes a <i>"first-in-first-out"</i> (FIFO) approach to the processing of elements. This is to say elements are retrieved in the same order in which they are inserted with the exception of objects that implement the priority queue discipline - which is an alternative queuing technique that subscribes to highest priority or greatest need. </p> <code class="prettyprint">Queue</code> objects can be used as a reliable means to comfortably transfer objects from one area of your program to another and they also possess the peculiar feature of including multiple forms for methods that provide a common functionality but behave differently. Whereby one form throws an exception in the event of an operation failure, the other returns some special value given similar circumstances. A <code class="prettyprint">LinkedList</code> object implements the <code class="prettyprint">Queue</code> interface and consequently has access to methods that support queue behaviour:-<br /> <pre class="prettyprint"><code>import java.util.*;<br /> <br />public class QueueOperations {<br /> public static void main(String[] args) {<br /> Queue<Character> qe = new LinkedList<Character>();<br /> for(char c : "Encyclopedia".toCharArray())<br /> qe.offer(c);<br /> System.out.println("Queue.offer() - "+ qe);<br /> qe.poll();<br /> System.out.println("Queue.poll() - "+ qe);<br /> System.out.println("queue.peek() " + qe.peek());<br /> qe.add(new Character('E'));<br /> System.out.println("queue.add() " + qe);<br /> }<br />/* Output<br />fer() - [E, n, c, y, c, l, o, p, e, d, i, a]<br />Queue.poll() - [n, c, y, c, l, o, p, e, d, i, a]<br />queue.peek() n<br />queue.add() [n, c, y, c, l, o, p, e, d, i, a, E]<br />*//</code></pre><br /> The example demonstrates some of the six or so methods that currently make up the <code class="prettyprint">Queue</code> interface. The <b>offer()</b> method inserts the specified element into the queue if possible and returns <code class="prettyprint">false</code> in the event of a failure. The <b>add()</b> method inherited from the <code class="prettyprint">Collection</code> interface performs the same operation which it can only fail by throwing an exception, as a result, the <b>offer()</b> method is normally the preferred option. The <b>poll()</b> method retrieves and removes the head of the queue, or returns <code class="prettyprint">null</code> if the queue is empty. The <b>remove()</b> method performs the same operation but throws an exception if the queue is empty. The <b>peek()</b> and <b>element()</b> methods perform the same operation but differ in much the same way as previously described. </p> <h1>Map Interface</h1> <p>A <code class="prettyprint">Map</code> is quite unlike those subtypes of <code class="prettyprint">Collection</code> with which we have by now become familiar, in fact it is a very distinct data structure. The <code class="prettyprint">Map</code> interface describes methods that operate on key/value pairs, providing you with an ability to map objects to other objects where each key maps to at most a single value, and for which duplicate keys are forbidden. <code class="prettyprint">Map</code> implementation may include objects of <code class="prettyprint">HashMap</code>, <code class="prettyprint">TreeMap</code> and <code class="prettyprint">LinkedHashMap</code>, and whose behaviour and performances are not very much different to those described within the <code class="prettyprint">Set</code> interface subheading. </p> <p>Some of the methods available to <code class="prettyprint">Map</code> objects include those for performing basic operations like <b>put()</b>, <b>get()</b>, <b>remove()</b>, <b>size()</b>, <b>containsKey()</b>, <b>containsValue()</b>, et cetera as well as other methods that perform bulk operations and provide collection views. Here is an example of how one may implement a <b>Map</b> object:-<br /> <pre class="prettyprint"><code>import java.util.*;<br /> <br />public class MapOperations {<br /> public static void main(String[] args) {<br /> Map<String, String> myPets = new HashMap<String, String>();<br /> myPets.put("new Dog ", " woof woof");<br /> myPets.put("new Cat ", " meeow meeow");<br /> myPets.put("new bunnyRabbit ", " honk honk");<br /> System.out.println("myPets: " + myPets);<br /> System.out.println("myPets.containsKey("new cat") = " + <br />myPets.containsKey("new Cat "));<br /> System.out.println("myPets.containsValue("honk honk") = " + <br />myPets.containsValue(" honk honk"));<br /> }<br /> <br />}<br />/* Output<br />myPets: {new Cat = meeow meeow, new Dog = woof woof, new bunnyRabbit = <br />honk honk}<br />myPets.containsKey("new cat") = true<br />myPets.containsValue("honk honk") = true<br />*//</code></pre> <p>A <code class="prettyprint">Map</code> provides a very powerful abstraction that simplify very many programming problems. The above example uses <b>String</b> objects to look up other <b>String</b> objects, but we might conceive of a slightly more complex program that tracks the pseudo-random nature of Java's <code class="prettyprint">Random</code> class, where the number produced by the <code class="prettyprint">Random</code> is assigned as the key and the number of times that number is produced is taken to be the value. It would be necessary to generate very many random numbers so that we may count those that fall into various categories:-<br /> <pre class="prettyprint"><code>import java.util.*;<br /> <br />public class MapOperations2 {<br /> public static void main(String[] args) {<br /> Map<Integer, Integer> lm = new LinkedHashMap<Integer, Integer>();<br /> Random rand = new Random(25);<br /> for (int i = 0; i < 2000; i++) {<br /> int x = rand.nextInt(10);<br /> Integer freq = lm.get(x);<br /> lm.put(x, freq == null ? 1 : freq + 1);<br /> }<br /> System.out.println(lm);<br /> }<br />}<br />/* Output<br />{1=200, 8=218, 7=194, 5=195, 6=198, 4=188, 0=198, 9=191, 2=220, 3=198}<br />*//</code></pre><br /> A ternary <code class="prettyprint"> if/else</code> operator is used to test and increment the frequency a particular number has been randomly generated. <code class="prettyprint">Map</code>s can also be expanded to include multiple dimensions by including other maps whose values also happen to be other containers or even further made up of other maps. A <code class="prettyprint">Map</code> may return a <code class="prettyprint">Collection</code> of its values, a <code class="prettyprint">Set</code> of its keys or a <code class="prettyprint">Set</code> of its pairs. </p> <h1>Iterator design pattern</h1> <p>An <code class="prettyprint">Iterator</code> object provides a way to access elements in a sequence without exposing its underlying implementation. It comes under the category of a behavioural design pattern and any discourse on Java's containers is incomplete without affording it adequate attention, since you may have perceived before now that it is possible for one to return an <code class="prettyprint">Iterator</code> object from a similarly named operation within a <code class="prettyprint">Collection</code> or any of it's subtypes.</p> <p>An <code class="prettyprint">Iterator</code> object provides you with a number of very useful methods that rest easily beside the concept of a container, like for instance the <b>next()</b> method which is a call to get the next element in the sequence, the <b>hasNext()</b> method which returns either of <code class="prettyprint">true</code> or <code class="prettyprint">false</code> according to whether the sequence remains empty or not, and of course, the <b>remove()</b> method which removes the last element returned by the iterator:-<br /> <pre class="prettyprint"><code>import java.util.*;<br /> <br />public class Iteration {<br /> public static void main(String[] args) {<br /> List<Integer> arraylist = new ArrayList<Integer>();<br /> Random rand = new Random(27);<br /> for(int i = 0; i < 8; i++)<br /> arraylist.add((Integer)rand.nextInt(20));<br /> System.out.println("arraylist: " + arraylist);<br /> Iterator al = arraylist.iterator();<br /> while(al.hasNext()) {<br /> Integer g = al.next();<br /> System.out.println("The double value for " + g + " is " + <br />g.doubleValue());<br /> }<br /> al.remove();<br /> System.out.println("arraylist: " + arraylist);<br /> }<br /> }<br />/* Output<br />arraylist: [10, 12, 17, 18, 11, 16, 10, 14]<br />The double value for 10 is 10.0<br />The double value for 12 is 12.0<br />The double value for 17 is 17.0<br />The double value for 18 is 18.0<br />The double value for 11 is 11.0<br />The double value for 16 is 16.0<br />The double value for 10 is 10.0<br />The double value for 14 is 14.0<br />arraylist: [10, 12, 17, 18, 11, 16, 10]<br />*//</code></pre><br /> In the above example, the <code class="prettyprint">Iterator</code> returned from the container object is used to perform operations on objects within the container in what is a very powerful programming ideology. The <code class="prettyprint">Iterator</code> object <b>al</b> allows us to step through the container sequentially, while printing each individual element to screen. At the close of the loop, the object's <b>remove()</b> method is used to remove the last element returned by this Iterator.</p> <p>Interesting as all this may sound, one may not readily appreciate the power of the iterator without a practical demonstration of how one may write code to iterate over a sequence while ignoring the underlying implementation of the container. Here is a peculiar <b>view()</b> method that does just that:-<br /> <pre class="prettyprint"><code>import java.util.*; <br /> <br />public class Iteration2 {<br /> static void view(Iterator<Character> it) {<br /> while(it.hasNext()) {<br /> Character p = it.next();<br /> System.out.print("[ " + p + " " + " ]");<br /> }<br /> System.out.println();<br /> }<br /> public static void main(String[] args) {<br /> char[] cc = {'d', 'e', 'c', 'x', 'd'};<br /> Random rand = new Random();<br /> ArrayList<Character> al = new ArrayList<Character>();<br /> LinkedList<Character> Ll = new LinkedList<Character>();<br /> HashSet<Character> hs = new HashSet<Character>();<br /> for(int i = 0; i < cc.length; i++) {<br /> al.add(cc[rand.nextInt(cc.length)]);<br /> Ll.add(cc[rand.nextInt(cc.length)]);<br /> hs.add(cc[rand.nextInt(cc.length)]);<br /> }<br /> view(al.iterator());<br /> view(Ll.iterator());<br /> view(hs.iterator());<br /> }<br /> <br />}<br />/* Output<br />[ d ][ d ][ c ][ d ][ c ]<br />[ e ][ c ][ e ][ e ][ d ]<br />[ d ][ x ]<br />*//</code></pre><br /> The <b>view()</b> method in this example has no knowledge of the underlying container or the type of sequence it is traversing, be it an <code class="prettyprint">ArrayList</code>, <code class="prettyprint">LinkedList</code> or <code class="prettyprint">HashSet</code>. The Iterator design pattern provides you with the ability to separate the operation of traversing a sequence from the underlying structure or implementation of the said sequence.</p> <p>You may have noticed that unlike arrays, you can easily print the elements within a container without the need of having to rely on some other method to provide some String representation. Also unlike arrays, containers do not work with primitives, but rely on wrapper classes to perform the necessary conversion over the course of the program.</p> <p>Containers can take some getting use to at first, but with constant practice you will find them very useful and even indispensable in the way they simplify your code and make your programs that much more powerful.</p> http://www.java.net/blog/hellofadude/archive/2014/02/12/overview-javas-container-classes#comments Blogs Global Education and Learning J2SE Open JDK Wed, 12 Feb 2014 18:41:02 +0000 hellofadude 901158 at http://www.java.net Java interfaces and the concept of multiple inheritance http://www.java.net/blog/hellofadude/archive/2014/01/18/java-interfaces-and-concept-multiple-inheritance <!-- | 0 --><p>Interfaces are completely abstract classes in Java that provide you with a uniform way to properly delineate the structure or inner workings of your program from its publicly available interface, with the consequence being a greater amount of flexibility and reusable code as well as more control over how you create and interact with other classes. More precisely, they are a special construct in Java with the additional characteristic that allow you to perform a kind of multiple inheritance i.e. classes that can be upcast to more than one <code class="prettyprint">class</code>; a technique from which you are severely restricted from undertaking (and with good reason) when working exclusively with classes.</p> <p>To create an interface in Java, you simply use the special <code class="prettyprint">interface</code> keyword like this:-</p> <code class="prettyprint">interface Transaction { ... }</code></br></p> <p>You may include the <code class="prettyprint">public</code> access identifier to the left of the <code class="prettyprint">interface</code> keyword if the interface is defined in a file of the same name. Without the <code class="prettyprint">public</code> keyword, the interface defaults to package access.<br /> Interfaces have no implementation and therefore relieve you of any concerns pertaining to storage; this is to say your interface can only be implemented in a concrete class. When you create an interface, you may determine method names, arguments types as well as return types, but your methods will have no body because that is left to the imagination of those classes that implement a particular interface. A <code class="prettyprint">class</code> is said to conform to an interface when it uses the special <code class="prettyprint">implements</code> keyword in relation to a specific interface:-</p> <code class="prettyprint">class Account implements Transaction { ... }</code></br></p> <p>Once you implement an interface in a class, it becomes usable as you would any other class. The methods you declare in an interface must be implemented in your inheriting classes and must be declared <code class="prettyprint">public</code>. Interfaces may also include fields which are implicitly <code class="prettyprint">static</code> and <code class="prettyprint">final</code>. For instance, you could create different types of bank accounts to conform to a <b>Transaction</b> interface in the following way:-<br /> <pre class="prettyprint"><code>package kingsley.java<br /><br />interface Transaction {<br /> int BALANCE = 500;<br /> Object transaction(Object input);<br />}<br />class CurrentAccount implements Transaction {<br /> int bal;<br /> public Object transaction(Object input) {<br /> this.bal = BALANCE - (int)input;<br /> return bal;<br /> }<br /> public String toString() { return "Current acc"; }<br />}<br />class SavingsAccount implements Transaction {<br /> int bal;<br /> public Object transaction(Object input) {<br /> this.bal = BALANCE + (int)input;<br /> return bal;<br /> }<br /> public String toString() { return "Savings acc"; }<br />}<br />public class Account {<br /> public static void payment(Transaction t, Object input) {<br /> System.out.println(t + " is debited: " + t.transaction(input));<br /> }<br /> public static void deposit(Transaction t, Object input) {<br /> System.out.println(t + " is credited: " + t.transaction(input));<br /> }<br /> public static void main(String[] args) {<br /> Integer input = new Integer(600);<br /> deposit(new SavingsAccount(), input);<br /> payment(new CurrentAccount(), input);<br /> }<br />/* Output<br />Savings acc is debited: 1100<br />Current acc is credited: -100<br />*//</code></pre><br /> Observe how both implementing classes <code class="prettyprint">CurrentAccount</code> and <code class="prettyprint">SavingsAccount</code> are automatically upcast in methods within the <code class="prettyprint">Account</code> class that accept a <code class="prettyprint">Transaction</code> interface. The <b>payment()</b> and <b>deposit()</b> methods utilise what we call a <i>Strategy design</i> pattern and represent an instance of the complete decoupling of interface from implementation. Strategy design pattern allows method to vary independently from the clients that use it. Theoretically, you can adapt any class to use these methods by simply making them conform to the <code class="prettyprint">Transaction</code> interface. Interfaces give you a considerable amount of flexibility, and release you from the sort of constraints inherent in using particular data types and their subclasses. Note the <b>BALANCE</b> field in the <code class="prettyprint">Transaction</code> interface which is a constant and is implicitly <code class="prettyprint">static</code> and <code class="prettyprint">final</code>. Finally, note that methods declared in interfaces have no body.</p> <p>One way to understand the utility of interfaces is to imagine how you might be able to utilise methods that accept interfaces within your own classes or how they might make it possible for others to make use of your own methods without having access to specific knowledge of its implementation, effectively keeping that part hidden. For example, imagine that a part of a program that allowed you to track the business activities of a property management company looked something like this:-<br /> <pre class="prettyprint"><code>package kingsley.java<br /><br />public class Activity {<br /> String name;<br /> public Property transact(Property input) { return input; }<br /> public String getName() { return name; }<br /> public static void main(String[] args) {<br /> System.out.println(new Sale(2).transact(new Property("1 Easy Street")));<br /> System.out.println(new Purchase(8).transact(new Property("5 Main Street")));<br /> }<br />}<br />class Property {<br /> private static long counter;<br /> private long id = counter++;<br /> private String address;<br /> public Property(String address) { this.address = address; }<br /> public String toString() { return "Property " + id + " at " + address; }<br />}<br />class Sale extends Activity {<br /> int id;<br /> String name = "Sale acc";<br /> public Sale(int i) { this.id = i; }<br /> public Property transact(Property input) {<br /> return input; // Dummy transaction<br /> }<br /> public String getName() { return name; }<br />}<br />class Purchase extends Activity {<br /> int id;<br /> String name = "Purchase acc";<br /> public Purchase(int i) { this.id = i; }<br /> public Property transact(Property input) { return input; }<br /> public String getName() { return name; }<br />}<br />/* Output<br />Property 0 at 1 Easy Street<br />Property 1 at 5 Main Street<br />*//</code></pre><br /> The purpose of this example is to demonstrate one of the ways by which one may reuse the previous <code class="prettyprint">Transaction</code> interface within a class that was perhaps written by a completely different individual. The <code class="prettyprint">Activity</code> class represents certain business transactions that would be relevant to some property development company. On closer examination, you might notice that the <code class="prettyprint">Activity</code> class contains some of the same elements as our earlier <code class="prettyprint">Transaction</code> interface. This gives us some sign that it might be possible to make use of the <code class="prettyprint">Transaction</code> interface to undertake some of those transactions defined as part of the public interface of the <code class="prettyprint">Account</code> class and which might be relevant to activities implemented in the <code class="prettyprint">Activity</code> class.</p> <p>One way to do this, would be to make use of the <i>Adapter design</i> pattern to create an adapter that would allow us to quickly add value to our program. An implementation of this idea would look something like this:-<br /> <pre class="prettyprint"><code>package kingsley.java<br /><br />public class Activity { <br /> String name;<br /> public Property transact(Property input) {<br /> return input;<br /> }<br /> public String getName() {<br /> return name;<br /> }<br /> public static void main(String[] args) {<br /> Account.deposit(new ActivityAdapter(new Sale(3)), new Property("1 Easy Street"));<br /> Account.payment(new ActivityAdapter(new Purchase(6)), new Property("5 Main Street"));<br /> }<br />}<br />class ActivityAdapter implements Transaction {<br /> Activity activity;<br /> public ActivityAdapter(Activity activity) {<br /> this.activity = activity;<br /> }<br /> public Property transaction(Object input) {<br /> return activity.transact((Property)input);<br /> }<br /> public String toString() {<br /> return activity.getName();<br /> }<br />}<br />/* Output<br />Sale acc is credited Property 0 at 1 Easy Street<br />Purchase acc is debited Property 1 at 5 Main Street<br />*//</code></pre> <p>The result has a more intuitive feel to it. Notice how the <code class="prettyprint">ActivityAdapter</code> class takes an <b>Activity</b> object in the constructor and produces an object that has the <b>Transaction</b> interface as a part of it. This is the a classic example of the Adapter design pattern without which it would not be possible for <code class="prettyprint">Activity</code> and <code class="prettyprint">Account</code> classes to work together. Adapter makes it possible for classes with incompatible interfaces to work together</p> <h1>Multiple Inheritance</h1> <p>When you work with classes, you are limited to inheriting from only one base class. Interfaces relax this constraint somewhat by allowing you undertake a kind of multiple inheritance by combining multiple interfaces within a class. To do this, you simply place interface names in sequence following the <code class="prettyprint">implements</code> keyword separated by commas:-<br /> <pre class="prettyprint"><code>package kingsley.java<br /><br />interface Forward { void drive(); }<br />interface Stop { void park(); }<br />interface Speed {<br /> void turbo();<br />}<br />class GearBox { public void move() { } }<br />class Automatic extends GearBox implements Forward, Stop, Speed {<br /> public void drive() { System.out.println("drive()"); }<br /> public void park() { System.out.println("park()");}<br /> public void turbo() { System.out.println("turbo()"); }<br /> public void move() { System.out.println("move()"); }<br />}<br />public class Car {<br /> public static void cruise(Forward x) { x.drive(); }<br /> public static void park(Stop x) { x.park(); }<br /> public static void race(Speed x) { x.turbo(); }<br /> public static void move(GearBox x) { x.move(); }<br /> public static void main(String[] args) {<br /> Automatic auto = new Automatic();<br /> cruise(auto); // Interface Forward<br /> park(auto); // Interface Stop<br /> race(auto); // Interface Speed<br /> move(auto); // class GearBox<br /> }<br />}<br />/* Output<br />drive()<br />park()<br />turbo()<br />move()<br />*//</code></pre><br /> In the preceding example, note the way by which the <b>Automatic</b> class implements multiple interfaces but inherits from only one class. Java will not allow you to inherit from more than one class, but you may implement as many interfaces as you need. The value in this example is in how an object of the <b>Automatic</b> class can be upcast to multiple types, as demonstrated in the <b>main()</b>, and hence my earlier reference to the concept of multiple inheritance.</p> <p>It is also possible to extend interfaces with inheritance in the same way you do classes by including new fields and methods. Unlike classes though, you can combine several interfaces into a new interface with inheritance. The result is a new interface with much of the same characteristics as those of its base interfaces.<br /> <pre class="prettyprint"><code>package kingsley.java <br /><br />interface SuperHero {<br /> void powers();<br />}<br />interface Alien {<br /> void planet();<br />}<br />interface SuperMan extends Alien, SuperHero {<br /> void xRayVision();<br /> <br />}<br />interface Human {<br /> void normal();<br />}<br />interface Monster extends Alien {<br /> void killHuman();<br />}<br />class Skrull implements Monster {<br /> void shapeShift() {}<br /> public void killHuman() {}<br /> public void planet() {}<br />}<br />class ClarkKent implements SuperMan, Human {<br /> public void normal() {}<br /> public void planet() {}<br /> public void xRayVision() {}<br /> public void powers() {}<br /> <br />}<br />class Hulk implements Human, SuperHero {<br /> void thunderClap() {};<br /> public void normal() {}<br /> public void powers() {}<br />}<br />public class Earth {<br /> static Alien invasion(Monster mt) {<br /> return mt;<br /> }<br /> static SuperMan change(ClarkKent ck) {<br /> return ck;<br /> }<br /> static void battle(SuperHero sh, Monster m) {<br /> sh.powers();<br /> m.planet();<br /> m.killHuman();<br /> }<br /> public static void main(String[] args) {<br /> Skrull ogre = new Skrull();<br /> invasion(ogre);<br /> ClarkKent christopherReeve = new ClarkKent();<br /> SuperMan superMan = change(christopherReeve);<br /> battle(superMan, ogre);<br /> battle(new Hulk(), ogre);<br /> }<br />}</code></pre><br /> One may also choose to nest interfaces within classes and other interfaces with very interesting results.<br /> <pre class="prettyprint"><code>package kingsley.java<br /><br />class OuterClass {<br /> interface Inner1 {<br /> void d();<br /> interface Inner2 { void d1(); }<br /> }<br /> public class InnerClass1 implements Inner1.Inner2 {<br /> public void d1() { }<br /> }<br /> public interface Inner3 {<br /> void d();<br /> }<br /> private interface Inner4 {<br /> void d();<br /> }<br /> private class InnerClass2 implements Inner4 {<br /> public void d() {}<br /> }<br /> public class InnerClass3 implements Inner4 {<br /> public void d() {}<br /> }<br /> public Inner4 getInner4() { return new InnerClass3(); }<br /> private Inner4 i4;<br /> public void getInner4(Inner4 in4) {<br /> i4 = in4;<br /> i4.d();<br /> }<br />}<br />interface Outer1 {<br /> void x();<br /> interface Outer2 {<br /> void d();<br /> }<br /> // Invalid identifier. Can only be public<br /> // within interface<br /> //! private interface Outer3 { }<br />}<br />public class NestingInterfaces {<br /> public class Class1 implements OuterClass.Inner1.Inner2 {<br /> public void d1() { }<br /> }<br /> class Class2 implements OuterClass.Inner3 {<br /> public void d() {}<br /> }<br /> // private interface not visible<br /> // from outside implementing class<br /> //! class Class3 implements OuterClass.Inner4 {<br /> //! public void d() { }<br /> //! }<br /> class Class3 implements Outer1 {<br /> public void x() {}<br /> class Class4 implements Outer1.Outer2 {<br /> public void d() {}<br /> }<br /> }<br /> public static void main(String[] args) {<br /> OuterClass oc = new OuterClass();<br /> // Error can't access private interface<br /> //! OuterClass.Inner4 = oc.getInner4();<br /> // Error can't access private interface members<br /> //! oc.getInner4().d();<br /> // To get to a nested private interface<br /> // use another OuterClass<br /> OuterClass oc2 = new OuterClass();<br /> oc2.getInner4(oc.getInner4());<br /> }<br />}</code></pre> <p>The example demonstrates some of the features available when working with nested interfaces. As you may observe for instance, it is possible to nest interfaces within interfaces which may have public or package access visibility same as non-nested interfaces do. Calling nested interfaces would then be a simple matter of placing a dot next to the surrounding class or interface, before making a call to the nested interface. Matters get a little complicated however with private interfaces, which provide a way to force the definition of methods without adding the benefit of type. In the main(), accessing private nested interface is not quite so straightforward, as attempts to use a returned reference to the private interface are unsuccessful. This is because in the first instance we are not able to access the private interface directly and secondly we can not access a member of that interface.<br /> We however can access the interface by handing the return value to an object that has permission to use it, in this case another OuterClass object.</p> <p>It is important to note that in implementing an interface, one is not required to implement any interfaces nested within and private interfaces can not be implemented outside of their defining classes. </p> <h1>Factory Methods </h1> <p>Interfaces have the added benefit of allowing you to make multiple implementations using the <i>Factory Method</i> design pattern. The Factory pattern allows you to define an interface for creating an object, but defer instantiation to implementing classes, making it so that your code is completely isolated from the interface implementation. This method is useful when creating a framework to be used by multiple types. Here is an example that uses a ticket factory to create the various types of tickets people might be interested to buy:-<br /> <pre class="prettyprint"><code>package kingsley.java;<br /> <br />interface Ticket {<br /> String type();<br /> int price();<br /> }<br />interface TicketFactory { Ticket getTicket(); }<br /> <br />class AdultTicket implements Ticket {<br /> int PRICE = 500;<br /> public String type() { return "Adult Ticket"; }<br /> public int price() {return PRICE;}<br />}<br />class AdultTicketFactory implements TicketFactory {<br /> public Ticket getTicket() { return new AdultTicket(); }<br />}<br />class ChildTicket implements Ticket {<br /> int PRICE = 40;<br /> public String type() { return "Child Ticket"; }<br /> public int price() { return PRICE; }<br />}<br />class ChildTicketFactory implements TicketFactory {<br /> public Ticket getTicket() { return new ChildTicket(); }<br />}<br />public class Tickets { <br /> public static void buyTicket(TicketFactory tf) {<br /> Ticket t = tf.getTicket();<br /> System.out.println("Paid - " + "1 " + t.type() + " £" + <br />t.price() + " - Thank you");<br /> }<br /> public static void main(String[] args) {<br /> buyTicket(new AdultTicketFactory());<br /> buyTicket(new ChildTicketFactory());<br /> }<br />}<br />/* Output<br />Paid - 1 Adult Ticket £500 - Thank you<br />Paid - 1 Child Ticket £40 - Thank you<br />*//</code></pre><br /> You can see how the Factory method allows you to easily swap implementation without reference to types. Essentially, you can apply this pattern to any other type that implements the required interface without having to worry about how you might call multiple constructors which would otherwise be the case, absent Factory. The Factory method represents an excellent way to write reusable code and is definitely worth the extra effort.</p> <p>Interfaces are a very useful tool that provide the opportunity to upcast to more than one base type, they can also be used to more effectively hide your implementation but they are not always necessary and sometimes an abstract class would work just as well relieving you of the additional complexity of interfaces. </p> <h1>Abstract classes</h1> <p>Abstract classes are a kind of mid way point between concrete classes and interfaces with the idea being to provide a common interface by which to manipulate a set of classes, this is to say they provide a basic form that establishes the commonality among classes. To create an abstract class, you simply include the abstract keyword like this:-</p> <code class="prettyprint">abstract class SuperHero { ... }</code> <p>A class that contains one or more abstract methods must be qualified as an abstract class and making a class abstract does not compel you to make all the methods abstract. An abstract method is an incomplete method, having only a declaration and no body.<br /> It is not possible to make an object of an abstract class, however other classes that inherit from an abstract class must provide method definitions for all the methods in the abstract base class. Likewise one may make an abstract class that does not include any abstract methods particularly in instances when you want to prevent instantiation of that class. Check out the following example:-</p> <pre class="prettyprint"><code>package kingsley.java;<br /> <br />abstract class SuperHero {<br /> void name() {}<br /> abstract void power();<br />}<br />class SuperMan extends SuperHero {<br /> public void power() {<br /> System.out.println("SuperMan(Heat Vision) ");<br /> }<br /> public void name() { System.out.println("Clark Kent"); }<br />}<br />class Hulk extends SuperHero {<br /> public void power() {<br /> System.out.println("Hulk(Thunderclap) ");<br /> }<br /> public void name() { System.out.println("Bruce Banner"); }<br />}<br />class SpiderMan extends SuperHero {<br /> public void power() {<br /> System.out.println("SpiderMan(Spider sense) ");<br /> }<br /> public void name() { System.out.println("Peter Parker"); }<br />}<br />class BatMan extends SuperHero {<br /> public void power() {<br /> System.out.println("BatMan(none) ");<br /> }<br /> public void skill() { System.out.println("BatMan(Law enforcement <br />techniques) "); }<br /> public void name() { System.out.println("Bruce Wayne"); }<br />}<br />public class Heroes {<br /> static void change(SuperHero sp) {<br /> sp.name();<br /> sp.power();<br /> }<br /> static void changeAll(SuperHero[] sh) {<br /> for(SuperHero sp : sh) {<br /> change(sp);<br /> }<br /> }<br /> public static void main(String[] args) {<br /> SuperHero[] heroes = {<br /> new SuperMan(),<br /> new Hulk(),<br /> new SpiderMan(),<br /> new BatMan()<br /> };<br /> changeAll(heroes);<br /> new BatMan().skill();<br /> }<br />}<br />/* Output<br />Clark Kent<br />SuperMan(Heat Vision)<br />Bruce Banner<br />Hulk(Thunderclap)<br />Peter Parker<br />SpiderMan(Spider sense)<br />Bruce Wayne<br />BatMan(none)<br />BatMan(Law enforcement techniques)<br />*//</code></pre><br /> And there you have it. The <code class="prettyprint">SuperHero</code> class is an abstract class whose methods are only partially implemented and not all of which are declared abstract. Further notice how the derived <code class="prettyprint">BatMan</code> class extends the interface by adding a new method, which does not effect the operation of other derived classes.</p> <p>A good way to remember the distinction between abstract classes and interfaces is to think of abstract classes as providing you with a common interface by which one may manipulate several classes, while interfaces on the other hand allow you to manipulate a class in more than one way.</p> http://www.java.net/blog/hellofadude/archive/2014/01/18/java-interfaces-and-concept-multiple-inheritance#comments Blogs Global Education and Learning J2SE Open JDK Sat, 18 Jan 2014 23:07:28 +0000 hellofadude 900862 at http://www.java.net Polymorphism in Java http://www.java.net/blog/hellofadude/archive/2014/01/07/polymorphism-java <!-- | 0 --><p> In object oriented programming, polymorphism is a feature that allows you to provide a single interface to varying entities of the same type. This is analogous to the interpretation of the same concept in the field of Biology.</p> <p>To understand how this works in Java, we must consider inheritance and the ways by which the Java programming language makes method calls.</p> <p>When you create a class in Java, you implicitly inherit the fields and methods of the root <b>Object</b> class even when you do not explicitly inherit from some other <code class="prettyprint">class</code>. This ability allows you to substitute an object for its base class and is proof of the saying - <i>"in Java everything is an object"</i>.<br /> To demonstrate, we consider the simple example of <b>Shape</b> objects:-<br /> <pre class="prettyprint"><code> class Shape { <br /> public String toString() {<br /> return " a Shape";<br /> }<br /> } <br /> class Triangle extends Shape { <br /> public String toString() {<br /> return "a Triangle";<br /> } <br /> }<br /> public class DrawShape { <br /> /**<br /> * @param args<br /> */<br /> static void draw(Shape s) {<br /> System.out.println("Drawing " + s);<br /> }<br /> public static void main(String[] args) {<br /> Triangle t = new Triangle();<br /> draw(t);<br /> // TODO Auto-generated method stub<br /> } <br /> }<br /><br /> /* Output<br /> Drawing a Triangle<br /> *//</code></pre><br /> What is interesting about this example is how in the <b>main()</b>, we pass a <b>Triangle</b> reference to the <b>draw()</b> method that expects a <b>Shape</b> reference. What is even more interesting is how the compiler accepts this without complaint and Java is able to make the correct method call at run time and without the need for upcasting. This is essentially how polymorphism works. Because the <b>draw()</b> is a dynamically bound method (as are all normal methods in Java), a programmer is able to ignore types and write methods that takes a base class as argument confident in the knowledge that such methods can also be applied to derived classes.<br /> Here is a slightly modified version of the example to better elucidate my point:-<br /> <pre class="prettyprint"><code> class Shape { <br /> public void draw() {<br /> System.out.println("Drawing a Shape");<br /> }<br /> } <br /> class Triangle extends Shape { <br /> public void draw() {<br /> System.out.println("Drawing a Triangle");<br /> }<br /> }<br /> public class DrawShape2 { <br /> /**<br /> * @param args<br /> */<br /> public static void main(String[] args) {<br /> Shape s = new Triangle();<br /> s.draw();<br /> // TODO Auto-generated method stub<br /> } <br /> }<br /><br /> /* Output<br /> Drawing a Triangle<br /> *//<br /> </code></pre><br /> In this example, we assign a <b>Triangle</b> object to a <b>Shape</b> reference. A call to the overloaded <b>draw()</b> method produces the correct behaviour, when it might be reasonable to expect that the <b>draw()</b> method in the <b>Shape</b> object might be bound to the call; afterall, we are speaking of a <b>Shape</b> reference. However on closer inspection, the output makes sense because a <b>Triangle</b> object is a kind of <b>Shape</b>, a fact concretised by the use of the <code class="prettyprint">extends</code> keyword that denotes the use of inheritance.<br /> You might wonder how it is that this is possible? I mean how does Java know what method to call in order to produce the correct behavior at run time? This is essentially what polymorphism is designed to achieve.</p> <p>To get a better understanding of how it works we might consider the concept of <i>binding</i> which refers to the process of connecting a method call to a method body.<br /> In programming, <i>early binding</i> references the point when the binding process is performed before the program is run. This is typically done by the compiler and is synonymous with some procedural languages. In the above example, it is clear that the compiler can not know which method to call when it only has a <b>Shape</b> reference from which we have derived at least one other type.</p> <p>To overcome this, Java uses the concept of <i>late binding</i> or <i>dynamic binding</i>, which allows binding to occur at run time. The term dynamic binding is often used to refer to polymorphism. When we say all method calls in Java happen polymorphically, we may correctly imagine the implementation of some mechanism that allows the correct object type to be determined at run time. The mechanism of this invention however, is not of itself of any interest to us, but the ways by which it might be perceived.</p> <h1>Polymorphism and constructors.</h1> <p>Constructors in Java are not known to be polymorphic and there is disagreement as to whether they constitute <code class="prettyprint">static</code> methods or not. Be that as it may, it has been suggested in other well regarded text that constructors are indeed implicitly <code class="prettyprint">static</code> and we are happy to retain this view in the absence of anything to suggest the contrary. In the previous example, we demonstrated how polymorphism allows us to substitute a simple <b>Triangle</b> object for its base class. In this section we aim to explain how polymorphism effects a more complex type and the order of construction of such objects. The example below gives an idea of what a complex class looks like:-<br /> <pre class="prettyprint"><code> class Books {<br /> Books() {<br /> System.out.println("Books");<br /> }<br /> class Computers {<br /> Computers() {<br /> System.out.println("Computers");<br /> }<br /> }<br /> class Address {<br /> Address() {<br /> System.out.println("Address");<br /> }<br /> }<br /> class Building extends Address {<br /> Building() {<br /> System.out.println("Building");<br /> }<br /> }<br /> public class Library extends Building { <br /> /**<br /> * @param args<br /> */<br /> private Books bs = new Books();<br /> private Computers cp = new Computers();<br /> public Library() {<br /> System.out.println("Library");<br /> }<br /> public static void main(String[] args) {<br /> // TODO Auto-generated method stub<br /> new Library();<br /> }<br /> }<br /> /* Output<br /> Address<br /> Building<br /> Books<br /> Computers<br /> Library<br /> *//</code></pre><br /> The example demonstrates the effect of polymorphism on the order of constructor calls in Java. The <b>Library</b> class is an example of a more complex type reflecting at least two levels of inheritance as well as being composed of a number of other objects. Notice how a call to the <b>Library()</b> constructor results in a call to its base classes in a sort of recursive manner as can be evidence by the output. The logic of this is inherent in the fact that for a derived class to be properly constructed, the base classes from which it is derived must first be properly initialised. In other words, an inherited class must know all about its base class or classes and be able to access all of its <code class="prettyprint">public</code> and <code class="prettyprint">protected</code> elements.<br /> What we are trying to say is when you have a complex class that makes use of composition and inheritance, the order of constructor calls begins by first calling the base class constructor at the root of the hierarchy, followed by other constructors within the hierarchy in a sort of recursive and sequential manner.</p> <p>It is only after this process has been completed that individual members within the class are initialised in the order in which they are written, after which the derived class constructor may then be called. There is sufficient evidence of this in the above output.</p> <p>As mentioned previously, Java applies late binding to all normal method calls. This means that all methods with the exception of constructors are polymorphic by default. It is important to remember that this has an impact when you try to call a dynamically bound method from within a class constructor. In these circumstances, the overridden method may be called before the object is fully constructed making it more likely that you might end up using an unintialised variable, which might result in hard to find bugs in your program.</p> <p>The best way to avoid such a scenario is to do as little as possible within your constructors to get the object into a proper state. However if you must call a dynamically bound method within your constructor, the only safe methods to call are those declared <code class="prettyprint">final</code> and <code class="prettyprint">private</code> which cannot be overridden. </p> <p>When you declare a method <code class="prettyprint">final</code> you effectively place a lock on it and prevent inheriting classes from changing its meaning. This is of course a design consideration. By the same token, methods declared <code class="prettyprint">private</code> are implicitly <code class="prettyprint">final</code> and therefore can not be accessed or overridden by inheriting classes.</p> <h1>Summary</h1> <p>In this post we have identified the effect of polymorphism on objects of derived classes, how such classes can be substituted for their base classes and as a consequence the order of constructor calls on objects of classes composed by means of composition and inheritance. Polymorphism is an essential construct in any OOP language as it works with essential features like inheritance and encapsulation and provides you with the ability to use the same interface with different forms. Having a proper understanding of how polymorphism works helps improve your code design with several desirable characteristics such as extensibility, readability and better code organisation.</p> <p>contact me @ <a href="mailto:[email protected]">[email protected]</a></p> <script src="//platform.linkedin.com/in.js" type="text/javascript"> lang: en_US </script><script type="IN/Share" data-counter="top"></script> http://www.java.net/blog/hellofadude/archive/2014/01/07/polymorphism-java#comments Blogs Global Education and Learning J2SE Tue, 07 Jan 2014 21:16:42 +0000 hellofadude 900736 at http://www.java.net How to reuse your Java classes http://www.java.net/blog/hellofadude/archive/2013/12/18/how-reuse-your-java-classes <!-- | 24 --><!--break--><!--break--><p> There are mainly two ways by which one may reuse classes in Java. The first is by way of <I>composition</I>. Composition provides a way to compose your classes from objects of existing classes, essentially making use of the objects' functionality as opposed to its form.</p> <p>The second method is by what we call <I>inheritance</I>, which describes how one may derive a new class as a type of an existing class. With inheritance you make use of not only the functionality of an existing class, but more importantly its form.</p> <h2> Composition</h2> <p>To make use of composition in your class, for non-primitives, you simply create a reference to that object; however for primitives you must define them directly.</p> <p>Here is an example of a class using composition:-<br /> <pre class="prettyprint"><code>class Dog {<br /> Dog() {<br /> System.out.println( "Dog()" );<br /> }<br />}<br />public class Kennel {<br /> private int i;<br /> private double d;<br /> private String Dog1, Dog2, Dog3, Dog4, Dog5, Dog6;<br /> private Dog poddle = new Dog();<br /><br /> public String toString() {<br /> return<br /> "Dog1 = " + Dog1 + " " +<br /> "Dog1 = " + Dog1 + " " +<br /> "Dog1 = " + Dog1 + " " +<br /> "Dog1 = " + Dog1 + " " +<br /> "Dog1 = " + Dog1 + " " +<br /> "Dog1 = " + Dog1 + " " + "\n" +<br /> "i = " + i + " " + "d = " + d;<br /> }<br /> public static void main(String[] args) {<br /> Kennel kn = new Kennel();<br /> System.out.print(kn);<br /> }<br />}<br />/* Output<br />Dog()<br />Dog1 = null Dog1 = null Dog1 = null Dog1 = null Dog1 = null Dog1 = null<br />i = 0 d = 0.0<br />*//</code></pre><br /> The <b>Kennel</b> class in this example is composed using primitives as well as non-primitives like the <b>String</b> and <b>Dog</b> object. Primitives that are fields in a class are automatically initialised to zero as can be gleaned from the output, while object references are initialised to <b>null</b>. As you may observe it is possible to print a <b>null</b> reference without throwing an exception, however an attempt to call a method on an uninitialised variable will induce the relevant compiler error. Further, take note of the special <b>toString()</b> method in the <b>Kennel</b> class which comes as standard in every non-primitive object and is useful in special situations when the compiler wants a string but has only an object.</p> <p>Composition is generally best suited to those situations where you require the functionality of an existing class inside of your new class but not its interface. In other words, composition allows you to embed an object in order to use it to implement features in your new class. To achieve this, you simply embed <code class="prettyprint">private</code> objects of an existing class inside your new class.</p> <h2> Inheritance</h2> <p>Inheritance on the other hand, is a way of taking one class and deriving from it, another class of the same type. The relationship between the two classes can be described as one class being like another or a type of another class. For instance, consider the relationship between a vehicle and a car. We could say, a car is a type of vehicle and our reasoning would make sense to most people. This is essentially how inheritance is designed to work.</p> <p>In Java, all classes you create implicitly inherit from the standard root class <b>Object</b>, from which all objects are ultimately derived. To make use of inheritance, you add the <code class="prettyprint">extends</code> keyword followed by the name of the base class:-<br /> <pre class="prettyprint"><code>class Pet {<br /> private String s = "Pet";<br /> public void append(String a) { s += a; }<br /> public void cuddle() { append(" cuddle()"); } <br /> public void run() { append("run()"); }<br /> public void Jump() { append(" jump()"); }<br /> public String toString() { return s; }<br /> public static void main(String[] args) {<br /> Pet p = new Pet();<br /> p.cuddle(); p.run(); p.Jump();<br /> System.out.print(p);<br /> }<br /> <br />}<br /><br />public class Cat extends Pet { <br /> //change a method <br /> public void Jump() {<br /> append(" Pet.jump()");<br /> super.Jump();<br /> }<br /> //add new members to interface<br /> public void purr() {<br /> append(" purr()");<br /> }<br /> public static void main(String[] args) {<br /> Cat c = new Cat();<br /> c.cuddle();<br /> c.run();<br /> c.Jump();<br /> c.purr();<br /> System.out.println(c);<br /> System.out.println("Test base class:");<br /> Pet.main(args);<br /> }<br />}<br />/* Output<br />Pet cuddle() run() Pet.jump() jump() purr()<br />Test base class:<br />Pet cuddle() run() jump()<br />*//</code></pre><br /> In the above example, the <b>Cat</b> class <code class="prettyprint">extends</code> or otherwise inherits the members of the <b>Pet</b> class and adds new members as you can see with the <b>purr()</b> method.</p> <p>It is also possible to observe within this example, how both classes include a <b>main()</b> method which is a way to allow for the easy testing of your classes. It is possible to include a <b>main()</b> method in every one of the classes in your program without breaking any rule. In such situations only the <b>main()</b> for the class invoked from the command line will be called though in this case, the <b>Pet.main()</b> method is invoked from the <b>main()</b> of the <b>Cat</b> class.</p> <p>Inheritance is clearly demonstrated in this example because the <b>Cat</b> class, by making use of the <code class="prettyprint">extends</code> keyword, ensures <b>Cat</b> objects automatically get all the methods defined in the <b>Pet</b> class - even if not explicitly defined - which to all intents is considered to be its base class. This explains why the <b>Cat</b> class is able to casually make a call to the <b>append()</b> method of its base class from within the <b>jump()</b> and <b>purr()</b> methods</p> <p>The example also demonstrates how to modify a method already defined in the base class, within the derived class as is demonstrated by the <b>jump()</b> method. The <code class="prettyprint">super</code> keyword is used to call the base class version of the <b>jump()</b> method from within the same method in the <b>Cat</b> class.</p> <p>Further, it should be noted how all methods within the <b>Pet</b> class are declared as <code class="prettyprint">public</code> access. This is important because the <b>Pet</b> class defaults to package access as is expected of any class for which you do not specify any access specifier and failure to make the methods <code class="prettyprint">public</code> might make them inaccessible to inheriting classes from other packages.</p> <h2> Base class initialisation</h2> <p>On the subject of inheritance, it is worth noting that when you create an object of a derived class, it automatically contains a sub-object of the base class within it. This is possible because Java automatically inserts a call to the base class constructor from within the derived class constructor. Here is an example that demonstrates my meaning:-<br /> <pre class="prettyprint"><code> class Hat {<br /> Hat() { System.out.println("Hat constructor"); }<br />}<br /><br />class Fedora extends Hat {<br /> Fedora() { System.out.println("Fedora constructor"); }<br />}<br />public class Stetson extends Fedora {<br /> Stetson() { System.out.println("Stetson constructor"); }<br /> public static void main(String[] args) {<br /> Stetson st = new Stetson();<br /> }<br />}<br />/* Output<br />Hat constructor<br />Fedora constructor<br />Stetson constructor<br />*//<br /> </code></pre><br /> The preceding example demonstrates how, when you use inheritance, the base class is always initialised before the derived class constructors are able to access it. In other words construction happens inside-out from the base class. This is how Java ensures the base-class subobject is initialised correctly and avoids many of the problems experienced in other programming languages. Note also that this example uses the default no args constructor. If however, your base class included only constructors with arguments, you would be forced in those circumstances to make an explicit call to the base class constructor using the <code class="prettyprint">super</code> keyword. In addition, the call to the base class would have to be the first thing you do in the derived class constructor.</p> <h2>Upcasting</h2> <p>Upcasting is a term that refers to the direction of movement from a derived type to a base type up the inheritance chain. It is based on the way class inheritance diagrams have traditionally been drawn i.e. with the root or base class at the top of the page, and extending downward.</p> <p>Upcasting is very relevant to inheritance, the most important aspect of which is the relationship expressed between the new class and the base class, which can be summarised by saying <I>"the new class is a type of this existing class".</I></p> <p>It is normally considered a safe operation because you are going from a more specific type to a more general type with very little chance of compromising your data. The derived class is said to be a superset of the base class and consequently might contain more methods than the base class, but it must contain at least the methods in the base class. Consider the following example:-<br /> <pre class="prettyprint"><code> class Instrument {<br /> public void play() {}<br /> static void tune(Instrument i) {<br /> i.play;<br /> }<br /> }<br /><br /> public class Piano extends Instrument {<br /> public static void main(String[] args) {<br /> Piano grandPiano = new Piano();<br /> Instrument.tune(grandpiano); //upcasting<br /> }<br /> }</code></pre><br /> The interesting thing about this example is that the <b>tune()</b> method, which accepts an <b>Instrument</b> object reference is passed a <b>Piano</b> object reference in the <b>main()</b>. How is this possible? You might well observe that an object of the <b>Piano</b> class is also a type of <b>Instrument</b> by virtue of the fact that it inherits all the methods of the <b>Instrument</b> class, and therefore has access to the <b>tune()</b> method of its base class.</p> <p>The act of converting a <b>Piano</b> object into an <b>Instrument</b> reference is what we refer to as upcasting. Understanding when you might need to upcast in your code allows you to make an informed decision about whether to use composition or inheritance when designing your classes. If you will ever need to upcast in your program, then inheritance is probably necessary, but if not, you should consider carefully whether you need inheritance as it is only really useful when there is a clear need for it.</p> <p>Finally, it should be well understood that inheritance has some implication for how Java loads and initialises your classes particularly when dealing with <code class="prettyprint">static</code>s; </p> <h2> Combining composition and inheritance</h2> <p>More often, one may combine both composition and inheritance to derive a more complex type. The following example demonstrates the use of both methods:-<br /> <pre class="prettyprint"><code>class Beverage {<br /> Beverage(int i) {<br /> System.out.println("Beverage constructor");<br /> }<br />}<br />class OrangeJuice extends Beverage {<br /> OrangeJuice(int i) {<br /> super(i);<br /> System.out.println("OrangeJuice constructor");<br /> }<br />}<br />class Beer extends Beverage {<br /> Beer(int i) {<br /> super(i);<br /> System.out.println("Beer constructor"); <br /> }<br />}<br />class Food {<br /> Food(int i) {<br /> System.out.println("Food constructor"); <br /> }<br />}<br />class Potatoes extends Food {<br /> Potatoes(int i) {<br /> super(i);<br /> System.out.println("Potato constructor");<br /> }<br />}<br />class Chicken extends Food {<br /> Chicken(int i) {<br /> super(i);<br /> System.out.println("Chicken constructor");<br /> }<br />}<br />class Dessert {<br /> Dessert(int i) {<br /> System.out.println("Dessert constructor");<br /> }<br />}<br />class Cake extends Dessert {<br /> Cake(int i) {<br /> super(i);<br /> System.out.println("Cake constructor");<br /> }<br />}<br />class Dinner {<br /> Dinner(int i) {<br /> System.out.println("Dinner constructor");<br /> }<br />}<br />public class Meal extends Dinner {<br /> private OrangeJuice oj;<br /> private Beer br;<br /> private Potatoes pt;<br /> private Chicken chk;<br /> private Cake ck;<br /> public Meal(int i) {<br /> super(i + 1);<br /> oj = new OrangeJuice(i + 1);<br /> br = new Beer(i + 2);<br /> pt = new Potatoes(i + 10);<br /> chk = new Chicken(i + 1);<br /> ck = new Cake(i + 1);<br /> } <br /> public static void main(String[] args) {<br /> Meal ml = new Meal(1);<br /> }<br /><br />}<br />/* Output<br />Dinner constructor<br />Beverage constructor<br />OrangeJuice constructor<br />Beverage constructor<br />Beer constructor<br />Food constructor<br />Potato constructor<br />Food constructor<br />Chicken constructor<br />Dessert constructor<br />Cake constructor<br />*//</code></pre><br /> The <b>Meal</b> class is constructed using both composition and inheritance and the example further demonstrates the importance of calling the base class constructor as the first thing you do in the derived class constructor using the <code class="prettyprint">super</code> keyword. This is particularly necessary when you are working with constructors that accept any number of arguments.</p> <h2> Summary</h2> <p>In this post, we have discussed the important subject of code reuse in Java as a mechanism by which one may quickly and easily add functionality to the classes one creates. We have also distinguished between two main ways by which this possible - composition and inheritance. Finally, we looked briefly at the manifestation of the inheritance mechanism in Java and its effects within constructor initialisation, upcasting and in the loading and initialisation of objects.</p> <p>contact me @ <a href="mailto:[email protected]">[email protected]</a></p> <script src="//platform.linkedin.com/in.js" type="text/javascript"> lang: en_US </script><script type="IN/Share" data-counter="top"></script> http://www.java.net/blog/hellofadude/archive/2013/12/18/how-reuse-your-java-classes#comments Blogs Global Education and Learning J2SE Open JDK Wed, 18 Dec 2013 19:24:24 +0000 hellofadude 900563 at http://www.java.net Controlling access with implementation hiding in Java http://www.java.net/blog/hellofadude/archive/2013/12/08/controlling-access-implementation-hiding-java <!-- | 0 --><p>A key consideration for the library designer in the normal conduct of operations is maintaining the ability to make changes or improvements to the library at any time without requiring the consumers (client programmers) of that library to do the same. In Java, a library consists of a logical grouping of <b>.class</b> files packaged together to make a working program.</p> <!--break--><!--break--><p>An apt analogy to this point may be to imagine the considerable inconvenience you might suffer, if, as a result of an update to the Java SE library, you were compelled to make wholesale changes to your existing classes that worked well with a previous version.<br /> As a consumer, you rely on the parts of a library on which your code depends staying the same, and a situation where this ceased to be the case would be very problematic to say the least.<br /> As a matter of custom, an 'unwritten' agreement exists whereby the library designer agrees not to alter or remove existing methods on which your code depends, when modifying some part of a library, since that would break the client programmers code, and make a mockery of years of trusted convention. To do this, the library designer must figure out a way to separate the parts that change in his code from the parts that do not. More precisely, the designer must look to the concept of <I>implementation hiding</I>.</p> <p>Implementation hiding in Java is closely aligned to the need for access control by providing a way to allow the library designer to say what is available to the client programmer (i.e. the parts that do not change) and what is not (i.e. the parts that do change) by putting boundaries within a data type that help separate the interface from the implementation.</p> <p>This interface/implementation distinction is an important point to understand as even though an interface is, by convention, accessible to the client programmer, the implementation which is a part of the structure of the library is hidden or not accessible to the user. In this way, the designer can make changes to the internal mechanism, which we now understand to be quite apart from the interface, without affecting or requiring the client programmer to make any changes in the way he(or she) uses such library.</p> <p>Java provides <I>access specifiers</I> that enable the library creator to say what is available to the client programmer and what is not by controlling access to data types and the members of which they are comprised. These access specifiers consist of the keywords <code class="prettyprint">public</code>, <code class="prettyprint">protected</code> and <code class="prettyprint">private</code>, each reflecting a different level of access from 'most access' to 'least access'. There is also the default <I>package access</I> which occurs when a programmer fails to specify any of the above access specifiers. In subsequent sections, I discuss these specifiers in more detail, however we would do better to begin with a discussion as to how components in Java are bundled together into a coherent library unit.</p> <h1> <code class="prettyprint">package</code>: Library unit</h1> <p>In Java, a package consists of a grouping of classes organised under a single namespace. We may perceive a package as a mechanism by which one may mitigate the risk that those classes one writes for the web do not collide with that of some other programmer simply because they may have the same <code class="prettyprint">class</code> name. The logic for this arrangement may be linked to the uniform interface constraint, a key constraint within <b>REST</b> that applies the principle of generality to component interfaces.</p> <p>In Java, a class file has a <b>.class</b> extension and is normally a product of your source code file. You see, when you write Java programs, you create source code files that comprise a compilation unit, usually with a <b>.java</b> extension. Each compilation unit must have only one public class with the same name as the file (without the <b>.java</b> extension) within which it exists. The Java compiler is responsible for producing output files with the <b>.class</b> extension, from a group of these source code files with the <b>.java</b> extension. These output files are what comprise the library.</p> <p>The <code class="prettyprint">package</code> keyword in Java is the mechanism by which you may package all your source files into a single namespace and ultimately reduce the risk of collisions. A package in Java would normally contain many <b>.class</b> files and the <code class="prettyprint">package</code> keyword is one way Java is able to reference them. Understanding the semantics behind this mechanism is important if you intend to program effectively in Java.</p> <p>The <code class="prettyprint">package</code> keyword must appear as the first non-comment line in your Java class file; and following the keyword should be the name of a directory on your system within which your files must reside and which must be searchable beginning from the <b>CLASSPATH</b>. By so doing, Java takes advantage of the hierarchical file structure of your operating system by allowing you to implicitly specify a directory structure when you assign a package, a name. For instance, suppose I wanted to create a package for my source code file <b>MySourceCode.java</b> in the <b>kingsley.java</b> directory, I would do something like this:-<br /> <pre class="prettyprint"><code> package kingsley.java;<br /><br /> public class MySourceCode { ... }</code></pre><br /> The above example says the file <b>MySourceCode.java</b> is located within the package <b>kingsley.java</b>. The Java compiler is responsible for producing the associated <b>MySourceCode.class</b> file, normally in a separate directory from the source file.</p> <p>Certain semantics allow for the concatenation of a string that describe the location of <b>.class</b> file directories relative to the root by which the Java interpreter may be able to find and load these files.</p> <p>The <code class="prettyprint">import</code> keyword allows you to specify or import a package to use within your own program. For instance, if you wanted to use my <b>MySourceCode.java</b> file in your own program, you would do something like this:-<br /> <pre class="prettyprint"><code> package user.java;<br /><br /> import kingsley.java.MySourceCode;<br /> import java.util.*;<br /> <br /> public class MyUserFile { ... }</code></pre><br /> With the <code class="prettyprint">import</code> statement, the <b>MySourceCode.java</b> file becomes available to your program. If there were, for instance more than one file in the <b>kingsley.java</b> package you were interested to use, instead of importing each file individually, you would use the wildcard character (<b>*</b>). Take the second <code class="prettyprint">import</code> statement for example which is used to import all the files <b>java.util</b> package.</p> <p>It is still possible in some rare instances where you may import two libraries that contain the same class name making a collision inevitable when you try to create a reference to an object. In these circumstances you will be required to specify the full class name including the package within which it resides. For instance, lets us suppose we have our own <b>Random</b> class in the <b>kingsley.java</b> package and then we also import the <b>java.util</b> library which also contains a <b>Random</b> class like so:-<br /> <pre class="prettyprint"><code> import kingsley.java.*;<br /> import java.util.*;</code></pre><br /> A collision would definitely occur if we tried to make a <b>Random</b> object reference like so:-</p> <code class="prettyprint"> Random rand = new Random();//! Error, Which Random</code><br /> <br>You would most likely receive a compiler error forcing you to be explicit. You can avoid this by specifying the full class name like this:-</p> <code class="prettyprint"> java.util.Random rand = new java.util.Random();</code><br /> <br>This gives the compiler the exact location of your intended <b>Random</b> class and ensures you avoid compiler errors.</p> <h1> Access Specifiers</h1> <p>Access specifiers in Java include the keywords <code class="prettyprint">public</code>, <code class="prettyprint">private</code> and <code class="prettyprint">protected</code> which are placed on the same line and immediately preceding each definition of the members of your class. Each specifier provides controlled access only to that particular member definition. </p> <p>If you do not specify any access control for a particular class, it automatically defaults to what we call package access. </p> <h2>Package access</h2> <p>Package access is the default access when you fail to specify any access control option, which means that class will become accessible only to those classes within the same package but will appear as private to other classes outside of the package. </p> <p>We can think of package access as a way to group related files together granting mutual access and interaction to classes within the same package while restricting access to others. There are four ways in Java to grant access to members within a particular class:-</p> <p>i. You might include the <code class="prettyprint">public</code> keyword to make a class member accessible to everyone, everywhere.</p> <p>ii. Not specify any access control thereby defaulting to package access.</p> <p>iii. Include the <code class="prettyprint">protected</code> keyword which allows an inherited class to access the member.</p> <p>iv. Provide accessor/mutator methods (get/set) that read and change members' value.</p> <h2><code class="prettyprint">public</code>: interface access</h2> <p>The <code class="prettyprint">public</code> keyword allows you to grant access to any member of your class to everyone, particularly those client programmers who rely on these portions of your code staying the same; examine the following class:-<br /> <pre class="prettyprint"><code> package kingsley.java;<br /><br /> public class Switch {<br /> public Switch() {<br /> System.out.println("Switch constructor");<br /> }<br /> public void on() {<br /> System.out.println("Switch on");<br /> }<br /> void off() {<br /> System.out.println("Switch off");<br /> }<br /> }</code></pre><br /> Now suppose you created another class outside of this <b>kingsley.java</b> package, you would be required to import the <b>Switch.java</b> class in order to access any of its members like so:-<br /> <pre class="prettyprint"><code> import kingsley.java.Switch;<br /><br /> public class Light {<br /> public static void main(String[] args) {<br /> Switch sw = new Switch();<br /> sw.on();<br /> sw.off(); //Error, can't access<br /> }<br /> }<br /> /* Output<br /> Switch constructor<br /> Switch on<br /> *//</code></pre><br /> You might glean from the output that the <b>Light</b> class is able to access the constructor member of the <b>Switch</b> class, as well as the <b>on()</b> method. This is because both methods have been declared <code class="prettyprint">public</code>. The <b>off()</b> method on the other hand, is inaccessible to the <b>Light</b> class because it has no access specifier, it defaults to package access. Note that when you declare a member public, anyone can access it.</p> <p><b>Tip</b>: To run the example, comment out the <b>off()</b> method</p> <h2><code class="prettyprint">private</code>: hidden access</h2> <p>When you specify the <code class="prettyprint">private</code> keyword in relation to a member of a class, you make that member inaccessible to anyone else, and the consumers of your library are restricted from using it. This is in effect a way of hiding implementation of the 'parts that change' in your program to give you, as the library creator, the ability to modify it without impacting on your consumers code. Further, private members of a class are only available to methods within the same class. Consider the following example:-<br /> <pre class="prettyprint"><code> package kingsley.java;<br /> <br /> class Egg {<br /> private Egg() {<br /> System.out.println("Egg constructor");<br /> }<br /> static Egg makeEgg() {<br /> System.out.println("in makeEgg()");<br /> return new Egg();<br /> }<br /> }<br /><br /> public class Breakfast () {<br /> public static void main(String[] args) {<br /> Egg egg1 = new Egg(); //! Error can not Make new Eggs<br /> Egg egg2 = Egg.makeEgg();<br /> }<br /> }<br /> /* Output<br /> in makeEgg()<br /> Egg constructor<br /> *//</code></pre><br /> In this example, both classes are actually in the same package and it demonstrates how to use the <code class="prettyprint">private</code> keyword to control how an object is created by preventing a user from directly accessing the <code class="prettyprint">class</code> constructor. In this instance, the user is prevented from directly creating an <b>Egg</b> object by applying the <code class="prettyprint">private</code> keyword to the <b>Egg</b> constructor. In the <b>main()</b> of the <b>Breakfast</b> class, any attempt to create an <b>Egg</b> object directly is certain to induce a compiler error, however in this instance, we are forced to use the <code class="prettyprint">static</code> <b>makeEgg()</b> method, a member of the same class to create an <b>Egg</b> object. In this way, a client programmer may use the <b>makeEgg()</b> method to construct a new <b>Egg</b> object even as the class designer is free to change the way this object is constructed without having to worry about affecting the consumers code.</p> <h2><code class="prettyprint">protected</code>: inherited access</h2> <p> The <code class="prettyprint">protected</code> access control specifier is relevant to the concept of <I>inheritance</I> in Java. Inheritance refers to the mechanism by which a derived class may inherit the members of it's base class. It is a fundamental concept in OOP that says take this existing class (base class) and add new members to a new class without interfering with the existing class in any way. Derived classes are subclasses of a base class and provide a simple way to extend the functionality of data types in Java.<br /> To cause your class to inherit the members of another class, you use the <code class="prettyprint">extends</code> keyword like so:- </p> <code class="prettyprint"> class Light extends Switch { ... }</code><br /> <br>The <code class="prettyprint">protected</code> keyword allows a class designer to grant access to derived classes but not to everyone in general. It is a kind of midway point between the <code class="prettyprint">private</code> and <code class="prettyprint">public</code> access control specifiers.<br /> In the previous example, the <b>Light</b> <code class="prettyprint">class</code> was unable to access the <b>off()</b> member in the <b>Switch</b> class which defaulted to package access. We could modify our <b>Light</b> class by inheriting from the <b>Switch</b> class and adding a constructor and a couple of public methods to elucidate further;-<br /> <pre class="prettyprint"><code> package kingsley.java;<br /><br /> public class Switch {<br /> public Switch() {<br /> System.out.println("Switch constructor");<br /> }<br /> public void on() {<br /> System.out.println("Switch on");<br /> }<br /> void off() {<br /> System.out.println("Switch off");<br /> }<br /> }<br /><br /> package kingsley.java.com;<br /><br /> import kingsley.java.*;<br /><br /> public class Light extends Switch {<br /> public Light() {<br /> System.out.println( "Light constructor" );<br /> }<br /> public void lightOn() {<br /> on(); <br /> }<br /> public void lightOff() {<br /> off();//!Error can not access <br /> }<br /> public static void main(String[] args) {<br /> Light ls = new Light();<br /> ls.lightOn();<br /> ls.lightOff(); <br /> }<br /> }<br /> /*Output<br /> Switch constructor<br /> Light constructor<br /> Switch on<br /> **//</code></pre><br /> The interesting thing to note in this example is you would expect the <b>Light</b> class being a derived class of <b>Switch</b> class to inherit all its members including the <b>off()</b> method. But this is not so in this case because the <b>Light</b> class exists in a different package to the <b>Switch</b> class and the lack of an access control specifier ensures the <b>off()</b> member remains inaccessible. To get around this, we could add the <code class="prettyprint">protected</code> keyword to our <b>off()</b> member in the following manner:-</p> <pre class="prettyprint"><code>package kingsley.java;<br /><br /> public class Switch {<br /> public Switch() {<br /> System.out.println("Switch constructor");<br /> }<br /> public void on() {<br /> System.out.println("Switch on");<br /> }<br /> protected void off() {<br /> System.out.println("Switch off");<br /> }<br /> }<br /><br /> package kingsley.java.com;<br /><br /> import kingsley.java.*;<br /><br /> public class Light extends Switch {<br /> public Light() {<br /> System.out.println( "Light constructor" );<br /> }<br /> public void lightOn() {<br /> on(); <br /> }<br /> public void lightOff() {<br /> off(); //protected method<br /> }<br /> public static void main(String[] args) {<br /> Light ls = new Light();<br /> ls.lightOn();<br /> ls.lightOff();<br /> }<br /> }<br /> /*Output<br /> Switch constructor<br /> Light constructor<br /> Switch on<br /> Switch off<br /> **//</code></pre><br /> The <code class="prettyprint">protected</code> keyword makes the <b>off()</b> member accessible to any class inheriting from the <b>Switch</b> class, even to those that exist in a different package.</p> <h2><code class="prettyprint"> class</code> access</h2> <p>It is also possible to use access specifiers to determine which classes are available within a library to a user of that library. To make classes available to a consumer of a library, just specify the <code class="prettyprint">public</code> keyword on the entire class definition like so:- </p> <code class="prettyprint"> public class Shape { ... }</code><br /> <br>However, if on the other hand you were inclined to deter the client programmer from becoming reliant on that class, so that you could re-engineer it at some later time, you would make it inaccessible or hidden from the consumer by leaving out the <code class="prettyprint">public</code> keyword in which case the class defaults to package access, making it like so:- </p> <code class="prettyprint"> class Shape { ... } </code><br /> <br>You should be aware that in this scenario, it will make sense to make fields of the class private - which is always good practice, even for your classes that are public. It is also considered within reason to give methods the same access as your class, which in this case is package access. Note that it is not possible for a class to be <code class="prettyprint">private</code> or <code class="prettyprint">protected</code>. If you must restrict anyone from having access to your class, the best way would be to make all constructors <code class="prettyprint">private</code>, thereby preventing anyone but yourself from making access from within a static member of the same class. </p> <h1> Summary</h1> <p>In this discussion, we have identified the convention that exists between the library designer and the client programmer. This 'unwritten' agreement provides rules that guide custom to ensure a certain amount of universality in the operation of software. </p> <p>We have also briefly discussed some of the mechanisms and components for access control that exist within the Java programming language at reasonably higher level of abstraction to simplify understanding.</p> <p>There are two primary motivations for controlling access to class members. The first motivation is to be able to separate the publicly accessible interfaces from the internal workings of the program. The public interfaces are only relevant to the client programmer and by restricting some members, you are actually making it easier for consumers to use your library.</p> <p>The second reason is to allow the library designer the flexibility to change the internal workings of the program without having to worry about affecting the consumers code.</p> <p>contact me @ <a href="mailto:[email protected]">[email protected]</a></p> <script src="//platform.linkedin.com/in.js" type="text/javascript"> lang: en_US </script><script type="IN/Share" data-counter="top"></script> http://www.java.net/blog/hellofadude/archive/2013/12/08/controlling-access-implementation-hiding-java#comments Blogs Global Education and Learning J2SE Open JDK Mon, 09 Dec 2013 02:28:34 +0000 hellofadude 900434 at http://www.java.net Understanding Java's flow of execution http://www.java.net/blog/hellofadude/archive/2013/12/04/understanding-javas-flow-execution <!-- | 0 --><p>Java uses conditional statements to determine the execution path. Conditional statements provide a way to determine the truth or falsehood of a conditional expression, by which we mean to describe expressions that make use of relational operators and such, and that are able to produce a <code class="prettyprint">boolean</code> value.</p> <!--break--><!--break--><p> These include the <code class="prettyprint">if-else</code> statement, which is a control statement most commonly used to control the flow of execution, also the <code class="prettyprint">while</code>, <code class="prettyprint">do-while</code>, and <code class="prettyprint">for</code> statements, which are mainly used to control iteration and loops, the <code class="prettyprint">return</code>, <code class="prettyprint">break</code> and <code class="prettyprint">continue</code> statements, which allow us to perform unconditional branching and finally the <code class="prettyprint">switch</code> statement, which, prior to Java SE 7 worked mainly with integral values and provides a way for multiway selection. </p> <h1> <code class="prettyprint">if-else</code> statement</h1> <p>The <code class="prettyprint">if-else</code> statement provides a way to control the flow or sequence of statements by allowing you to test for certain conditions within the conventions of a <code class="prettyprint">boolean</code> expression. It is a way of saying if <b>x == y</b> then do <b>z</b>. Note the use of the relational equivalence operator which works with primitives as well as objects, though in the latter case it actually compares object references as opposed to the content of the object themselves. Here is an example of the use of an <code class="prettyprint">if-else</code> statement that imitates a simple traffic control system:-<br /> <pre class="prettyprint"><code> public class TrafficLights {<br /> public static void main(String[] args) {<br /> boolean condition = false;<br /> if(condition) {<br /> System.out.println("Walk") <br /> } else {<br /> System.out.println("Don't walk")<br /> }<br /> }<br /> }<br /> /* Output<br /> Don't Walk<br /> *//</code></pre><br /> In the <b>main()</b>, the <code class="prettyprint">if</code> statement is used to evaluate the <I>boolean-expression</I>, in this case, if the <code class="prettyprint">boolean</code> variable <b>condition</b> happens to be <code class="prettyprint">true</code> or <code class="prettyprint">false</code>. If <b>condition</b> evaluates to <code class="prettyprint">true</code>, it prints <b>"Walk"</b> to system console, otherwise, if it is <code class="prettyprint">false</code> it prints <b>"Don't walk"</b>. The placement of curly braces should be well noted, as a common error is when a programmer gets confused as to how to properly situate curly braces particularly when there is a subsequent <code class="prettyprint">else</code> statement. </p> <p>Curly braces are optional, without an <code class="prettyprint">else</code> statement, otherwise an <code class="prettyprint">if</code> statement should be properly enclosed within an open <b>{</b> and close <b>}</b> curly brace before any subsequent <code class="prettyprint">else</code> statement.<br /> We can extend the <code class="prettyprint">if-else</code> statement in a manner that allows us to provide for several execution paths like so:-</p> <pre class="prettyprint"><code> import java.util.Random;<br /><br /> public class ThrowDice {<br /> public static void main(String[] args) {<br /> Random rand = new Random(8);<br /> for (int i = 1; i < 6; i++) {<br /> int spin = rand.nextInt(7);<br /> if(spin == 1) {<br /> System.out.println(spin + ": Loser.. Low number");<br /> } else if(spin == 6) {<br /> System.out.println(spin + ": Winner! High number");<br /> } else if (spin > 1 && spin < 6){<br /> System.out.println(spin + ": Try again..");<br /> } else {<br /> System.out.println(spin + ": Not allowed" );<br /> }<br /> } <br /> }<br /> }<br /> /* Output<br /> 1: Loser.. low number<br /> 2: Try again..<br /> 4: Try again..<br /> 0: Not allowed<br /> 6: Winner! High number<br /> *//<br /> </code></pre><br /> Here is a class that uses a random object to imitate a dice throwing game. A random number generator is set with a long seed, which is just a way of giving it an initial value to keep our results consistent.<br /> The random object rand uses the <b>nextInt()</b> method to return a pseudorandom value between and including 0 and, but excluding 7. What this means is the value of our <b>spin</b> variable may be anything from 0 to 6, which does not strictly conform to the six sides of a standard die. </p> <p>Note how we extend the use of the <code class="prettyprint">if-else</code> statement to deal with a number of possible scenarios, depending on the number generated. An additional <code class="prettyprint">if</code> is added to the <code class="prettyprint">else</code> to test for the high and low numbers and everything else in-between. Also note the final <code class="prettyprint">else</code> statement which acts as a default and deals the number 0. You may also wish to note the use of conditional and logical operators to test for values between 1 and 6, in this instance printing <b>"Try again..</b>" to the screen if <b>spin</b> is greater than 1 but less than 6.<br /> Further, we place our <code class="prettyprint">if-else</code> statement in a <code class="prettyprint">for</code> loop that allows us to have 6 throws of the dice. </p> <h1>Iteration and looping </h1> <p>Iteration or looping is a way to allow certain code in your program be repeated multiple times. This is usually achieved by means of a sequence of quantities returned from a test condition. These quantities act in a way to determine the number of iterations required in your loop. Java uses the keywords <code class="prettyprint">while</code>, <code class="prettyprint">do-while</code> and <code class="prettyprint">for</code>, to implement iteration statements.</p> <h2><code class="prettyprint">while</code> loop</h2> <p>The <code class="prettyprint">while</code> loop evaluates an expression once, at the beginning of every iteration. It is of the form:-<br /> <pre class="prettyprint"><code> while(Boolean-expression)<br /> statement</code></pre><br /> The <code class="prettyprint">while</code> statement continues until the <I>boolean-expression</I> evaluates to <code class="prettyprint">false</code>. Here is a demonstration that prints a sequence of numbers to screen as long as the condition <b>count < 10</b> evaluates to <code class="prettyprint">true</code>.<br /> <pre class="prettyprint"><code> import java.util.Random;<br /><br /> public class Counter {<br /> public static void main(String[] args) {<br /> Random rand = new Random(8);<br /> int count = rand.nextInt(10);<br /> while(count < 10) {<br /> System.out.println("count is " + count);<br /> count++;<br /> } <br /> }<br /> }<br /> /* Output<br /> count is 4<br /> count is 5<br /> count is 6<br /> count is 7<br /> count is 8<br /> count is 9<br /> //*</code></pre> <p>The reader will note how it is necessary to manually increment the <b>count</b> variable within the <code class="prettyprint">while</code> block, otherwise the test condition will never evaluate to <code class="prettyprint">false</code> and then you might get into what we call an infinite loop.</p> <h2><code class="prettyprint">do-while</code> loop</h2> <p>The <code class="prettyprint">do-while</code> loop works slightly differently from the <code class="prettyprint">while</code> loop because it always executes the statement at least once, even if the test condition evaluates the <code class="prettyprint">false</code> the first time. This is so, because the <code class="prettyprint">do-while</code> evaluates the expression after the statement, like so:-<br /> <pre class="prettyprint"><code> do <br /> statement<br /> while(Boolean-expression);</code></pre><br /> We could modify our <b>Counter</b> class to implement a <code class="prettyprint">do-while</code> loop like this:-<br /> <pre class="prettyprint"><code> import java.util.Random;<br /><br /> public class Counter {<br /> public static void main(String[] args) {<br /> Random rand = new Random(8);<br /> int count = rand.nextInt(10);<br /> do {<br /> System.out.println("count = " + count);<br /> count++;<br /> } while(count < 10);<br /> <br /> }<br /> }<br /> /* Output<br /> count is 4<br /> count is 5<br /> count is 6<br /> count is 7<br /> count is 8<br /> count is 9<br /> //*</code></pre> <h2> <code class="prettyprint">for</code> loop</h2> <p>The <code class="prettyprint">for</code> loop is one most commonly used for iteration and whose logic is divided into three distinct segments, each terminated by a semi-colon, that speak to the initialisation of a variable, a termination condition, and then a step - that is the actual incrementing of the variable proper after each iteration. It is of the form:-<br /> <pre class="prettyprint"><code> for(initialisation; termination-condition; step)<br /> statement</code></pre><br /> Here is an example using our <b>Counter</b> class:- </p> <pre class="prettyprint"><code> import java.util.Random;<br /><br /> public class Counter {<br /> public static void main(String[] args) {<br /> Random rand = new Random(8);<br /> for(int count = rand.nextInt(10); count < 10; count++) {<br /> System.out.println("count is " + count);<br /> }<br /> }<br /> }<br /> /* Output<br /> count is 4<br /> count is 5<br /> count is 6<br /> count is 7<br /> count is 8<br /> count is 9<br /> //*</code></pre><br /> As you will observe, incrementation is performed automatically, in a manner of speaking, at the end of every iteration so there is little need to manually increment the <b>count</b> variable within the loop as in the <code class="prettyprint">while</code> and <code class="prettyprint">do-while</code> loops.<br /> With the <code class="prettyprint">for</code> loop, You can define multiple variables separated by a comma operator like this:-<br /> <pre class="prettyprint"><code> import java.util.Random;<br /><br /> public class Counter {<br /> public static void main(String[] args) {<br /> Random rand = new Random(8);<br /> for(int count = rand.nextInt(10), i = count * 2; count <<br /> 10; count++, i = i - count ) {<br /> System.out.println("count = " + count + ", i is " + i);<br /> <br /> }<br /> }<br /> }<br /> /* Output<br /> count is 4, i is 8<br /> count is 5, i is 3<br /> count is 6, i is -3<br /> count is 7, i is -10<br /> count is 8, i is -18<br /> count is 9, i is -27<br /> *//</code></pre><br /> In the above example, we initialise two <code class="prettyprint">int</code> variables <b>count</b> and <b>i</b>, separated by the comma operator. At each step, <b>count</b> is incremented by 1 while the <b>i</b> variable is divided by the current value of the <b>count</b> variable.</p> <h2> foreach syntax</h2> <p>Another variation of the <code class="prettyprint">for</code> loop exists specifically for use with arrays and containers. Sometimes called the <b>foreach</b> syntax it allows you to count through a sequence of items in a container and looks like this:-</p> <code class="prettyprint"> for(type variable : arrayVariable)</code><br /> <br>Here is an example of a <code class="prettyprint">class</code> using the foreach syntax:-<br /> <pre class="prettyprint"><code> import java.util.Random;<br /><br /> public class ForEach {<br /> public static void main(String[] args) {<br /> Random rand = new Random(8);<br /> int[] count = new int[5];<br /> for(int i = 0; i < 5; i++) {<br /> count[i] = rand.nextInt();<br /> }<br /> for(int x : count)<br /> System.out.println(x);<br /> }<br /> }<br /> /* Output<br /> count is 4<br /> count is 6<br /> count is 0<br /> count is 1<br /> count is 2<br /> *//</code></pre><br /> In this example, the <code class="prettyprint">for</code> loop is first used to populate an array <b>[ ]</b> of <code class="prettyprint">int</code>. The <b>foreach</b> syntax iterates through the array and assigns each element within to a variable <b>x</b> also of type <code class="prettyprint">int</code>; in other words the variable <b>x</b>, holds the current element in the array. The <b>foreach</b> syntax is ideally suited to any method that returns an array.</p> <h1>Unconditional branching</h1> <p>Unconditional branching statements allow us to break from the current iteration in a loop or exit from some method without any test conditions. Java uses the keywords <code class="prettyprint">return</code>, <code class="prettyprint">break</code> and <code class="prettyprint">continue</code> are used to achieve this effect.</p> <h2><code class="prettyprint">return</code> statement</h2> <p>The <code class="prettyprint">return</code> statement is used to specify the return value of a method i.e. one that is not declared <code class="prettyprint">void</code>, as well as causing the program to exit from that method with or without a return value:-<br /> <pre class="prettyprint"><code> static int Counter() {<br /> return ++count;<br /> }</code></pre><br /> The <code class="prettyprint">return</code> statement causes the method to exit and returns control to the point where the method was invoked. A method declared as <code class="prettyprint">void</code> has an implicit return so adding a <code class="prettyprint">return</code> statement would be superfluous but not necessarily illegal.</p> <h2><code class="prettyprint">break</code> statement</h2> <p>The <code class="prettyprint">break</code> statement is used to break out of a loop without executing the rest of the statements; It can be used with either of the <code class="prettyprint">for</code>, <code class="prettyprint">do</code>, <code class="prettyprint">do-while</code> or <code class="prettyprint">switch</code> statements. To terminate a loop, you simply place the <code class="prettyprint">break</code> keyword at the point where you wish for this to happen:-<br /> <pre class="prettyprint"><code> import java.util.Random;<br /><br /> public class BreakLoop {<br /> <br /> public static void main(String[] args) {<br /> Random rand = new Random(6);<br /> for(int i = 0; i < 6; i++) {<br /> int x = rand.nextInt(6);<br /> if(x == i) {<br /> System.out.println("x == i " + "x is " + x + ", i is " <br /> + i + ", breaking out of loop!");<br /> break;<br /> }<br /> System.out.println("x != i: " + "x is " + x + ", i is " + i);<br /> }<br /> } <br /> }<br /> /* Output<br /> x != i: x is 1, i is 0<br /> x != i: x is 0, i is 1<br /> x == i: x is 2, i is 2, breaking out of loop!<br /> *//</code></pre><br /> The <code class="prettyprint">break</code> statement can also be used in the <code class="prettyprint">while</code> and <code class="prettyprint">do-while</code> loops, but in this example, it is used to break out of the <code class="prettyprint">for</code> loop if <b>x == i</b>, otherwise, it carries on until termination condition. In a situation where we might have nested loops i.e. loops within loops, the <code class="prettyprint">break</code> statement will terminate the inner most loop returning control to the outermost loop. To terminate the outermost loop from within an inner loop, we may use a labelled <code class="prettyprint">break</code> statement like so:-<br /> <pre class="prettyprint"><code> public class LabelledBreak {<br /> public static void main(String[] args) {<br /> outer:<br /> for(int i=0; i < 10; i++) {<br /> for(int x=i * 10; x < 100; x++) {<br /> if (x % 9 == 0) break outer;<br /> System.out.print(x + " ");<br /> <br /> }<br /> }<br /> }<br /> }<br /> /* Output<br /> 10 11 12 13 14 15 16 17<br /> *//</code></pre><br /> The label <b>outer</b> is an identifier used to mark the start of the outermost for loop, which signals the point where the <code class="prettyprint">break</code> statement must terminate. This is to say control is returned to statement immediately following the labelled statement. From the example, it is clear that <b>x</b> never gets 100 as the <code class="prettyprint">break</code> statement terminates the loop when there is no remainder from the modulus operator. Note the use of the print statement which prints horizontally across the screen as opposed to the println statement which prints vertically.</p> <h2> <code class="prettyprint">continue</code> statement</h2> <p>The <code class="prettyprint">continue</code> statement causes control to return to the beginning of the next iteration of the innermost loop, without completing the current iteration. Unlike the <code class="prettyprint">break</code> statement it does not break out of the loop completely.<br /> <pre class="prettyprint"><code> public class ContinueStatement {<br /> public static void main(String[] args) {<br /> for(int i = 0; i < 100; i++) { <br /> if(i % 10 != 0) continue; //Top of the loop<br /> System.out.println(i + " ");<br /> }<br /> }<br /> }<br /> /* Output<br /> 10 20 30 40 50 60 70 80 90 <br /> *//<br /> </code></pre><br /> The <code class="prettyprint">continue</code> statement in this example, is used to exit the current iteration if the condition <b>i % 10</b> is not equal to 0, and only prints when the value of the expression evaluates to 0. </p> <h2><code class="prettyprint">switch</code> statement</h2> <p>The <code class="prettyprint">switch</code> statement is a kind of selection statement that allows you to select from a number of possible execution paths. The <code class="prettyprint">switch</code> statement works with integral values like <code class="prettyprint">int</code>, <code class="prettyprint">char</code> primitive types, as well as enumerated types, the <b>String</b> class and certain other wrapper classes. </p> <p>The following code uses a <code class="prettyprint">switch</code> statement to print the value of <b>i</b> to screen:-<br /> <pre class="prettyprint"><code> public class SwitchStatement {<br /> public static void main(String[] args) {<br /> for(int i = 0; i < 6; i++) {<br /> switch(i) {<br /> case 1: System.out.println("i is " + i); break;<br /> case 2: System.out.println("i is " + i); break;<br /> case 3: System.out.println("i is " + i); break;<br /> case 4: System.out.println("i is " + i); break;<br /> case 5: System.out.println("i is " + i); break;<br /> default: System.out.println("i is " + i);<br /> }<br /> }<br /> }<br /> }<br /> /* Output<br /> i is 0<br /> i is 1<br /> i is 2<br /> i is 3<br /> i is 4<br /> i is 5<br /> *//</code></pre><br /> You may observe the body of a <code class="prettyprint">switch</code> statement is enclosed between curly braces that comprise the <code class="prettyprint">switch</code> block. Following the switch, One may have multiple case labels whereby a matching case is executed based on the evaluation of the test expression.</p> <p>The preceding example compares the value of <b>i</b> with each case label and prints the appropriate value to screen, so for instance, if <b>i == 1</b>, the statements in case 1 are executed.</p> <p>Note that each case ends with a <code class="prettyprint">break</code> clause, which causes execution to jump to the end of the <code class="prettyprint">switch</code> block. Also note the last statement which is the default statement, handles all scenarios not explicitly provided for in any of the case labels and does not require a <code class="prettyprint">break</code> sytatement, even though it would comprise no ill-effect were you to choose to include one; in this case, the default statements prints 0 to screen. </p> <p>It is also possible to stack case labels in a way that provides multiple matches for a particular piece of code. Run the following example to see the result:-<br /> <pre class="prettyprint"><code> public class SwitchStatement2 {<br /> public static void main(String[] args) {<br /> for(int i = 0; i < 10; i++) {<br /> switch(i) {<br /> case 1:<br /> case 3:<br /> case 5:<br /> case 7:<br /> case 9: System.out.println("odd number " + i);<br /> break;<br /> default: System.out.println("even number " + i);<br /> }<br /> }<br /> }<br /> }</code></pre><br /> It is essential to put a <code class="prettyprint">break</code> statement at the end of a particular case otherwise control will simply drop through and continue processing until a <code class="prettyprint">break</code> is encountered. In the above example, the cases are stacked on top of the other when <b>i</b> is an odd number, otherwise the default statement is used to print out even numbers.</p> <h1>Summary</h1> <p> We have fully explored the basics of controlling the flow of execution in your Java program. We should by now be well acquainted with how to use the <code class="prettyprint">if-else</code> statements to control flow, the <code class="prettyprint">switch</code> statement for multiway selection, implementing loops like <code class="prettyprint">for</code>, <code class="prettyprint">while</code> and the <code class="prettyprint">do-while</code>, terminating loops using the <code class="prettyprint">break</code> statement, breaking out of the current iteration with the <code class="prettyprint">continue</code> statement and how to use the <code class="prettyprint">return</code> statement to return required values in methods that specify a return type.</p> <p>contact me @ <a href="mailto:[email protected]">[email protected]</a></p> <script src="//platform.linkedin.com/in.js" type="text/javascript"> lang: en_US </script><script type="IN/Share" data-counter="top"></script> http://www.java.net/blog/hellofadude/archive/2013/12/04/understanding-javas-flow-execution#comments Wed, 04 Dec 2013 22:49:36 +0000 hellofadude 900376 at http://www.java.net
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Commit 6056ebd4 authored by Jeroen F.J. Laros's avatar Jeroen F.J. Laros
Simplified counting for now.
parent 35176088
...@@ -16,8 +16,8 @@ def _add(root, word, count): ...@@ -16,8 +16,8 @@ def _add(root, word, count):
node = node[char] node = node[char]
if '' not in node: if '' not in node:
node[''] = {'count': 0} node[''] = 0
node['']['count'] += count node[''] += count
def _find(root, word): def _find(root, word):
...@@ -49,8 +49,8 @@ def _remove(node, word, count): ...@@ -49,8 +49,8 @@ def _remove(node, word, count):
""" """
if not word: if not word:
if '' in node: if '' in node:
node['']['count'] -= count node[''] -= count
if node['']['count'] < 1 or count == -1: if node[''] < 1 or count == -1:
node.pop('') node.pop('')
return True return True
return False return False
...@@ -78,7 +78,7 @@ def _iterate(path, node, unique): ...@@ -78,7 +78,7 @@ def _iterate(path, node, unique):
""" """
if '' in node: if '' in node:
if not unique: if not unique:
for _ in range(1, node['']['count']): for _ in range(1, node['']):
yield path yield path
yield path yield path
...@@ -99,7 +99,7 @@ def _fill(node, alphabet, length): ...@@ -99,7 +99,7 @@ def _fill(node, alphabet, length):
{alphabet}. {alphabet}.
""" """
if not length: if not length:
node[''] = {'count': 1} node[''] = 1
return return
for char in alphabet: for char in alphabet:
......
...@@ -17,11 +17,11 @@ class TestTrie(object): ...@@ -17,11 +17,11 @@ class TestTrie(object):
assert self._trie.root == { assert self._trie.root == {
'a': { 'a': {
'b': { 'b': {
'c': {'': {'count': 1}}, 'c': {'': 1},
'd': {'': {'count': 2}}}}, 'd': {'': 2}}},
't': {'e': { 't': {'e': {
'': {'count': 1}, '': 1,
's': {'t': {'': {'count': 1}}}}}} 's': {'t': {'': 1}}}}}
def test_word_present(self): def test_word_present(self):
assert 'abc' in self._trie assert 'abc' in self._trie
...@@ -61,18 +61,18 @@ class TestTrie(object): ...@@ -61,18 +61,18 @@ class TestTrie(object):
assert 'abx' in self._trie assert 'abx' in self._trie
def test_get_present(self): def test_get_present(self):
assert self._trie.get('abc')['count'] == 1 assert self._trie.get('abc') == 1
def test_get_absent(self): def test_get_absent(self):
assert not self._trie.get('abx') assert not self._trie.get('abx')
def test_add_twice(self): def test_add_twice(self):
self._trie.add('abc') self._trie.add('abc')
assert self._trie.get('abc')['count'] == 2 assert self._trie.get('abc') == 2
def test_add_multiple(self): def test_add_multiple(self):
self._trie.add('abc', 2) self._trie.add('abc', 2)
assert self._trie.get('abc')['count'] == 3 assert self._trie.get('abc') == 3
def test_remove_present(self): def test_remove_present(self):
assert self._trie.remove('test') assert self._trie.remove('test')
...@@ -93,14 +93,14 @@ class TestTrie(object): ...@@ -93,14 +93,14 @@ class TestTrie(object):
def test_remove_twice(self): def test_remove_twice(self):
self._trie.add('abc') self._trie.add('abc')
assert not self._trie.remove('abc') assert not self._trie.remove('abc')
assert self._trie.get('abc')['count'] == 1 assert self._trie.get('abc') == 1
assert self._trie.remove('abc') assert self._trie.remove('abc')
assert 'abc' not in self._trie assert 'abc' not in self._trie
def test_remove_multile(self): def test_remove_multile(self):
self._trie.add('abc', 3) self._trie.add('abc', 3)
assert not self._trie.remove('abc', 2) assert not self._trie.remove('abc', 2)
assert self._trie.get('abc')['count'] == 2 assert self._trie.get('abc') == 2
def test_remove_force(self): def test_remove_force(self):
self._trie.add('abc') self._trie.add('abc')
......
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Sorry if this seems like a hazy question but it's something that's been bugging me for a little while.
In my day job, some of the code I write gets pretty complex. Not that it's usually very technical but the problem domain itself is a complex matter dealing with spacial data amongst many other things. I'm pretty sure my NDA would prohibit me from giving any details of what I'm working on so, unfortunately, I'll have to keep this pretty general.
Now, I'm all for reducing complexity so I try to find the right abstractions when I can but is there any way to reduce it further by explicitly not dealing with the actual matter at hand but rather some metaphor that could be operated on and then translated into the actual result I want later?
Of course, since the area is so complicated in itself,..I've tried but failed (many times!)to find the right metaphor :-(
So my question is, has someone already done this work and found (or even half found) a structured way to extrapolate an appropriate metaphor or heuristic for a set of given programming problems?
Again, my apologies if this seems like a bit of an odd question. I'm just trying to find new ways of being a better programmer.
Thanks in advance.
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1
Isn't this what object orientation is all about? – anon Feb 10 '10 at 15:21
Sure, I could do it using OO but it's not directly OO that I'm talking about. I'm talking about expressing a real world problem in completely different terms than it is in the real world...dealing with a something that represents the problem but in much simpler terms. Maybe I'm thinking too much! :-) – Stimul8d Feb 10 '10 at 15:26
What you are talking about is (I think) a model. And modelling is what OO is all about. – anon Feb 10 '10 at 16:06
I am talking about a model but not of the actual domain i'm interested in. Take the 3D pallet loading problem for example. You could model it in terms of pallets and boxes from the offset but when you introduce new rules (say you want to guarantee that box 110 is on the bottom tier) it starts to become very involved. OR you could choose to model it in an entirely different way where boxes, tiers or 3D space aren't involved until the very end. You'd use something much simpler to manipulate your output, then transform that into your desired result at the end. Hope that makes more sense :-) – Stimul8d Feb 10 '10 at 17:06
1 Answer 1
up vote 1 down vote accepted
So my question is, has someone already done this work and found (or even half found) a structured way to extrapolate an appropriate metaphor or heuristic for a set of given programming problems?
I may have the start of an answer to your query. This consists of multiple levels of abstraction; descriptions of multiple domains; and the application of "modelling" techniques in a particular way - really rather different from what is normally done in modelling. The overall approach is, I believe, what you are seeking - it gives metaphors that are operated on, and then translated into the actual result. It is based on a number of published approaches, and relies heavily on some of these approaches and methods.
What follows is subject to these caveats:
1. This is very much "work in progress".
2. I have been on the receiving end of derogatory comments that I am an "architecture astronaut", with the consequent implication that I am out of touch with the nitty-gritty of real system development. This is one of the reasons this is work in progress - my current project is designed to validate this concept, and demonstrate that it has real utility. In order to do this I need to develop a system of sufficient scale and complexity, using my approach, that would normally be out of reach of an individual developer. I am only part way through such proof of concept development.
3. While the concepts I describe are derived from consideration of complex problem domains (sonar contact tracking system, and chip design systems), my examples relate to a simpler domain - basically an information system, constrained by a rules system. The rules base includes not only rules about the domain, but also rules about the applicability of other rules.
4. I suspect, from your question, that I am coming to it from the opposite direction to you. You ask for a way to extrapolate an appropriate metaphor or heuristic for a set of given programming problems - which to me implies starting with the programming problems. My impetus has been from the analysis side - trying to find and formulate ways of describing a proposed system so that it can be implemented as a real system.
5. When I use the words "model" and "modelling" I mean modelling as described below. This description is somewhat different from common usage of these terms.
The three main consituents needed for this approach are:
Multiple domains
My definition of a domain is wider than that normally adopted:
A domain is a separate real, hypothetical, or abstract world inhabited by a distinct set of objects and phenomena that behave according to rules and policies characteristic of the domain. In problem analysis.the domain is a useful unit of consideration when developing complex systems.
Under this definition, there are multiple domains for consideration within a system, Often when developers refer to a domain, they mean the problem (or application) domain (referred to hereafter as P). However for this approach, any aspect of the system, or system development is a potential subject for modelling. This includes the system architecture (A); system production artefacts (code, make scripts, database schemas, etc) (C); DBA functions; etc. To approach P via metaphor requires development of several such domains - relating to the metaphor and transformations from the metaphor to a model of the real world, or to the code realisation of the real world in the developed system. When multiple such models are developed, they are all developed to the same degree of scope and precision.
Multiple levels of abstraction
To describing a problem and system, one models not only P, but also models appropriate higher levels of abstraction. Thus the metaphor chosen to describe P is modelled (M). In a similar way, the formalism of A (F) is modelled, and if deemed necessary the transformation process between P and C using A (R). So one abstracts the problem domain; abstracts the abstraction and so on.
The application of multiple models is similar to colour separations - they lie on top of each other, and the system has to meet all the descriptions (the "complete picture") of all the layers. Again this differs from common ways of modelling which tend to meet such multiple requirements by elaborating the original model to take in the different constraints. This has particular implications when all of the architecture domains are effectively applied to all the elements of all the problem domains.
Modelling
What I mean by modelling differs from more usual approaches to modelling in the following respects:
1. The subject matter of the model is highly likely to be different from domain to domain. This contrasts with modelling where initial models are elaborations of other models. Indeed, one test of the validity of a model is that it represents an extreme version of the DRY principles - if something is defined in more than one place, it indicates a deficiency in the model. This extends to all domains under consideration, so the way the system is built is defined only in one place.
2. A domain, once modelled, is likely to be fairly static. The higher the level of abstraction, the less likely it is to change.
3. The scope of a model is likely to be much narrower, and much deeper than those conventionally developed. There are likely to be fewer objects within a particular domain than in conventional modelling; but these models must be complete and there are tests which give some indication of the completness of the model (obviously, one can never prove that a system description is complete).
4. P is described in terms defined by M. The model might be expressed as diagrams, OO representations, mathematical formulae, or whatever is appropriate for the domain M.
The following example, derived from my proof of concept project, may give some flesh to my descriptions above. I list some of my domains, together with candidate contents of the domain models.
• P [Vehicles, units, movements, exhaustion, speed...]
• P1 [Rules, applicability, states, priorities...] (a subsidiary problem domain)
• M [Object types, attributes, relationships, domains...]
• A [Events, tables, columns, database domains, classes...]
• F [Persistence mechanism, processing,.....]
• C [Database schemas, source code, SQL scripts, build scripts,....]
• R [Formalisms, transforms, mappings...]
There is at least one major disadvantage with this approach - the work involved in developing the models can be seen by management as non-productive, with no real deliverables being produced.
The sources for this answer are many and varied, but do rely heavily on works by:
1. Michael Jackson on structured programming; system analysis; and problem domain descriptions.
2. The Shlaer-Mellor method of system development by transformation rather than elaboration.
3. The Kennedy-Carter method consultancy.
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May 25, 2019
Hot Topics:
Java vs. C++: The Performance Showdown
• January 6, 2010
• By Liviu Tudor
• Send Email »
• More Articles »
Since the early days of the Java programming language, some have argued that Java's being an interpreted language has made it inferior to the likes of C and C++ in terms of performance. Of course, C++ devotees would never, ever even consider Java a “proper” language, while the Java crowd always throws “write once, run everywhere” in the faces of the C++ programmers.
First things first, how well does Java perform when it comes to basic integer arithmetic? If I asked you how much 2 x 3 is, you would probably answer in no time. How long would it take a program? To check, here’s a basic test:
1. Generate first X numbers of random integer numbers.
2. Multiply those numbers with every number from 2 to Y.
3. Compute how long it takes to perform the whole set.
Because you’re not interested in how long it takes to generate a random number, it is important that you generate the random numbers upfront, before you start measuring.
Putting Java to the Test
In Java, generating the random numbers is a very simple task:
private void generateRandoms()
{
randoms = new int[N_GENERATED];
for( int i = 0; i < N_GENERATED; i++)
{
randoms[i] = (int)(i * Math.random());
}
}
The computations are just as easy:
private void javaCompute()
{
int result = 0;
for(int i = 2; i < N_MULTIPLY; i++)
{
for( int j = 0; j < N_GENERATED; j++ )
{
result = randoms[j] * i;
result++;
}
}
}
Now, you could simply measure the time it took you to execute the javaCompute method shown above and consider that a valid test. However, that wouldn’t be fair for a few reasons. First, during the execution of a program, the JVM will load classes as needed by the classloaders. The OS itself also will prepare various data structures needed by the JVM as it requests them. So, chances are the first execution of the above function actually spends a lot of time preparing the execution. That's why measuring the time as a one-off is probably not a good idea.
You could instead run the program a few times and average the results. However, the first execution of the function will still suffer from the same problems in terms of execution time, which will prevent you from getting an accurate measurement of timing. On the other hand, the more you execute the method above, the greater the chances that some data will get cached—some by the JVM, some by the OS. As a result, you will end up with some data reflecting not necessarily the execution time but rather the result of OS code running optimization, which again will affect your averaging.
To counterbalance these problems, you can run the method above a few times and simply eliminate the “freakiest” cases: the lowest and the highest times. That should get you closer to the true average. And assuming you store the length of each time you ran the javaCompute method above in an array, here’s a simple method to achieve this:
private static long testTime( long diffs[] )
{
long shortest = Long.MAX_VALUE;
long longest = 0;
long total = 0;
for( int i = 0; i < diffs.length; i++ )
{
if( shortest > diffs[i] )
shortest = diffs[i];
if( longest < diffs[i] )
longest = diffs[i];
total += diffs[i];
}
total -= shortest;
total -= longest;
total /= ( diffs.length - 2 );
return total;
}
Now, putting all of this together, you get the following code (also found in IntMaths.java):
public static void main(String[] args)
{
IntMaths maths = new IntMaths();
maths.generateRandoms();
//compute in Java
long timeJava[] = new long[N_ITERATIONS];
long start, end;
for( int i = 0; i < N_ITERATIONS; i++ )
{
start = System.currentTimeMillis();
maths.javaCompute();
end = System.currentTimeMillis();
timeJava[i] = (end - start);
}
System.out.println( "Java computing took " + testTime(timeJava) );
}
Notice that you’re measuring time in milliseconds here.
On my laptop (which is not quite high-spec but new enough to be representative), running the above code shows an average of about 25 milliseconds. That is, it takes my laptop about 25 milliseconds to multiply 10,000 random integer numbers with each number from 2 to 1,000, which basically means it takes 25ms to perform about 10,000,000 integer arithmetic operations.
Putting C++ to the Test
Now let’s try something similar in C++:
void generate_randoms( int randoms[] )
{
for( int i = 0; i < N_GENERATED; i++ )
randoms[i] = rand();
}
To be precise, the numbers generated in Java are not the same numbers generated in C++. Therefore, you cannot ensure that both implementations will use the same set of numbers. However, the differences between the two should be quite small.
The C++ computation is similar to the Java one:
void nativeCompute(int randoms[])
{
int result = 0;
for(int i = 2; i < N_MULTIPLY; i++)
{
for( int j = 0; j < N_GENERATED; j++ )
{
result = randoms[j] * i;
result++;
}
}
}
Because C++ doesn't seem to offer a standard way to get millisecond precision for timing the operations, this code is specifically targeted to Windows platforms. Using the QueryPerformanceCounter function provides access to high-resolution timers. To use this function, the code utilizes a simple “stopwatch” class that has just two methods (Start and Stop). Based on the timing, this class records when each of these methods is called and returns the time difference in milliseconds:
// Stop watch class.
class CStopWatch
{
public:
// Constructor.
CStopWatch()
{
// Ticks per second.
QueryPerformanceFrequency( &liPerfFreq );
}
// Start counter.
void Start()
{
liStart.QuadPart = 0;
QueryPerformanceCounter( &liStart );
}
// Stop counter.
void Stop()
{
liEnd.QuadPart = 0;
QueryPerformanceCounter( &liEnd );
}
// Get duration.
long double GetDuration()
{
return ( (liEnd.QuadPart - liStart.QuadPart) / long double(liPerfFreq.QuadPart) ) * 1000; //return result in milliseconds
}
private:
LARGE_INTEGER liStart;
LARGE_INTEGER liEnd;
LARGE_INTEGER liPerfFreq;
};
You can apply a similar pattern for finding the average of the times the stopwatch class took:
long double test_times( long double diffs[] )
{
long double shortest = 65535;
long double longest = -1;
long double total = 0;
for( int i = 0; i < N_ITERATIONS; i++ )
{
if( shortest > diffs[i] )
shortest = diffs[i];
else if( longest < diffs[i] )
longest = diffs[i];
total += diffs[i];
}
total -= shortest;
total -= longest;
total /= ( N_ITERATIONS - 2 );
return total;
}
This pattern leaves you with the following implementation (present in IntMaths.c):
int main(int argc, char* argv[])
{
int randoms[N_GENERATED];
generate_randoms( randoms );
CStopWatch watch;
long double timeNative[N_ITERATIONS];
for( int i = 0; i < N_ITERATIONS; i++ )
{
watch.Start();
nativeCompute(randoms);
watch.Stop();
timeNative[i] = watch.GetDuration();
}
printf( "C computing took %lf\n", test_times(timeNative) );
return 0;
}
Find the above code in the IntMaths project included in the JavaVsCPP.zip code download.
Running the above code on my laptop returns the following (the measurement is in milliseconds):
C computing took 0.001427
So, it took the C++ code about 1/1000th of a millisecond to perform about 10,000,000 arithmetic operations!
To compare: 25ms in Java, 0.001ms in C++. That's quite a difference! However, bear in mind that the C++ code was compiled using full compiler and linker optimization for speed. Simply disabling the optimization will you return the following result:
C computing took 70.179901
Ouch! That is three times slower than the Java version! The moral of the story is: yes, C++ performs better at first glance, but (and this is a big but) only if the compiler optimizes the code well!
Most C++ compilers nowadays will perform a decent optimization of the generated code. However, the difference between 1/1000th of a millisecond and 70 milliseconds is left in the hands of the compiler. Always bear that in mind when you switch to C++ for speed reasons!
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__label__pos
| 0.996703 |
Go to main content
Oracle® x86 Servers Diagnostics and Troubleshooting Guide
Exit Print View
Updated: January 2020
Introduction to System Diagnostics and Troubleshooting
Oracle provides a wide spectrum of diagnostic and troubleshooting tools for use with Oracle x86 servers. These tools include integrated log file information, operating system diagnostics, standalone software packages such as Oracle VTS software, and hardware LED indicators, all of which contain clues helpful in narrowing down the possible sources of a problem.
Some diagnostic tools stress the system by running tests in parallel, while other tools run sequential tests, enabling the system to continue its normal functions. Some diagnostic tools function on Standby power or when the system is offline, while others require the operating system to be up and running.
This section describes the Oracle diagnostic tools for x86 servers equipped with Oracle ILOM Firmware Releases 4.x and 5.x. It includes the following topics:
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__label__pos
| 0.583346 |
簡単なenum書いた
<?php
class Enum
{
protected $namelist = array(), $currentnum = 0;
static function create(Array $arr)
{
$ins = new self;
foreach ($arr as $name) $ins->add($name);
return $ins;
}
function add($name, $num = null)
{
if (!is_null($num)) $this->currentnum = $num;
if (isset($this->namelist[$name])) throw new Exception;
$this->namelist[$name] = $this->currentnum++;
return $this;
}
function get($name)
{
if (!isset($this->namelist[$name])) throw new Exception;
return $this->namelist[$name];
}
function __get($name)
{
return $this->get($name);
}
}
function enum()
{
return Enum::create(func_get_args());
}
使い方
<?php
$hoge = @enum(Hoge, Fuga, Piyo, Foo);
// もしくは $hoge = enum('Hoge', 'Fuga', 'Piyo', 'Foo');
$hoge->Hoge; //=>0
$hoge->Piyo; //=>2
$hoge->add(@Bar, 10);
// もしくは $hoge->add('Bar', 10);
$hoge->Bar; //=>10
@を使うと結構スマートにかける。ただし、予約語や定数として登録されているものを使いたい時は素直にクオートする必要あり。
|
__label__pos
| 0.961041 |
We use cookies to personalise content and advertisements and to analyse access to our website. Furthermore, our partners for online advertising receive pseudonymised information about your use of our website. cookie policy and privacy policy.
+0
0
119
1
avatar
If a, b, c and d are positive real numbers such that log_a b= 8/9, log_b c= -(3/4), log_c d = 2,
find the value of log_d (abc)
Feb 22, 2019
#1
avatar+23076
+3
If a, b, c and d are positive real numbers such that
log_a b= 8/9,
log_b c= -(3/4),
log_c d = 2,
find the value of log_d (abc)
\(\begin{array}{|rclcl|} \hline \log_a(b) &=& \dfrac{\log_d(b)}{\log_d(a)} &=& \dfrac{\log_b(b)}{\log_b(a)} \\\\ & & \dfrac{\log_d(b)}{\log_d(a)} &=& \dfrac{\log_b(b)}{\log_b(a)} \quad | \quad \log_b(b) = 1 \\\\ & & \dfrac{\log_d(b)}{\log_d(a)} &=& \dfrac{1}{\log_b(a)} \quad | \quad \log_a(b)\log_b(a)=1 \\\\ & & \dfrac{\log_d(b)}{\log_d(a)} &=& \log_a(b) \\\\ & & \dfrac{\log_d(a)} {\log_d(b)}&=& \dfrac{1}{\log_a(b)} \\\\ & & \mathbf{\log_d(a)} & \mathbf{=} & \mathbf{ \dfrac{\log_d(b)}{\log_a(b)} } \qquad (1) \\ \hline \log_b(c) &=& \dfrac{\log_d(c)}{\log_d(b)} &=& \dfrac{\log_c(c)}{\log_c(b)} \\\\ & & \dfrac{\log_d(c)}{\log_d(b)} &=& \dfrac{\log_c(c)}{\log_c(b)} \quad | \quad \log_c(c) = 1 \\\\ & & \dfrac{\log_d(c)}{\log_d(b)} &=& \dfrac{1}{\log_c(b)} \quad | \quad \log_b(c)\log_c(b)=1 \\\\ & & \dfrac{\log_d(c)}{\log_d(b)} &=& \log_b(c) \\\\ & & \dfrac{\log_d(b)}{\log_d(c)} &=& \dfrac{1}{\log_b(c)} \\\\ & & \mathbf{\log_d(b)} &\mathbf{=}& \mathbf{\dfrac{\log_d(c)}{\log_b(c)}} \qquad (2) \\ \hline && \log_c(d) &=& \dfrac{\log_d(d)}{\log_d(c)} \quad | \quad \log_d(d) = 1 \\\\ && \log_c(d) &=& \dfrac{1}{\log_d(c)} \\\\ & & \mathbf{\log_d(c)} &\mathbf{=}& \mathbf{\dfrac{1}{\log_c(d)}} \qquad (3) \\ \hline \end{array}\)
\(\begin{array}{|rcll|} \hline \log_d(abc) &=& \log_d(a)+\log_d(b)+\log_d(c) \quad | \quad \mathbf{\log_d(a)=\dfrac{\log_d(b)}{\log_a(b)} } \qquad (1) \\ &=& \dfrac{\log_d(b)}{\log_a(b)}+\log_d(b)+\log_d(c) \\ &=& \log_d(b) \left( \dfrac{1}{\log_a(b)}+1 \right) +\log_d(c) \quad | \quad \mathbf{\log_d(b)=\dfrac{\log_d(c)}{\log_b(c)}} \qquad (2) \\ &=& \dfrac{\log_d(c)}{\log_b(c)} \left( \dfrac{1}{\log_a(b)}+1 \right) +\log_d(c) \\ &=&\log_d(c)\left( \dfrac{1}{\log_b(c)} \left( \dfrac{1}{\log_a(b)}+1 \right) + 1 \right) \quad | \quad \mathbf{\log_d(c)=\dfrac{1}{\log_c(d)}} \qquad (3) \\ \mathbf{\log_d(abc)} & \mathbf{=} & \mathbf{\dfrac{1}{\log_c(d)}\left( \dfrac{1}{\log_b(c)} \left( \dfrac{1}{\log_a(b)}+1 \right) + 1 \right)} \\ \hline \end{array}\)
\(\begin{array}{|rcll|} \hline \mathbf{\log_d(abc)} & \mathbf{=} & \mathbf{\dfrac{1}{\log_c(d)}\left( \dfrac{1}{\log_b(c)} \left( \dfrac{1}{\log_a(b)}+1 \right) + 1 \right)} \\\\ & = & \dfrac{1}{2}\left( \dfrac{1}{ -\dfrac{3}{4} } \left( \dfrac{1}{\dfrac{8}{9}}+1 \right) + 1 \right) \\\\ & = & \dfrac{1}{2}\left( \dfrac{-4}{3} \Big( \dfrac{1}{8}+1 \Big) + 1 \right) \\\\ & = & \dfrac{1}{2}\left( \dfrac{-4}{3} \left( \dfrac{17}{8} \right) + 1 \right) \\\\ & = & \dfrac{1}{2}\left( \dfrac{-17}{6} + 1 \right) \\\\ & = & \dfrac{1}{2}\left( \dfrac{-11}{6} \right) \\\\ & \mathbf{=} & -\mathbf{\dfrac{11}{12}} \\ \hline \end{array}\)
laugh
Feb 22, 2019
edited by heureka Feb 22, 2019
12 Online Users
avatar
|
__label__pos
| 0.999193 |
open-falcon安装使用监控树莓派
简介
open-falcon是一款用golang和python写的监控系统，由小米启动这个项目。 官方网址 http://open-falcon.org/ 文档 https://book.open-falcon.org/zh_0_2/ 今天我要安装的是0.2版本，监控的是树莓派。系统是阿里云ecs centos7.4
安装准备
• 安装redis yum install redis 启动redis服务 systemctl start redis 让redis开机自启 systemctl enable redis
• 安装mariadb yum install mariadb-server 启动mariadb服务 systemctl start mariadb 让mariadb开机启动 systemctl enable mariadb 初始化mariadb密码
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[[email protected] ~]# mysql_secure_installation
NOTE: RUNNING ALL PARTS OF THIS SCRIPT IS RECOMMENDED FOR ALL MariaDB
SERVERS IN PRODUCTION USE! PLEASE READ EACH STEP CAREFULLY!
In order to log into MariaDB to secure it, we'll need the current
password for the root user. If you've just installed MariaDB, and
you haven't set the root password yet, the password will be blank,
so you should just press enter here.
Enter current password for root (enter for none):
ERROR 1045 (28000): Access denied for user 'root'@'localhost' (using password: YES)
Enter current password for root (enter for none):
OK, successfully used password, moving on...
Setting the root password ensures that nobody can log into the MariaDB
root user without the proper authorisation.
Set root password? [Y/n] y
New password:
Re-enter new password:
Password updated successfully!
Reloading privilege tables..
... Success!
By default, a MariaDB installation has an anonymous user, allowing anyone
to log into MariaDB without having to have a user account created for
them. This is intended only for testing, and to make the installation
go a bit smoother. You should remove them before moving into a
production environment.
Remove anonymous users? [Y/n] y
... Success!
Normally, root should only be allowed to connect from 'localhost'. This
ensures that someone cannot guess at the root password from the network.
Disallow root login remotely? [Y/n] y
... Success!
By default, MariaDB comes with a database named 'test' that anyone can
access. This is also intended only for testing, and should be removed
before moving into a production environment.
Remove test database and access to it? [Y/n] y
- Dropping test database...
... Success!
- Removing privileges on test database...
... Success!
Reloading the privilege tables will ensure that all changes made so far
will take effect immediately.
Reload privilege tables now? [Y/n] y
... Success!
Cleaning up...
All done! If you've completed all of the above steps, your MariaDB
installation should now be secure.
Thanks for using MariaDB!
我解释下上面每步都是做什么的 Enter current password for root (enter for none): 当你第一次运行时直接回车就好，就是问你原来的root密码是什么，因为第一次运行，所以直接回车就好 Set root password? [Y/n] y 问你是不是设置root密码 Remove anonymous users? [Y/n] y 问你是不是移除匿名用户 Disallow root login remotely? [Y/n] y 禁止root远程登录 Remove test database and access to it? [Y/n] y 移除test数据库 Reload privilege tables now? [Y/n] y 重新加载权限表 最后运行mysql -uroot -p登入mariadb验证下
• 导入open-falcon数据表 安装git yum install git 下载falcon-plus里面有open-falcon的数据库导入就好 git clone https://github.com/open-falcon/falcon-plus.git 进入下面这个目录，然后执行下面命令 cd /root/falcon-plus/scripts/mysql/db_schema mysql -h 127.0.0.1 -u root -p < 1_uic-db-schema.sql mysql -h 127.0.0.1 -u root -p < 2_portal-db-schema.sql mysql -h 127.0.0.1 -u root -p < 3_dashboard-db-schema.sql mysql -h 127.0.0.1 -u root -p < 4_graph-db-schema.sql mysql -h 127.0.0.1 -u root -p < 5_alarms-db-schema.sql
之后你会发现数据库中多出了下面这些数据库
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+--------------------+
| Database |
+--------------------+
| information_schema |
| alarms |
| dashboard |
| falcon_portal |
| graph |
| mysql |
| performance_schema |
| uic |
+--------------------+
• 下载open-falcon的安装包
如果你是其他平台的比如arm或是32位系统的要自己编译下，我就直接使用二进制包安装好了 wget https://github.com/open-falcon/falcon-plus/releases/download/v0.2.1/open-falcon-v0.2.1.tar.gz 因为网速较慢所以建议本地挂代理下载之后上传到服务器上
启动后端
因为是编译好的包，所以我们就不需要各种复杂的设置了，直接启动就好了 我把所有文件放在home文件夹中work下 mkdir -p /home/work/open-falcon tar -zxvf open-falcon-v0.2.1.tar.gz -C /home/work/open-falcon/ 修改配置文件，其实就是修改所有组件的cfg.json的mariadb密码，我的意思是如果这个组件用到mariadb的话 首先修改aggregator的 vim /home/work/open-falcon/aggregator/config/cfg.json 改为
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"database": {
"addr": "root:[email protected](127.0.0.1:3306)/falcon_portal?loc=Local&parseTime=true",
"idle": 10,
"ids": [1, -1],
"interval": 55
},
注意第二行Yuncan1803是密码，root是用户名其他的都一样 接着修改graph的 vim graph/config/cfg.json
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"db": {
"dsn": "root:[email protected](127.0.0.1:3306)/graph?loc=Local&parseTime=true",
"maxIdle": 4
},
修改hbs的 vim hbs/config/cfg.json
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{
"debug": true,
"database": "root:[email protected](127.0.0.1:3306)/falcon_portal?loc=Local&parseTime=true",
"hosts": "",
"maxConns": 20,
"maxIdle": 15,
"listen": ":6030",
"trustable": [""],
"http": {
"enabled": true,
"listen": "0.0.0.0:6031"
}
}
修改nodata的 vim nodata/config/cfg.json
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"config": {
"enabled": true,
"dsn": "root:[email protected](127.0.0.1:3306)/falcon_portal?loc=Local&parseTime=true&wait_timeout=604800",
"maxIdle": 4
},
修改api的 vim api/config/cfg.json
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"db": {
"falcon_portal": "root:[email protected](127.0.0.1:3306)/falcon_portal?charset=utf8&parseTime=True&loc=Local",
"graph": "root:[email protected](127.0.0.1:3306)/graph?charset=utf8&parseTime=True&loc=Local",
"uic": "root:[email protected](127.0.0.1:3306)/uic?charset=utf8&parseTime=True&loc=Local",
"dashboard": "root:[email protected](127.0.0.1:3306)/dashboard?charset=utf8&parseTime=True&loc=Local",
"alarms": "root:[email protected](127.0.0.1:3306)/alarms?charset=utf8&parseTime=True&loc=Local",
"db_bug": true
},
修改alarm的 vim alarm/config/cfg.json
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"falcon_portal": {
"addr": "root:[email protected](127.0.0.1:3306)/alarms?charset=utf8&loc=Asia%2FChongqing",
"idle": 10,
"max": 100
},
最好，终于他妈最后了，启动后端 ./open-falcon start 你可以用下面的命令检查所有组件的状态 ./open-falcon check
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[[email protected] open-falcon]# ./open-falcon check
falcon-graph UP 27849
falcon-hbs UP 27858
falcon-judge UP 27868
falcon-transfer UP 27876
falcon-nodata UP 27883
falcon-aggregator UP 27891
falcon-agent UP 27901
falcon-gateway UP 27908
falcon-api UP 27915
falcon-alarm UP 27929
更多命令的用法 ./open-falcon [start|stop|restart|check|monitor|reload] module 比如./open-falcon start agent 这样就启动了agent ./open-falcon check这个是检查组件状态的 所有组件的日志都在组件目录下logs目录中
安装前端
首先把代码clone下来 cd /home/work/open-falcon git clone https://github.com/open-falcon/dashboard.git 安装依赖包 yum install -y python-virtualenv yum install -y python-devel yum install -y openldap-devel yum install -y mysql-devel yum groupinstall "Development tools"
新建虚拟环境 cd dashboard virtualenv ./env 安装python依赖 ./env/bin/pip install -r pip_requirements.txt 修改配置 vim rrd/config.py PORTAL_DB_PASS = os.environ.get("PORTAL_DB_PASS","Yuncan1803") ALARM_DB_PASS = os.environ.get("ALARM_DB_PASS","Yuncan1803") 在生产环境启动
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[[email protected] dashboard]# bash control start
falcon-dashboard started..., pid=8190
下面是一些其他操作 开发者模式启动 ./env/bin/python wsgi.py
停止 bash control stop
查看日志 bash control tail
账号管理
首先我们用ip:8081访问open-falcon，接着要知道的是falcon默认没有任何用户，新建的第一个root用户就是管理员，为了安全你可以修改api组件的配置文件 vim api/config/cfg.json 中的 "signup_disable": false, 设置为true就可以禁止注册了
树莓派上安装
这个肯定是要编译的了，所以先下载golang安装 因为实在树莓派下，所以golang要编译安装，但是现在最新版本的golang都是用go编译的，所以因为我们系统中没有go，只能先安装1.4版本的c语言版本go，之后安装最新版本的go 首先下载 wget https://dl.google.com/go/go1.4.3.src.tar.gz 解压 tar -zxvf go1.4.3.src.tar.gz 编译 cd go/src ./make.bash 因为是树莓派所以编译肯定花点时间 移动go到/root/go1.4 mv go /root/go1.4 /root/go1.4是默认$GOROOT_BOOTSTRAP的值 接着下载最新版本的go wget https://dl.google.com/go/go1.9.3.src.tar.gz 解压 tar -zxvf go1.9.3.src.tar.gz 编译 cd go/src ./make.bash 编译好后接着移动到/usr/local/目录下 mv go /usr/local 设置环境变量 vim ~/.zshrc
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export GOROOT=/usr/local/go
export GOPATH=/root/go
export PATH=$PATH:GOROOT/bin
使环境变量生效 source ~/.zshrc
接着编译安装open-falcon的agent 新建一个目录下载源码 mkdir -p $GOPATH/src/github.com/open-falcon cd $GOPATH/src/github.com/open-falcon 下载源码 git clone https://github.com/open-falcon/falcon-plus.git 进入agent源码目录 cd falcon-plus/modules/agent 下载依赖 go get 编译 ./control build 接着把agent这个文件夹放入/usr/loca/目录 mv agent /usr/local 启动agent生成配置文件 cd /usr/local/agent ./control start 编辑配置文件
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{
"debug": true,
"hostname": "raspberry",
"ip": "",
"plugin": {
"enabled": false,
"dir": "./plugin",
"git": "https://github.com/open-falcon/plugin.git",
"logs": "./logs"
},
"heartbeat": {
"enabled": true,
"addr": "ip:6030",
"interval": 60,
"timeout": 1000
},
"transfer": {
"enabled": true,
"addrs": [
"ip:8433"
],
"interval": 60,
"timeout": 1000
},
"http": {
"enabled": true,
"listen": ":1988",
"backdoor": false
},
"collector": {
"ifacePrefix": ["eth", "em"],
"mountPoint": []
},
"default_tags": {
},
"ignore": {
"cpu.busy": true,
"df.bytes.free": true,
"df.bytes.total": true,
"df.bytes.used": true,
"df.bytes.used.percent": true,
"df.inodes.total": true,
"df.inodes.free": true,
"df.inodes.used": true,
"df.inodes.used.percent": true,
"mem.memtotal": true,
"mem.memused": true,
"mem.memused.percent": true,
"mem.memfree": true,
"mem.swaptotal": true,
"mem.swapused": true,
"mem.swapfree": true
}
}
重启 ./control restart 和zabbix比起来这个配置真的少了很多，除了HOSTNAME以外就是ip要改，而且挺简单的，dashboard上也不用设置，等会就会有数据出来
欢迎关注我的博客 www.bboy.app Have Fun
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This is documentation for Caché & Ensemble. See the InterSystems IRIS version of this content.
For information on migrating to InterSystems IRIS, see Why Migrate to InterSystems IRIS?
Users, Roles, and Privileges
Caché has both system-level security, and an additional set of SQL-related security features. Caché security provides an additional level of security capabilities beyond its database-level protections. Some of the key differences between SQL and system-level security are:
• SQL protections are more granular than system-level protections. You can define privileges for tables, views, and stored procedures.
• SQL privileges can be granted to users as well as to roles. System-level privileges are only assigned to roles.
• Holding an SQL privilege implicitly grants any related system privileges that are required to perform the SQL action. (Conversely, system-level privileges do not imply table-level privileges.) The different types of privileges are described in the “SQL Privileges and System Privileges” section.
InterSystems SQL enforces privilege checking for ODBC, JDBC, Dynamic SQL, and the Caché SQL Shell interface. Embedded SQL statements do not perform privilege checking; it is assumed that applications using Embedded SQL will check for privileges before using Embedded SQL statements.
This chapter discusses the following topics:
SQL Privileges and System Privileges
To manipulate tables or other SQL entities through SQL-specific mechanisms, a user must have the appropriate SQL privileges. System-level privileges are not sufficient. A user may be granted SQL privileges directly, or the user may belong to a role that has SQL privileges.
Note:
Roles are shared by SQL and system level security: a single role can include both system and SQL privileges.
Consider the following example for an instance of Caché on a Windows machine:
• There is a persistent class in the USER namespace called User.MyPerson. This class is projected to SQL as the SQLUser.MyPerson table.
• There is a user called Test, who belongs to no roles (and therefore has no system privileges) and who has all privileges on the SQLUser.MyPerson table (and no other SQL privileges).
• There is a second user, called Test2. This user is assigned to the following roles: %DB_USER (and so can read or write data on the USER database); %SQL (and so has SQL access through the %Service_Bindings service); and, through a custom role, has privileges for using the Console and %Development.
If the Test user attempts to read or write data in the SQLUser.MyPerson table through any SQL-specific mechanism (such as one that uses ODBC), the attempt succeeds. This is because Caché makes the Test user a member of the %SQL role (which includes the %Service_SQL:Use privilege) and the %DB_USER role, so the user has the necessary privileges to establish the connection; this is visible in audit events that the connection generates, such as the %System/%Login/Login event. (If the Test user attempts to use Terminal object mechanisms, these attempts fail, because the user lacks sufficient privilege for these.)
If the Test2 user attempts to read or write data in the SQLUser.MyPerson table through any SQL-specific mechanism (such as one that uses ODBC), the attempt fails because the user does not have sufficient privileges for the table. (If the Test2 user attempts to view the same data in the Terminal using object mechanisms, the attempt succeeds — because the user is sufficiently privileged for this type of connection.)
Users
An InterSystems SQL user is the same as a user defined for InterSystems security. You can define a user using either SQL commands or the Management Portal.
• In SQL you use the CREATE USER statement to create a user. This simply creates a user name and user password. The newly created user has no roles. You must use the GRANT statement to assign privileges and roles to the user. You can use the ALTER USER and DROP USER statements to modify existing user definitions.
• In the Management Portal Select System Administration select Security, then select Users. Click the Create New User button at the top of the page. This takes you to the Edit User page where you can specify the user name, user password, and other parameters. Once you create a user, the other tabs become available, where you can specify which roles a user holds, which general SQL privileges the user holds, which table-level privileges the user holds, which views are available, and which stored procedures can be executed.
If a user has SQL table privileges, or general SQL privileges, then roles granted or revoked on the user’s Roles tab do not affect a user’s access to tables through SQL-based services, such as ODBC. This is because, in the SQL-based services, table-based privileges take precedence over resource-based privileges.
You can use %Library.SQLCatalogPriv class queries to list:
• All users SQLUsers()
• All privileges granted to a specified user SQLUserPrivs(“username”)
• All system privileges granted to a specified user SQLUserSysPrivs(“username”)
• All roles granted to a specified user SQLUserRole(“username”)
The following example lists the privileges granted to the current user:
SET statemt=##class(%SQL.Statement).%New()
SET cqStatus=statemt.%PrepareClassQuery("%Library.SQLCatalogPriv","SQLUserPrivs")
IF cqStatus'=1 {WRITE "%PrepareClassQuery failed:" DO $System.Status.DisplayError(cqStatus) QUIT}
SET rset=statemt.%Execute($USERNAME)
WRITE "Privileges for ",$USERNAME
DO rset.%Display()
User Name as Schema Name
Under some circumstances, a username can be implicitly used as an SQL schema name. This may pose problems if the username contains characters that are forbidden in an SQL identifier. For example, in a multiple domain configuration the username contains the “@” character.
Caché handles this situation differently depending on the setting of the Delimited Identifiers configuration parameter:
• If the use of delimited identifiers is enabled, no special processing occurs.
• If the use of delimited identifiers is disabled, then any forbidden characters are removed from the username to form a schema name. For example, the username “[email protected]” would become the schema name “documentationintersystemscom”.
This does not affect the value returned by the SQL CURRENT_USER function. It is always the same as $USERNAME.
Roles
SQL privileges are assigned to a user or role. A role enables you to set the same privileges for multiple users. Roles are shared by SQL and system level security: a single role can include both system privileges and SQL privileges.
The Management Portal, System Administration, Security, Roles page provides a list of role definitions for a Caché instance. To view or change details on a particular role, select the Name link for the role. On the Edit Role page that appears, there is information regarding the roles privileges, and which users or roles hold it.
The General tab lists a role’s privileges for InterSystems security resources. If a role only holds SQL privileges, the General tab’s Resources table lists the role’s privileges as “None defined.”
The SQL Privileges tab lists a role’s privileges for InterSystems SQL resources, where a drop-down list of namespaces allows you to view each namespace’s resources. Because privileges are listed by namespace, the listing for a role holding no privileges in a particular namespace displays “None.”
Note:
You should define privileges using roles and associate specific users with these roles. There are two reasons for this:
1. It is much more efficient for the SQL Engine to determine privilege levels by checking a relatively small role database than by checking individual user entries.
2. It is much easier to administer a system using a small set of roles as compared with a system with many individual user settings.
For example, you can define a role called “ACCOUNTING” with certain access privileges. As the Accounting Department grows, you can define new users and associate them with the ACCOUNTING role. If you need to modify the privileges for ACCOUNTING, you can do it once and it will automatically cover all the members of the Accounting Department.
A role can hold other roles. For example, the ACCOUNTING role can hold the BILLINGCLERK role. A user granted the ACCOUNTING role would have the privileges of both the ACCOUNTING role and the BILLINGCLERK role.
You can also define users and roles with the following SQL commands: CREATE USER, CREATE ROLE, ALTER USER, GRANT, DROP USER, and DROP ROLE.
You can use %Library.SQLCatalogPriv class queries to list:
• All roles SQLRoles()
• All privileges granted to a specified role SQLRolePrivileges(“rolename”)
• All roles or users granted to a specified role SQLRoleUser(“rolename”)
• All roles granted to a specified user SQLUserRole(“username”)
SQL Privileges
SQL privileges are assigned to a user or role. A role enables you to set the same privileges for multiple users.
InterSystems SQL supports two types of privileges: administrative and object.
• Administrative privileges are namespace-specific.
Administrative privileges cover the creation, altering, and deleting of types of objects, such as the %CREATE_TABLE privilege required to create tables. The %ALTER_TABLE privilege is required not only to alter a table, but to create or drop an index and to create or drop a trigger.
Administrative privileges also include %NOCHECK, %NOINDEX, %NOLOCK, and %NOTRIGGER, which determine whether the user can apply the corresponding keyword restrictions when performing an INSERT, UPDATE, INSERT OR UPDATE, or DELETE. Assigning the %NOTRIGGER administrative privilege is required for a user to perform a TRUNCATE TABLE.
• Object privileges are specific to a table, view, or stored procedure. They specify the type of access to specific named SQL objects (in the SQL sense of the word: a table, a view, a column, or a stored procedure). If the user is the Owner (creator) of the SQL object, the user is automatically granted all privileges for that object.
Table-level object privileges provide access (%ALTER, DELETE, SELECT, INSERT, UPDATE, EXECUTE, REFERENCES) to the data in all columns of a table or view, both those columns that currently exist and any subsequently added columns.
Column-level object privileges provide access to the data in only the specified columns of a table or view. You do not need to assign column-level privileges for columns with system-defined values, such as RowID and Identity.
Stored procedure object privileges permit the assignment of EXECUTE privilege for the procedure to specified users or roles.
For further details, refer to the GRANT command.
Granting SQL Privileges
You can grant privileges in the following ways:
• Use the Management Portal. From System Administration select Security, then select either Users or Roles. Select the desired user or role, then select the appropriate tab: SQL Privileges for administrative privileges, SQL Tables, SQL Views, or SQL Procedures for object privileges.
• From SQL, use the GRANT command to grant specific administrative privileges or object privileges to a specified user or role (or list of users or roles). You can use the REVOKE command to remove privileges.
• From ObjectScript, use the %SYSTEM.SQL.GrantObjPriv() method to grant specific object privileges to a specified user (or list of users).
Listing SQL Privileges
• Use the Management Portal. From System Administration select Security, then select either Users or Roles. Select the desired user or role, then select the appropriate tab: SQL Privileges for administrative privileges, SQL Tables, SQL Views, or SQL Procedures for object privileges.
• From SQL, use the %CHECKPRIV command to determine if the current user has a specific administrative or object privilege.
• From ObjectScript, use the $SYSTEM.SQL.CheckPriv() method to determine if a specified user has a specific object privilege.
Feedback
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Moving /var to a new drive & renaming mount point
Discussion in 'Technical' started by tdd_topdog, Jul 9, 2006.
1. tdd_topdog
tdd_topdog New Member
I am a novice linux user. Everything is working perfectly. I am knee deep in working on the website. I need to address my available hard drive space at some point. I used the perfect debian howto and ispconfig. I have two hard drives and five partitions. When I partitioned I dedicated a hard drive to /backup and /home. When I installed ispconfig I followed the recomendation and changed /home/www to /var/www. /dev/hda is an old 6.4gb western digital which is going to run out of space for my www directories which contain howto videos (non-pornographic :) ). It's not mission critical because I am only using this box as a test server and not a production server but it would be nice if I can create enough space to test everything before I upload to a web host.
/etc/fstab:
/proc /proc proc defaults 0 0
/dev/hda2 / ext3 defaults,errors=remount-ro,usrquota,grpquota 0 1
/dev/hdc2 /backup ext3 defaults 0 2
/dev/hdc3 /home ext3 defaults 0 2
/dev/hda1 none swap sw 0 0
/dev/hdc1 none swap sw 0 0
/dev/hdb /media/cdrom0 iso9660 ro,user,noauto
I know at some point I will have to rename and move /var and I think I can do this:
mkdir /xvar
cp -av /var/* /xvar
rm -r /var
Here are my questions:
1. Can I resize /home partition, ext3 file system, and create a new partition with mount point /var?
2. Is there a way to resize the partition and file system at the same time?
3. Or can I rename /backup mount point to /var?
I can unmount /backup but cant remount it with a new name doing this:
umount /dev/hdc2 /backup
mount /dev/hdc2 /var
this gives me an error message
if i would have been able to remount i am under the impression i can just change the /etc/fstab file at that point.
4. Would one option be more reliable or easier than the other?
Any suggestions and or help appreciated. Please keep any replies on a 'newbies' level :confused:
2. falko
falko Super Moderator Howtoforge Staff Moderator HowtoForge Supporter ISPConfig Developer
You can try to reconfigure your partitions with
Code:
cfdisk
But please keep in mind that it's possible that you lose data!
3. tdd_topdog
tdd_topdog New Member
Thank you falko I will read up on cfdisk
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Tweak QgsProcessingUtils::combineLayerExtents for future proofing, re…
…move deprecated usage
• Loading branch information
nyalldawson committed Apr 17, 2019
1 parent 82f2cb1 commit b6bc1ee2d48c9e19e60a84e30b64a848010f93ba
@@ -177,11 +177,11 @@ temporary layer store.
%End
static QgsRectangle combineLayerExtents( const QList<QgsMapLayer *> &layers, const QgsCoordinateReferenceSystem &crs, const QgsCoordinateTransformContext &transformContext );
static QgsRectangle combineLayerExtents( const QList<QgsMapLayer *> &layers, const QgsCoordinateReferenceSystem &crs, QgsProcessingContext &context );
%Docstring
Combines the extent of several map ``layers``. If specified, the target ``crs``
will be used to transform the layer's extent to the desired output reference system
using the specified ``transformContext``.
using the specified ``context``.
.. versionadded:: 3.10
%End
@@ -489,9 +489,9 @@ def processInputs(self, parameters, context, feedback):
elif layer.type() == QgsMapLayerType.VectorLayer:
self.loadVectorLayer(layerName, layer, external=None, feedback=feedback)
self.postInputs()
self.postInputs(context)
def postInputs(self):
def postInputs(self, context):
"""
After layer imports, we need to update some internal parameters
"""
@@ -500,7 +500,7 @@ def postInputs(self):
# Build GRASS region
if self.region.isEmpty():
self.region = QgsProcessingUtils.combineLayerExtents(self.inputLayers)
self.region = QgsProcessingUtils.combineLayerExtents(self.inputLayers, context)
command = 'g.region n={} s={} e={} w={}'.format(
self.region.yMaximum(), self.region.yMinimum(),
self.region.xMaximum(), self.region.xMinimum()
@@ -33,7 +33,7 @@ def processInputs(alg, parameters, context, feedback):
# Use v.in.ogr
for name in ['first', 'second']:
alg.loadRasterLayerFromParameter(name, parameters, context, False, None)
alg.postInputs()
alg.postInputs(context)
def processOutputs(alg, parameters, context, feedback):
@@ -37,7 +37,7 @@ def processInputs(alg, parameters, context, feedback):
# Use v.in.ogr
for name in ['first', 'second']:
alg.loadRasterLayerFromParameter(name, parameters, context, False, None)
alg.postInputs()
alg.postInputs(context)
def processCommand(alg, parameters, context, feedback):
@@ -52,7 +52,7 @@ def processInputs(alg, parameters, context, feedback):
parameters, context,
False, None)
alg.loadRasterLayerFromParameter('map', parameters, context)
alg.postInputs()
alg.postInputs(context)
def processCommand(alg, parameters, context, feedback):
@@ -54,7 +54,7 @@ def processInputs(alg, parameters, context, feedback):
if raster:
alg.loadRasterLayerFromParameter('raster', parameters, context, False, None)
alg.postInputs()
alg.postInputs(context)
def processCommand(alg, parameters, context, feedback):
@@ -35,7 +35,7 @@ def processInputs(alg, parameters, context, feedback):
# We need to import all the bands and color tables of the input raster
alg.loadRasterLayerFromParameter('map', parameters, context, False, None)
alg.postInputs()
alg.postInputs(context)
def processCommand(alg, parameters, context, feedback):
@@ -42,7 +42,7 @@ def processInputs(alg, parameters, context, feedback):
# We need to import without r.external
alg.loadRasterLayerFromParameter('map', parameters, context, False)
alg.postInputs()
alg.postInputs(context)
def processCommand(alg, parameters, context, feedback):
@@ -32,7 +32,7 @@ def processInputs(alg, parameters, context, feedback):
# We need to import all the bands and color tables of the input raster
alg.loadRasterLayerFromParameter('input', parameters, context, False, None)
alg.postInputs()
alg.postInputs(context)
def processCommand(alg, parameters, context, feedback):
@@ -34,4 +34,4 @@ def processInputs(alg, parameters, context, feedback):
# and we can use r.external for the raster
alg.loadVectorLayerFromParameter('input', parameters, context, feedback, False)
alg.loadRasterLayerFromParameter('raster', parameters, context, True)
alg.postInputs()
alg.postInputs(context)
@@ -42,4 +42,4 @@ def processInputs(alg, parameters, context, feedback):
# We need to import the vector layer with v.in.ogr
alg.loadVectorLayerFromParameter('input', parameters, context, feedback, False)
alg.postInputs()
alg.postInputs(context)
@@ -127,7 +127,7 @@ def processAlgorithm(self, parameters, context, feedback):
bbox = transform.transformBoundingBox(bbox)
if bbox.isNull() and layers:
bbox = QgsProcessingUtils.combineLayerExtents(layers, crs)
bbox = QgsProcessingUtils.combineLayerExtents(layers, crs, context)
cellsize = self.parameterAsDouble(parameters, self.CELLSIZE, context)
if cellsize == 0 and not layers:
@@ -627,7 +627,7 @@ void QgsProcessingUtils::createFeatureSinkPython( QgsFeatureSink **sink, QString
}
QgsRectangle QgsProcessingUtils::combineLayerExtents( const QList<QgsMapLayer *> &layers, const QgsCoordinateReferenceSystem &crs, const QgsCoordinateTransformContext &transformContext )
QgsRectangle QgsProcessingUtils::combineLayerExtents( const QList<QgsMapLayer *> &layers, const QgsCoordinateReferenceSystem &crs, QgsProcessingContext &context )
{
QgsRectangle extent;
for ( const QgsMapLayer *layer : layers )
@@ -638,7 +638,7 @@ QgsRectangle QgsProcessingUtils::combineLayerExtents( const QList<QgsMapLayer *>
if ( crs.isValid() )
{
//transform layer extent to target CRS
QgsCoordinateTransform ct( layer->crs(), crs, transformContext );
QgsCoordinateTransform ct( layer->crs(), crs, context.transformContext() );
try
{
QgsRectangle reprojExtent = ct.transformBoundingBox( layer->extent() );
@@ -662,7 +662,8 @@ QgsRectangle QgsProcessingUtils::combineLayerExtents( const QList<QgsMapLayer *>
// Deprecated
QgsRectangle QgsProcessingUtils::combineLayerExtents( const QList<QgsMapLayer *> &layers, const QgsCoordinateReferenceSystem &crs )
{
return QgsProcessingUtils::combineLayerExtents( layers, crs, QgsCoordinateTransformContext( ) );
QgsProcessingContext context;
return QgsProcessingUtils::combineLayerExtents( layers, crs, context );
}
QVariant QgsProcessingUtils::generateIteratingDestination( const QVariant &input, const QVariant &id, QgsProcessingContext &context )
@@ -217,10 +217,10 @@ class CORE_EXPORT QgsProcessingUtils
/**
* Combines the extent of several map \a layers. If specified, the target \a crs
* will be used to transform the layer's extent to the desired output reference system
* using the specified \a transformContext.
* using the specified \a context.
* \since QGIS 3.10
*/
static QgsRectangle combineLayerExtents( const QList<QgsMapLayer *> &layers, const QgsCoordinateReferenceSystem &crs, const QgsCoordinateTransformContext &transformContext );
static QgsRectangle combineLayerExtents( const QList<QgsMapLayer *> &layers, const QgsCoordinateReferenceSystem &crs, QgsProcessingContext &context );
/**
* Combines the extent of several map \a layers. If specified, the target \a crs
@@ -6236,7 +6236,8 @@ void TestQgsProcessing::checkParamValues()
void TestQgsProcessing::combineLayerExtent()
{
QgsRectangle ext = QgsProcessingUtils::combineLayerExtents( QList< QgsMapLayer *>() );
QgsProcessingContext context;
QgsRectangle ext = QgsProcessingUtils::combineLayerExtents( QList< QgsMapLayer *>(), context );
QVERIFY( ext.isNull() );
QString testDataDir = QStringLiteral( TEST_DATA_DIR ) + '/'; //defined in CmakeLists.txt
@@ -6248,20 +6249,20 @@ void TestQgsProcessing::combineLayerExtent()
QFileInfo fi2( raster2 );
std::unique_ptr< QgsRasterLayer > r2( new QgsRasterLayer( fi2.filePath(), "R2" ) );
ext = QgsProcessingUtils::combineLayerExtents( QList< QgsMapLayer *>() << r1.get() );
ext = QgsProcessingUtils::combineLayerExtents( QList< QgsMapLayer *>() << r1.get(), context );
QGSCOMPARENEAR( ext.xMinimum(), 1535375.000000, 10 );
QGSCOMPARENEAR( ext.xMaximum(), 1535475, 10 );
QGSCOMPARENEAR( ext.yMinimum(), 5083255, 10 );
QGSCOMPARENEAR( ext.yMaximum(), 5083355, 10 );
ext = QgsProcessingUtils::combineLayerExtents( QList< QgsMapLayer *>() << r1.get() << r2.get() );
ext = QgsProcessingUtils::combineLayerExtents( QList< QgsMapLayer *>() << r1.get() << r2.get(), context );
QGSCOMPARENEAR( ext.xMinimum(), 781662, 10 );
QGSCOMPARENEAR( ext.xMaximum(), 1535475, 10 );
QGSCOMPARENEAR( ext.yMinimum(), 3339523, 10 );
QGSCOMPARENEAR( ext.yMaximum(), 5083355, 10 );
// with reprojection
ext = QgsProcessingUtils::combineLayerExtents( QList< QgsMapLayer *>() << r1.get() << r2.get(), QgsCoordinateReferenceSystem::fromEpsgId( 3785 ) );
ext = QgsProcessingUtils::combineLayerExtents( QList< QgsMapLayer *>() << r1.get() << r2.get(), QgsCoordinateReferenceSystem::fromEpsgId( 3785 ), context );
QGSCOMPARENEAR( ext.xMinimum(), 1995320, 10 );
QGSCOMPARENEAR( ext.xMaximum(), 2008833, 10 );
QGSCOMPARENEAR( ext.yMinimum(), 3523084, 10 );
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On-premise users: click in-app to access the full platform documentation for your version of DataRobot.
Define custom model runtime parameters
Add runtime parameters to a custom model through the model metadata, making your custom model code easier to reuse. To define runtime parameters, you can add the following runtimeParameterDefinitions in model-metadata.yaml:
Key Description
fieldName Define the name of the runtime parameter.
type Define the data type the runtime parameter contains: string, boolean, numeric credential, and deployment.
defaultValue (Optional) Set the default string value for the runtime parameter (the credential type doesn't support default values).
minValue (Optional) For numeric runtime parameters, set the minimum numeric value allowed in the runtime parameter.
maxValue (Optional) For numeric runtime parameters, set the maximum numeric value allowed in the runtime parameter.
credentialType (Optional) For credential runtime parameters, set the type of credentials the parameter should contain.
allowEmpty (Optional) Set the empty field policy for the runtime parameter:
• True: (Default) Allows an empty runtime parameter.
• False: Enforces providing a value for the runtime parameter before deployment.
description (Optional) A description of the purpose or contents of the runtime parameter.
Default values
If you define a runtime parameter without specifying a defaultValue, the default value is None.
Before you define runtimeParameterDefinitions in model-metadata.yaml, define the custom model metadata required for the target type:
Example: model-metadata.yaml for a binary model
name: binary-example
targetType: binary
type: inference
inferenceModel:
targetName: target
positiveClassLabel: "1"
negativeClassLabel: "0"
Example: model-metadata.yaml for a regression model
name: regression-example
targetType: regression
type: inference
Example: model-metadata.yaml for a text generation model
name: textgeneration-example
targetType: textgeneration
type: inference
Example: model-metadata.yaml for an anomaly detection model
name: anomaly-example
targetType: anomaly
type: inference
Example: model-metadata.yaml for an unstructured model
name: unstructured-example
targetType: unstructured
type: inference
Example: model-metadata.yaml for a multiclass model
name: multiclass-example
targetType: multiclass
type: inference
Then, below the model information, you can provide the runtimeParameterDefinitions:
Example: runtimeParameterDefinitions in model-metadata.yaml
name: runtime-parameter-example
targetType: regression
type: inference
runtimeParameterDefinitions:
- fieldName: my_first_runtime_parameter
type: string
description: My first runtime parameter.
- fieldName: runtime_parameter_with_default_value
type: string
defaultValue: Default
description: A string-type runtime parameter with a default value.
- fieldName: runtime_parameter_boolean
type: boolean
defaultValue: true
description: A boolean-type runtime parameter with a default value of true.
- fieldname: runtime_parameter_numeric
type: numeric
defaultValue: 0
minValue: -100
maxValue: 100
description: A boolean-type runtime parameter with a default value of 0, a minimum value of -100, and a maximum value of 100.
- fieldName: runtime_parameter_for_credentials
type: credential
credentialType: basic
allowEmpty: false
description: A runtime parameter containing a dictionary of credentials; credentials must be provided before registering the custom model.
- fieldName: runtime_parameter_for_connected_deployment
type: deployment
description: A runtime parameter defined to accept the deployment ID of another deployment to connect to the deployed custom model.
Connected deployments
When you define a deployment runtime parameter, you can provide a deployment ID through the defaultValue or through the the UI. This deployment ID can be used for various purposes, such as connected deployments.
Provide credentials through runtime parameters
The credential runtime parameter type supports any credentialType value available in the DataRobot REST API. The credential information included depends on the credentialType, as shown in the examples below:
Note
For more information on the supported credential types, see the API reference documentation for credentials.
Credential Type Example
basic
basic:
credentialType: basic
description: string
name: string
password: string
user: string
azure
azure:
credentialType: azure
description: string
name: string
azureConnectionString: string
gcp
gcp:
credentialType: gcp
description: string
name: string
gcpKey: string
s3
s3:
credentialType: s3
description: string
name: string
awsAccessKeyId: string
awsSecretAccessKey: string
awsSessionToken: string
api_token
api_token:
credentialType: api_token
apiToken: string
name: string
Provide override values during local development
For local development with DRUM, you can specify a .yaml file containing the values of the runtime parameters. The values defined here override the defaultValue set in model-metadata.yaml:
Example: .runtime-parameters.yaml
my_first_runtime_parameter: Hello, world.
runtime_parameter_with_default_value: Override the default value.
runtime_parameter_for_credentials:
credentialType: basic
name: credentials
password: password1
user: user1
When using DRUM, the new --runtime-params-file option specifies the file containing the runtime parameter values:
Example: --runtime-params-file
drum score --runtime-params-file .runtime-parameters.yaml --code-dir model_templates/python3_sklearn --target-type regression --input tests/testdata/juniors_3_year_stats_regression.csv
Import and use runtime parameters in custom code
To import and access runtime parameters, you can import the RuntimeParameters module in your code in custom.py:
Example: custom.py
from datarobot_drum import RuntimeParameters
def mask(value, visible=3):
return value[:visible] + ("*" * len(value[visible:]))
def transform(data, model):
print("Loading the following Runtime Parameters:")
parameter1 = RuntimeParameters.get("my_first_runtime_parameter")
parameter2 = RuntimeParameters.get("runtime_parameter_with_default_value")
print(f"\tParameter 1: {parameter1}")
print(f"\tParameter 2: {parameter2}")
credentials = RuntimeParameters.get("runtime_parameter_for_credentials")
if credentials is not None:
credential_type = credentials.pop("credentialType")
print(
f"\tCredentials (type={credential_type}): "
+ str({k: mask(v) for k, v in credentials.items()})
)
else:
print("No credential data set")
return data
Updated May 14, 2024
|
__label__pos
| 0.826188 |
Elixir/Phoenix — Build a simple chat room
With Coherence, Channels and Presence
Hello, how are you?
In this blog post I want to discuss how to write a very simple chat room with a list of online users. The user will need to register an account before being able to enter the chat room. We will use both elixir and JavaScript, as well as a brand new authentication package I’ve been dabbling a bit with.
Modules and dependencies
Coherence
Coherence is a full featured, configurable authentication
system for Phoenix”
For those of you who are coming from Ruby on Rails, this package will look familiar to the lovely Devise gem. Be aware that Coherence is in the early release stages, and the project is under active development. Things may change! Read more at https://github.com/smpallen99/coherence
Phoenix Channels
For client-server communication over web sockets.
Phoenix Presence
Provides tracking for processes and channels.
Setting up the project
We’ll start off by creating our new project, I will call mine Chatourious:
~/$ mix phoenix.new chatourius
* creating chatourius/config/config.exs
* creating chatourius/config/dev.exs
Fetch and install dependencies? [Yn] y
...
~/$ cd chatourius/
~/chatourius$ mix ecto.create
...
The database for Chatourius.Repo has been created
Next, we’ll add Coherence, run mix deps.get and restart our server:
#mix.exs
......
def application do
[mod: {Chatourius, []},
applications: [:phoenix, :phoenix_pubsub, :phoenix_html, :cowboy, :logger, :gettext,
:phoenix_ecto, :postgrex, :coherence]]
end
......defp deps do
[{:phoenix, “~> 1.2.1”},
{:phoenix_pubsub, “~> 1.0”},
{:phoenix_ecto, “~> 3.0”},
{:postgrex, “>= 0.0.0”},
{:phoenix_html, “~> 2.6”},
{:phoenix_live_reload, “~> 1.0”, only: :dev},
{:gettext, “~> 0.11”},
{:cowboy, “~> 1.0”},
{:coherence, “~> 0.3”}]
end
......
Coherence comes with different modules just like Devise- Authenticatable, Invitable, Registerable, Confirmable and so on. We’ll use the built in Coherence installer to generate some boilerplate files. We’ll run the installer without the confirmable option:
~/chatourius$ mix coherence.install — full-invitable
Your config/config.exs file was updated.
Compiling 12 files (.ex)
The installer also gave us some instructions on updating some files manually.
This is my updated routes:
# web/router.exdefmodule Chatourius.Router do
use Chatourius.Web, :router
use Coherence.Router
pipeline :browser do
plug :accepts, ["html"]
plug :fetch_session
plug :fetch_flash
plug :protect_from_forgery
plug :put_secure_browser_headers
plug Coherence.Authentication.Session # Add this
end
# Add this block
pipeline :protected do
plug :accepts, ["html"]
plug :fetch_session
plug :fetch_flash
plug :protect_from_forgery
plug :put_secure_browser_headers
plug Coherence.Authentication.Session, protected: true # Add this
end
pipeline :api do
plug :accepts, ["json"]
end
scope "/" do
pipe_through :browser
coherence_routes
end
# Add this block
scope "/" do
pipe_through :protected
coherence_routes :protected
end
scope "/", Chatourius do
pipe_through :browser # Use the default browser stack
end
# Add this block
scope "/", Chatourius do
pipe_through :protected
# Add protected routes below
get "/", PageController, :index
end
end
I’ve also moved our page_controller route to the protected scope, so that the users need to log in to see it. Adding and editing model fields are beyond the scope of this blog, however, I just want to make a tiny change to the registration field name label:
# web/templates/coherence/registration/form.html.eex
......
<div class="form-group">
<%= required_label f, :username, class: "control-label" %>
<%= text_input f, :name, class: "form-control", required: ""%>
<%= error_tag f, :name %>
</div>
......
Also, we’ll add the jQuery CDN url at the bottom our app.html.eex file:
# web/templates/page/index.html.eex
......
<script src="https://cdnjs.cloudflare.com/ajax/
libs/jquery/2.2.4/jquery.min.js"></script>
<script src="<%= static_path(@conn, "/js/app.js") %>"></script>
</body>
</html>
We’ll run mix ecto.migrate, and restart our application. Now, after we’ve admired the amazing boilerplate templates for a while, we create a new account and sign into our application.
Building a simple interface
For simplicity, we’ll just stash our chat inside our Page Controller’s index template. First, delete everything inside the template located in web/templates/page/index.html.eex.
We’ll also delete the customized container css in web/static/css/phoenix.css:
This block of css code needs to go...@media (min-width: 768px) {
.container {
max-width: 730px;
}
}
After removing the content of index template and removing the container block
Perfect. A nice, white and spotless canvas for us to fill with amazing design! Jokes aside, let’s start writing some html. We will use minimal css, and stick with the building blocks Twitter Bootstrap already provides us with:
# web/templates/page/index.html.eex
<div class="chat container">
<div class="col-md-9">
<div class="panel panel-default chat-room">
<div class="panel-heading">
Hello <%= Coherence.current_user(@conn).name %>
<%= link "Sign out", to: session_path(@conn, :delete),
method: :delete %>
</div>
<div id="chat-messages" class="panel-body panel-messages">
</div>
<input type="text" id="message-input" class="form-control"
placeholder="Type a message…">
</div>
</div>
<div class="col-md-3">
<div class="panel panel-default chat-room">
<div class="panel-heading">
Online Users
</div>
<div class="panel-body panel-users" id="online-users">
</div>
</div>
</div>
</div>
Notice that we can fetch our current user with the help of a built in Coherence helper, Coherence.current_user(@conn).name .
Our css goes into our app.css file:
.chat {
margin-top: 0em;
}
.chat-room {
margin-top: 1em;
}
.panel-messages, .panel-users {
height: 400px;
}
#chat-messages {
min-height: 400px;
overflow-y: scroll;
}
And this is what we’ll end up with:
Two windows for chat messages and online users. An input field for typing messages.
Connecting to our room with channels
We’ll start of by making our server ready to receive connections from the client through web sockets.
Phoenix has a built in tool for creating boilerplate channel files.
mix phoenix.gen.channel name_of_channel. This creates two files, a file which contains our channel code, and a file which contains test code for our channel. However, we’ll just write everything by hand.
The first thing we’ll do, is adding a room_channel.ex file with the following content:
# web/channels/room_channel.exdefmodule Chatourius.RoomChannel do
use Phoenix.Channel
def join("room", _payload, socket) do
{:ok, socket}
end
end
We also need to add our room channel to user_socket.ex:
# web/channels/user_socket.exdefmodule Chatourius.UserSocket do
use Phoenix.Socket
......
channel "room", Chatourius.RoomChannel
......
end
In our room channel, we write a function called join/3. This function will handle all client authentication for the the given topic. In this case, we only have a “room” channel, and at this moment we’ll allow anyone to join, and we’ll return a {:ok, socket} tuple to authorize the socket.
Now, obviously if you look in your development console, you won’t find anything there, since we haven’t added the client side code yet.
Let’s tacle that now.
In our app.js file, we have to uncomment import socket from “./socket”:
# web/static/js/app.jsimport “phoenix_html”import socket from “./socket”
Now, if you take a look in your development console, you’ll notice an error:
Unable to join Object {reason: “unmatched topic”}
This is a good error! It means our client is trying to join our room.
For simplicity, we are just going to use our socket.js file for all our client code. If we take a look in this file (web/static/js/socket.js), we’ll notice that our client is trying to connect to “topic:subtopic”. We’ll change this to our “room”, which we wrote server side:
# web/static/js/socket.jsimport {Socket} from "phoenix"let socket = new Socket("/socket", {
params: {token: window.userToken}})
socket.connect()let channel = socket.channel("room", {}) # Edit this linechannel.join()
.receive("ok", resp => { console.log("Joined successfully", resp) })
.receive("error", resp => { console.log("Unable to join", resp) })
export default socket
Sweet, if we refresh our application, we get a better looking message:
Joined successfully Object {}
This means our client and server are talking to each other over web sockets.
Now, let’s figure out how to dish out messages in our chat application. Since we don’t have a send button, we’ll just rely on sending the message when we press enter. We then push the message to our channel, and sending them to the server:
# web/static/js/socket.jsimport {Socket} from "phoenix"let socket = new Socket("/socket", {
params: {token: window.userToken}})
socket.connect()
let channel = socket.channel("room", {})
let message = $('#message-input')
let nickName = "Nickname"
let chatMessages = document.getElementById("chat-messages")
message.focus();message.on('keypress', event => {
if(event.keyCode == 13) {
channel.push('message:new', {message: message.val(),
user: nickName})
message.val("")
}
});
channel.on('message:new', payload => {
let template = document.createElement("div");
template.innerHTML = `<b>${payload.user}</b>:
${payload.message}<br>`
chatMessages.appendChild(template);
chatMessages.scrollTop = chatMessages.scrollHeight;
})
channel.join()
.receive("ok", resp => { console.log("Joined successfully", resp) })
.receive("error", resp => { console.log("Unable to join", resp) })
export default socket
First, we create some variables for our DOM elements (input field and the window for our chat messages), and add “message.focus()”. This function will focus the input field so that the user doesn’t have to physically click it each time they want to send a message. We also write a listener function, where we listen for a keypress on key code 13. Key code 13 is the enter key, which means we’ll do something whenever the enter key is pressed.
Once the enter key is pressed, we push the value of the input field in “message”, and our hard coded nick name in “user” to the channel. Next,
we reset the input field so that it’s empty and ready for a new message.
We write another listener function, where we listen for a ‘message:new’ on our channel. Our channel will pick this up every time we push a ‘message:new’ from our key press function.
When we receive a new message, we put it into a new div element, and append it to our chat window.
This should be all we need for now. We are sending a message to our server, addressing it as ‘message:new’. We are ready to pick it up server side.
Conversations between the client and server
# web/channels/room_channel.ex
......
def handle_in("message:new", payload, socket) do
broadcast! socket, "message:new", %{user: payload["user"],
message: payload["message"]}
{:noreply, socket}
end
The handle_in/3 function handles every incoming event to the server. We are handling everything addressed as “message:new”, and send it out to every client connected to our room. The payload parameter holds the information about the user (where we stored our nickname from the client side) and the message (where we stored the chat message).
We are now able to dish out messages. From the client, to the server, and back to all connected clients
If you open up another browser and create a new account, you’ll be able to send messages back and forth between the browsers. The nickname is hardcoded, and won’t change even though we are logged into two different accounts. However the messages are broadcasted correctly from the server.
Now, let’s try and figure out how to display our user name instead of the hard coded nickname. We are already authenticated. Can we send this information over the channel? What we’ll do, is adding a user token with the user’s id in it, when the user signs in.
We’ll add a plug to our router, which generates a token
when the user signs in:
# web/router.ex
......
defp put_user_token(conn, _) do
current_user = Coherence.current_user(conn).id
user_id_token = Phoenix.Token.sign(conn, "user_id",
Coherence.current_user(conn).id)
conn
|> assign(:user_id, user_id_token)
IO.inspect user_id_token
end
......
We create a token and add the current user’s id in it, before storing it in the connection as :user_id.
We also have to add the plug to our protected pipeline:
# web/router.expipeline :protected do
plug :accepts, ["html"]
plug :fetch_session
plug :fetch_flash
plug :protect_from_forgery
plug :put_secure_browser_headers
plug Coherence.Authentication.Session, protected: true
plug :put_user_token
end
If we refresh our app, it’s going to crash, however if you look in your server console, you’ll find the generated user token:
“SFMyNTY.g3QAAAACZAAEZGF0YWEBZAAGc2lnbmVkbgYA8XevCFkB.o3buJ7JW9IvDhW4Zslp2wiyuO8JgMiGw1upGrm8XaV0”
Perfect. Now, remove the IO.inspect user_id_token.
I don’t know if you noticed, but in socket.js, we already have the following:
let socket = new Socket("/socket", {
params: {token: window.userToken}})
Our client is expecting a window.userToken. Let’s add that now. Add this line at the end of our app.html.eex file:
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/2.2.4/jquery.min.js"></script>
<script>window.userToken = "<%= assigns[:user_id] %>"</script>
<script src="<%= static_path(@conn, "/js/app.js") %>"></script>
</body>
</html>
we put our user token that we added to the connection as :user_id in window.userToken. If you look at the source code in your development tools, you’ll find that our user token is available client side.
We need to authenticate the token when the client connects to our chat:
# web/channels/user_socket.ex
......
def connect(%{"token" => user_id_token}, socket) do
case Phoenix.Token.verify(socket,
"user_id",
user_id_token,
max_age: 1000000) do
{:ok, user_id} ->
{:ok, assign(socket, :user_id, user_id)}
{:error, _reason} ->
:error
end
end
......
We receive the token in our connect function, we verify it, and store it in our socket.
# web/channels/room_channel.exdefmodule Chatourius.RoomChannel do
use Phoenix.Channel
alias Chatourius.Repo
alias Chatourius.User
...... def handle_in("message:new", payload, socket) do
user = Repo.get(User, socket.assigns.user_id)
broadcast! socket, "message:new", %{user: user.name,
message: payload["message"]}
{:noreply, socket}
end
end
Now that we have the user id stored in the socket, we can query the database and collect the correct user. We then broadcast the user.name instead of the nickname we hardcoded in JavaScript.
Each user’s username is correctly displayed in the chat room.
Awesome! If you try it in both your browsers you can see the correct username in the chat. Try it out!
Feeding online users to the client
We have come far. We have a very simple chat, but we have no way to track who’s online. We will use a Phoenix’s built in channel tracker for this.
We start off by using a built in generator:
~chatourius$ mix phoenix.gen.presence
* creating web/channels/presence.ex
Add your new module to your supervision tree,in lib/chatourius.ex:children = [...supervisor(Chatourius.Presence, []),]You're all set! See the Phoenix.Presence docs for more details:http://hexdocs.pm/phoenix/Phoenix.Presence.html
We do as told, and add presence to our supervision tree:
# lib/chatourius.ex
......
def start(_type, _args) do
......
children = [
supervisor(Chatourius.Repo, []),
supervisor(Chatourius.Endpoint, []),
supervisor(Chatourius.Presence, []), #Added line
]
......
end
Now, let’s set up the server side first. We need to add some changes to our room channel:
# web/channels/room_channel.exdefmodule Chatourius.RoomChannel do
use Phoenix.Channel
alias Chatourius.Presence #Added alias
alias Chatourius.Repo
alias Chatourius.User
def join("room", _payload, socket) do
send(self, :after_join) #Added
{:ok, socket}
end
def handle_info(:after_join, socket) do
user = Repo.get(User, socket.assigns.user_id)
{:ok, _} = Presence.track(socket, user.name, %{
online_at: inspect(System.system_time(:seconds))
})
push socket, "presence_state", Presence.list(socket)
{:noreply, socket}
end
......
We added a new function, handle_info/2. This function will handle changes when the user joins and leaves the channel. It gets triggered by the newly added line in join/3. When a user joins the channel, handle_info/2 gets invoked. We then use Phoenix.Presence to track the user, and push the state to the socket. We can now pick it up in the client.
Fetching online users from the server
It’s time to write some JavaScript. The Presence state is sent over the channel, and our client can pick it up. Let’s write some more code in our socket.js file:
import {Socket, Presence} from "phoenix" // import Presencelet socket = new Socket("/socket", {
params: {token: window.userToken}})
socket.connect()
let channel = socket.channel("room", {})
let message = $('#message-input')
let chatMessages = document.getElementById("chat-messages")
// Added variables
let presences = {}
let onlineUsers = document.getElementById("online-users")
// Added block
let listUsers = (user) => {
return {
user: user
}
}
// Added block
let renderUsers = (presences) => {
onlineUsers.innerHTML = Presence.list(presences, listUsers)
.map(presence => `
<li>${presence.user}</li>`).join("")
}
message.focus();message.on('keypress', event => {
if(event.keyCode == 13) {
channel.push('message:new', {message: message.val()})
message.val("")
}
});
channel.on('message:new', payload => {
let template = document.createElement("div");
template.innerHTML = `<b>${payload.user}</b>: ${payload.message}<br>`
chatMessages.appendChild(template);
chatMessages.scrollTop = chatMessages.scrollHeight;
});
// Added block
channel.on('presence_state', state => {
presences = Presence.syncState(presences, state)
renderUsers(presences)
});
// Added block
channel.on('presence_diff', diff => {
presences = Presence.syncDiff(presences, diff)
renderUsers(presences)
});
channel.join()
.receive("ok", resp => { console.log("Joined successfully", resp) })
.receive("error", resp => { console.log("Unable to join", resp) })
export default socket
First, we added two variables. An empty object to hold our presence data, and a variable to contain our window which displays online users.
We added a block, listUsers, which will pluck out the user from the presence data. We then added another block, renderUsers, which will use the presence object containing our online users, and listUsers, to render the username within some li tags.
We also added two more listener functions. The first function is listening on “presence_state”. Presence state contains a map of the presence information sent from the server. It sends the information to renderUsers, which then displays the users in our chat room. If you try and log in and out, the list of users online won’t change. If you refresh your application, the users online will be displayed. So, how can we display the online users automatically without refreshing our application? “presence_diff”, also sent to the channel from the handle_info/2 function, holds join and leave event information.
The second listener function listens for a “presence_diff”. If it receives one, it sends it to the renderUsers function, which then updates the list of online users realtime.
Online users are displayed in real time.
Awesome. It didn’t take that long, or that much code to write this simple chat app. Phoenix provides of with amazing tools to help developers create functioning applications.
I hope you learned something from this post, that it may help you build awesome real time applications. Channels can be used to create a whole lot more than just online chats. One example could be to display real time inventory in e-commerce application.
That’s it for now.
Until next time!
Stephan Bakkelund Valois
--
--
--
I’m a Norwegian developer and a motorcycle enthusiast. I love building cool stuff, and playing with exciting technology.
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Stephan Bakkelund Valois
Stephan Bakkelund Valois
I’m a Norwegian developer and a motorcycle enthusiast. I love building cool stuff, and playing with exciting technology.
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In Transition
Forgiveness is For Your Well-Being
|
__label__pos
| 0.688291 |
Random text generator
Demo to preview the plugin:
Introduction
Random text placeholder generator. Lorem ipsum is a placeholder text commonly used to demonstrate the visual form of a document or a typeface without relying on meaningful content.
How to setup
• Set up the min and max words per sentence or paragraphs
• Indicate your own list of words to generate from (Optional)
• Set up the amount of data to be generated (words,paragraphs...)
Generated data will be available in states.
What is Lorem Ipsum used for? In publishing and graphic design, lorem ipsum is a filler text commonly used to demonstrate the graphic elements of a document or visual presentation. Replacing meaningful content that could be distracting with placeholder text may allow viewers to focus on graphic aspects such as font, typography, and page layout.
Optionally, for each action you can indicate your own list of words to generate from.
Plugin Element Properties
The plugin contains a Lorem visual element that should be used on a page.
Image without caption
Element Actions
1. Generate words - Generates words according to provided options
Image without caption
Title
Description
Count
Total words
Custom words
Words separated by a comma, if not provided, will be used default words
1. Get Random Number - Generates a random number in the provided range
Image without caption
Title
Description
From
Inclusive
To
Inclusive
1. Get Random word - Generates a random word
Image without caption
Title
Description
Custom words
Words separated by a comma, if not provided, will be used default words
1. Generate Sentences - Generates sentences with provided options
Image without caption
Title
Description
Count
Total sentences
Min Words per Sentence
Minimum Words per Sentence
Max Words per Sentence
Maximum Words per Sentence
Custom words
Words separated by a comma, if not provided, will be used default words
1. Generate Paragraphs - Generates paragraphs with the provided options
Image without caption
Title
Description
Count
Total paragraphs
Min Words per Paragraph
Minimum Words per Paragraph
Max Words per Paragraph
Maximum Words per Paragraph
Custom words
Words separated by a comma, if not provided, will be used default words
Exposed states
Name
Description
Type
Random number
Generated random number
Number
Random word
Generated random word
Text
Random words
Generated random words
Text
Random Sentences
Generated random sentences
Text
Random Paragraphs
Generated random paragraphs
Text
Workflow example
1. Drag and drop the "Random Text Generator" element onto your page.
Image without caption
1. Set a workflow to trigger the action, select the desired action from the lorem element, set the options
Image without caption
1. Use the returned state in your desired element
Image without caption
Changelogs
|
__label__pos
| 0.993532 |
++ed by:
RWSTAUNER BRUNOV PERLOVER OVID GENEHACK
149 PAUSE users
109 non-PAUSE users.
Karen Etheridge 🐾 🌋
and 1 contributors
NAME
Moose::Spec::Role - Formal spec for Role behavior
VERSION
version 2.1005
DESCRIPTION
NOTE: This document is currently incomplete.
Components of a Role
Excluded Roles
A role can have a list of excluded roles, these are basically roles that they shouldn't be composed with. This is not just direct composition either, but also "inherited" composition.
This feature was taken from the Fortress language and is really of most use when building a large set of role "building blocks" some of which should never be used together.
Attributes
A roles attributes are similar to those of a class, except that they are not actually applied. This means that methods that are generated by an attributes accessor will not be generated in the role, but only created once the role is applied to a class.
Methods
These are the methods defined within the role. Simple as that.
Required Methods
A role can require a consuming class (or role) to provide a given method. Failure to do so for classes is a fatal error, while for roles it simply passes on the method requirement to the consuming role.
Required Attributes
Just as a role can require methods, it can also require attributes. The requirement fulfilling attribute must implement at least as much as is required. That means, for instance, that if the role requires that the attribute be read-only, then it must at least have a reader and can also have a writer. It means that if the role requires that the attribute be an ArrayRef, then it must either be an ArrayRef or a subtype of an ArrayRef.
Overridden Methods
The override and super keywords are allowed in roles, but their behavior is different from that of its class counterparts. The super in a class refers directly to that class's superclass, while the super in a role is deferred and only has meaning once the role is composed into a class. Once that composition occurs, super then refers to that class's superclass.
It is key to remember that roles do not have hierarchy, so they can never have a super role.
Method Modifiers
These are the before, around and after modifiers provided in Moose classes. The difference here is that the modifiers are not actually applied until the role is composed into a class (this is just like attributes and the override keyword).
Role Composition
Composing into a Class
Excluded Roles
Required Methods
Required Attributes
Attributes
Methods
Overridden methods
Method Modifiers (before, around, after)
Composing into a Instance
Composing into a Role
Excluded Roles
Required Methods
Required Attributes
Attributes
Methods
Overridden methods
Method Modifiers (before, around, after)
Role Summation
When multiple roles are added to another role (using the with @roles keyword) the roles are composed symmetrically. The product of the composition is a composite role (Moose::Meta::Role::Composite).
Excluded Roles
Required Methods
Required Attributes
Attributes
Attributes with the same name will conflict and are considered a unrecoverable error. No other aspect of the attribute is examined, it is enough that just the attribute names conflict.
The reason for such early and harsh conflicts with attributes is because there is so much room for variance between two attributes that the problem quickly explodes and rules get very complex. It is my opinion that this complexity is not worth the trouble.
Methods
Methods with the same name will conflict, but no error is thrown, instead the method name is added to the list of required methods for the new composite role.
To look at this in terms of set theory, each role can be said to have a set of methods. The symmetric difference of these two sets is the new set of methods for the composite role, while the intersection of these two sets are the conflicts. This can be illustrated like so:
Role A has method set { a, b, c }
Role B has method set { c, d, e }
The composite role (A,B) has
method set { a, b, d, e }
conflict set { c }
Overridden methods
An overridden method can conflict in one of two ways.
The first way is with another overridden method of the same name, and this is considered an unrecoverable error. This is an obvious error since you cannot override a method twice in the same class.
The second way for conflict is for an overridden method and a regular method to have the same name. This is also an unrecoverable error since there is no way to combine these two, nor is it okay for both items to be composed into a single class at some point.
The use of override in roles can be tricky, but if used carefully they can be a very powerful tool.
Method Modifiers (before, around, after)
Method modifiers are the only place where the ordering of role composition matters. This is due to the nature of method modifiers themselves.
Since a method can have multiple method modifiers, these are just collected in order to be later applied to the class in that same order.
In general, great care should be taken in using method modifiers in roles. The order sensitivity can possibly lead to subtle and difficult to find bugs if they are overused. As with all good things in life, moderation is the key.
Composition Edge Cases
This is a just a set of complex edge cases which can easily get confused. This attempts to clarify those cases and provide an explanation of what is going on in them.
Role Method Overriding
Many people want to "override" methods in roles they are consuming. This works fine for classes, since the local class method is favored over the role method. However in roles it is trickier, this is because conflicts result in neither method being chosen and the method being "required" instead.
Here is an example of this (incorrect) type of overriding.
package Role::Foo;
use Moose::Role;
sub foo { ... }
package Role::FooBar;
use Moose::Role;
with 'Role::Foo';
sub foo { ... }
sub bar { ... }
Here the foo methods conflict and the Role::FooBar now requires a class or role consuming it to implement foo. This is very often not what the user wants.
Now here is an example of the (correct) type of overriding, only it is not overriding at all, as is explained in the text below.
package Role::Foo;
use Moose::Role;
sub foo { ... }
package Role::Bar;
use Moose::Role;
sub foo { ... }
sub bar { ... }
package Role::FooBar;
use Moose::Role;
with 'Role::Foo', 'Role::Bar';
sub foo { ... }
This works because the combination of Role::Foo and Role::Bar produce a conflict with the foo method. This conflict results in the composite role (that was created by the combination of Role::Foo and Role::Bar using the with keyword) having a method requirement of foo. The Role::FooBar then fulfills this requirement.
It is important to note that Role::FooBar is simply fulfilling the required foo method, and **NOT** overriding foo. This is an important distinction to make.
Now here is another example of a (correct) type of overriding, this time using the excludes option.
package Role::Foo;
use Moose::Role;
sub foo { ... }
package Role::FooBar;
use Moose::Role;
with 'Role::Foo' => { -excludes => 'foo' };
sub foo { ... }
sub bar { ... }
By specifically excluding the foo method during composition, we allow Role::FooBar to define its own version of foo.
SEE ALSO
Traits
Roles are based on Traits, which originated in the Smalltalk community.
http://www.iam.unibe.ch/~scg/Research/Traits/
This is the main site for the original Traits papers.
Class::Trait
I created this implementation of traits several years ago, after reading the papers linked above. (This module is now maintained by Ovid and I am no longer involved with it).
Roles
Since they are relatively new, and the Moose implementation is probably the most mature out there, roles don't have much to link to. However, here is some bits worth looking at (mostly related to Perl 6)
http://www.oreillynet.com/onlamp/blog/2006/08/roles_composable_units_of_obje.html
This is chromatic's take on roles, which is worth reading since he was/is one of the big proponents of them.
http://svn.perl.org/perl6/doc/trunk/design/syn/S12.pod
This is Synopsis 12, which is all about the Perl 6 Object System. Which, of course, includes roles.
AUTHOR
Moose is maintained by the Moose Cabal, along with the help of many contributors. See "CABAL" in Moose and "CONTRIBUTORS" in Moose for details.
COPYRIGHT AND LICENSE
This software is copyright (c) 2013 by Infinity Interactive, Inc..
This is free software; you can redistribute it and/or modify it under the same terms as the Perl 5 programming language system itself.
|
__label__pos
| 0.681404 |
SheafSystem 0.0.0.0
name_multimap.cc
Go to the documentation of this file.
1
2 //
3 // Copyright (c) 2014 Limit Point Systems, Inc.
4 //
5 // Licensed under the Apache License, Version 2.0 (the "License");
6 // you may not use this file except in compliance with the License.
7 // You may obtain a copy of the License at
8 //
9 // http://www.apache.org/licenses/LICENSE-2.0
10 //
11 // Unless required by applicable law or agreed to in writing, software
12 // distributed under the License is distributed on an "AS IS" BASIS,
13 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
14 // See the License for the specific language governing permissions and
15 // limitations under the License.
16 //
17
20
21 #include "SheafSystem/name_multimap.h"
22
23 #include "SheafSystem/sheaf_dll_spec.h"
24 #include "SheafSystem/assert_contract.h"
25 #include "SheafSystem/deep_size.h"
26
27 using namespace std;
28
29 // =============================================================================
30 // NAME_MULTIMAP FACET
31 // =============================================================================
32
33 // PUBLIC FUNCTIONS
34
37 {
38 // Preconditions:
39
40 // Body:
41
42 // Nothing to do.
43
44 // Postconditions:
45
46 ensure(invariant());
47
48 // Exit:
49
50 return;
51 }
52
55 {
56 // Preconditions:
57
58 // Body:
59
60 (*this) = xother;
61
62 // Postconditions:
63
64 ensure(invariant());
65 ensure(*this == xother);
66
67 // Exit:
68
69 return;
70 }
71
73 name_multimap(const std::string xnames[], size_type xnames_ub)
74 {
75 // Preconditions:
76
77 // Body:
78
79 for(index_type i = 0; i < xnames_ub; ++i)
80 {
81 if(!xnames[i].empty())
82 {
83 _index_to_name_map[i].push_back(xnames[i]);
84 _name_to_index_map[xnames[i]] = i;
85 }
86 }
87
88 // Postconditions:
89
90 ensure(invariant());
91 ensure_for_all(i, 0, xnames_ub, !xnames[i].empty() ? name(index_type(i)) == xnames[i] : true);
92
93 // Exit:
94
95 return;
96 }
97
100 {
101 // Preconditions:
102
103 // Body:
104
105 // Nothing to do.
106
107 // Postconditions:
108
109 // Exit:
110
111 return;
112 }
113
114 std::string
116 name(index_type xindex) const
117 {
118 // Preconditions:
119
120 // Body:
121
122 index_to_name_map_type::const_iterator map_result =
123 _index_to_name_map.find(xindex);
124
125 if(map_result != _index_to_name_map.end())
126 {
127 // This index has a name.
128
129 // Exit
130
131 return map_result->second.front();
132 }
133 else
134 {
135 // This index has no name.
136
137 string result; // empty string
138
139 // Postcondition:
140
141 ensure(result.empty() == !contains_index(xindex));
142
143 // Exit
144
145 return result;
146 }
147
148 }
149
150 std::string
152 name(index_type xindex, int xi) const
153 {
154 // Preconditions:
155
156 // Body:
157
158 string result;
159
160 const name_list_type& lnames = all_names(xindex);
161 const_name_iterator litr = lnames.begin();
162 int i = 0;
163 while(litr != lnames.end())
164 {
165 if(i == xi)
166 {
167 result = *litr;
168 break;
169 }
170 ++litr;
171 ++i;
172 }
173
174 // Postconditions:
175
176 ensure((!result.empty()) == ((0 <= xi) && (xi < name_ct(xindex))) );
177
178 // Exit:
179
180 return result;
181 }
182
185 all_names(index_type xindex) const
186 {
187 // Preconditions:
188
189 // Body:
190
191 static const name_list_type empty_list;
192
193 const_iterator map_result = _index_to_name_map.find(xindex);
194
195 const name_list_type& result =
196 (map_result != _index_to_name_map.end()) ? map_result->second : empty_list;
197
198 // Postconditions:
199
200 // Exit:
201
202 return result;
203 }
204
205 void
207 all_names(index_type xindex, block<std::string>& xresult) const
208 {
209 // Preconditions:
210
211 // Body:
212
213 xresult.reserve(4); // Arbitrary; will resize as needed.
214 xresult.set_ct(0);
215
216 const name_list_type& lnames = all_names(xindex);
217
218 for(const_name_iterator itr = lnames.begin(); itr != lnames.end(); ++itr)
219 {
220 xresult.push_back(*itr);
221 }
222
223 // Postconditions:
224
225 ensure(xresult.ct() == name_ct(xindex));
226
227 // Exit:
228
229 return;
230 }
231
234 index(const std::string& xname) const
235 {
236 index_type result;
237
238 // Preconditions:
239
240 require(!xname.empty());
241
242 // Body:
243
244 // Lookup the name in the name-to-index map.
245
246 typedef name_to_index_map_type::const_iterator itr_type;
247
248 itr_type litr = _name_to_index_map.find(xname);
249
250 if(litr != _name_to_index_map.end())
251 {
252 // Name is in the map; result is the value of the map.
253
254 result = litr->second;
255 }
256 else
257 {
258 // Name not in the map; result is invalid.
259
260 result = invalid_pod_index();
261 }
262
263 // Postconditions:
264
265 // Must be unexecutable or create infinite recursion.
266
267 ensure(unexecutable(is_valid(result) == !contains_name(xname)));
268
269 // Exit
270
271 return result;
272 }
273
274 void
276 put_entry(const entry_type& xentry, bool xunique)
277 {
278 // Preconditions:
279
280 require(!xentry.second.empty());
281 require(!contains_name(xentry.second));
282
283 // Body:
284
285 if(xunique)
286 {
287 // Delete all other entries for this index.
288
289 delete_index(xentry.first);
290 }
291
292
293 _index_to_name_map[xentry.first].push_back(xentry.second);
294 _name_to_index_map[xentry.second] = xentry.first;
295
296 // Postconditions:
297
298 ensure(contains_entry(xentry));
299
300 // Exit
301 }
302
303 void
305 put_entry(index_type xindex, const std::string& xname, bool xunique)
306 {
307 // Preconditions:
308
309 require(!xname.empty());
310 require(!contains_name(xname));
311
312 // Body:
313
314 entry_type ltmp(xindex, xname);
315 put_entry(ltmp, xunique);
316
317 // Postconditions:
318
319 ensure(contains_entry(xindex, xname));
320
321 // Exit:
322
323 return;
324 }
325
326 bool
328 contains_name(const std::string& xname) const
329 {
330 bool result;
331
332 // Preconditions:
333
334 require(!xname.empty());
335
336 // Body:
337
338 result = _name_to_index_map.find(xname) != _name_to_index_map.end();
339
340 // Postconditions:
341
342 // Exit
343
344 return result;
345 }
346
347 bool
350 {
351 // Preconditions:
352
353 // Body:
354
355 // Postconditions:
356
357 // Exit:
358
359 return _index_to_name_map.find(xindex) != _index_to_name_map.end();
360 }
361
362 bool
364 contains_entry(index_type xindex, const std::string& xname) const
365 {
366 // Preconditions:
367
368 require(!xname.empty());
369
370 // Body:
371
372 bool result;
373
374 entry_type entry(xindex, xname);
375
376 result = contains_entry(entry);
377
378 // Postconditions:
379
380 // Exit:
381
382 return result;
383 }
384
385 bool
387 contains_entry(const entry_type& xentry) const
388 {
389 // Preconditions:
390
391 require(!xentry.second.empty());
392
393 // Body:
394
395 bool result;
396
397 result = contains_name(xentry.second) && (index(xentry.second) == xentry.first);
398
399 // Postconditions:
400
401 // Exit:
402
403 return result;
404 }
405
406 void
409 {
410 // Preconditions:
411
412 // Body:
413
414 // Remove xindex from the name to index map.
415
416 const name_list_type& lnames = all_names(xindex);
417
418 for(const_name_iterator itr = lnames.begin(); itr != lnames.end(); ++itr)
419 {
420 _name_to_index_map.erase(*itr);
421 }
422
423 // Remove xindex from the index to name map.
424
425 _index_to_name_map.erase(xindex);
426
427 // Postcondition
428
429 ensure(!contains_index(xindex));
430
431 // Exit
432
433 return;
434 }
435
436 void
438 delete_name(const std::string& xname)
439 {
440 // Preconditions:
441
442 require(!xname.empty());
443
444 // Find entry in name to index map.
445
446 name_to_index_map_type::iterator itr = _name_to_index_map.find(xname);
447
448 if(itr != _name_to_index_map.end())
449 {
450 // Found name to index entry;
451 // find and delete corresponding index to name entry.
452
453 iterator lindex_entry = _index_to_name_map.find(itr->second);
454 name_list_type& lnames = lindex_entry->second;
455
456 name_iterator lname_itr = lnames.begin();
457 while(lname_itr != lnames.end())
458 {
459 if((*lname_itr) == xname)
460 {
461 lnames.erase(lname_itr);
462 break;
463 }
464 ++lname_itr;
465 }
466
467 if(lnames.empty())
468 {
469 // List of names for this index is now empty;
470 // delete the entry.
471
472 _index_to_name_map.erase(lindex_entry);
473 }
474
475 // Now erase the name to index entry.
476
477 _name_to_index_map.erase(itr);
478 }
479
480 // Postcondition
481
482 ensure(!contains_name(xname));
483
484 // Exit
485
486 return;
487 }
488
489 void
492 {
493 // cout << endl << "Entering name_multimap::clear." << endl;
494
495 // Preconditions:
496
497
498 // Body:
499
500 _name_to_index_map.clear();
501 _index_to_name_map.clear();
502
503 // Postconditions:
504
505 ensure(empty());
506
507 // Exit:
508
509 // cout << "Leaving name_multimap::clear." << endl;
510 return;
511 }
512
513 bool
515 empty() const
516 {
517 // cout << endl << "Entering name_multimap::empty." << endl;
518
519 // Preconditions:
520
521
522 // Body:
523
524 bool result = (_name_to_index_map.empty() && _index_to_name_map.empty());
525
526 // Postconditions:
527
528
529 // Exit:
530
531 // cout << "Leaving name_multimap::empty." << endl;
532 return result;
533 }
534
535
536
539 begin() const
540 {
541 return _index_to_name_map.begin();
542 }
543
546 end() const
547 {
548 return _index_to_name_map.end();
549 }
550
551 int
553 ct() const
554 {
555 return _index_to_name_map.size();
556 }
557
558 void
560 print() const
561 {
562 print(cout, *this);
563 }
564
565 void
567 print(std::ostream& xos, const name_multimap& xm) const
568 {
569 // Preconditions:
570
571 // Body:
572
574 while(itr != xm.end())
575 {
576 xos << "index: " << itr->first << "\tnames: " ;
577
578 const name_multimap::name_list_type& lnames = itr->second;
579 name_multimap::const_name_iterator name_itr = lnames.begin();
580 while(name_itr != lnames.end())
581 {
582 xos << " \"" << *name_itr << "\"";
583 ++name_itr;
584 }
585 xos << endl;
586 ++itr;
587 }
588
589 // Postconditions:
590
591 // Exit:
592
593 return;
594 }
595
598 name_ct(const index_type& xindex) const
599 {
600 return all_names(xindex).size();
601 }
602
603 // PROTECTED FUNCTIONS
604
605 // PRIVATE FUNCTIONS
606
607
608 // ===========================================================
609 // ANY FACET
610 // ===========================================================
611
612 // PUBLIC FUNCTIONS
613
616 clone() const
617 {
618 name_multimap* result;
619
620 // Preconditions:
621
622 // Body:
623
624 result = new name_multimap(*this);
625
626 // Postconditions:
627
628 ensure(result != 0);
629 ensure(is_same_type(result));
630
631 // Exit:
632
633 return result;
634 }
635
636 bool
638 invariant() const
639 {
640 bool result = true;
641
642 // Preconditions:
643
644 // Body:
645
646 // Must satisfy base class invariant
647
648 result = result && any::invariant();
649
650 if(invariant_check())
651 {
652 // Prevent recursive calls to invariant
653
654 disable_invariant_check();
655
656 invariance(_index_to_name_map.size() <= _name_to_index_map.size());
657
658 // Finished, turn invariant checking back on.
659
660 enable_invariant_check();
661 }
662
663 // Postconditions:
664
665 // Exit
666
667 return result;
668 }
669
670 bool
672 is_ancestor_of(const any* xother) const
673 {
674
675 // Preconditions:
676
677 require(xother != 0);
678
679 // Body:
680
681 // True if other conforms to this.
682
683 bool result = dynamic_cast<const name_multimap*>(xother) != 0;
684
685 // Postconditions:
686
687 return result;
688 }
689
690 bool
692 operator==(const name_multimap& xother)
693 {
694 // Preconditions:
695
696 // Body:
697
698 bool result =
699 (_name_to_index_map == xother._name_to_index_map) &&
700 (_index_to_name_map == xother._index_to_name_map);
701
702 // Postconditions:
703
704 // Exit
705
706 return result;
707 }
708
711 operator=(const name_multimap& xother)
712 {
713
714 // Preconditions:
715
716 // Body:
717
718 _name_to_index_map = xother._name_to_index_map;
719 _index_to_name_map = xother._index_to_name_map;
720
721 // Postconditions:
722
723 ensure(invariant());
724 ensure(*this == xother);
725
726 // Exit
727
728 return *this;
729 }
730
731 // PROTECTED FUNCTIONS
732
733 // PRIVATE FUNCTIONS
734
735
736 // ===========================================================
737 // NON-MEMBER FUNCTIONS
738 // ===========================================================
739
740 ostream&
741 sheaf::
742 operator<<(ostream& xos, const name_multimap& xm)
743 {
744 // Preconditions:
745
746 // Body:
747
748 xm.print(xos, xm);
749 return xos;
750
751 // Postconditions:
752
753 // Exit:
754
755 return xos;
756 }
757
758 size_t
759 sheaf::
760 deep_size(const name_multimap& xp, bool xinclude_shallow)
761 {
762 size_t result;
763
764 // Preconditions:
765
766 // Body:
767
768 result = xinclude_shallow ? sizeof(xp) : 0;
769
770 // Add the deep size of the data members.
771
772 typedef name_multimap::index_type index_type;
773
774 typedef key_deep_size_policy<map<string, index_type> > name_to_index_policy;
775 result += deep_size<string, index_type, name_to_index_policy>
776 (xp._name_to_index_map, false);
777
778 typedef value_deep_size_policy<map<index_type, list<string> > > index_to_name_policy;
779 result += deep_size<index_type, list<string>, index_to_name_policy>
780 (xp._index_to_name_map, false);
781
782 // Postconditions:
783
784 ensure(result >= 0);
785
786 // Exit
787
788 return result;
789 }
790
791
bool contains_entry(const entry_type &xentry) const
True if this already contains an entry equal to xentry.
size_type name_ct(const index_type &xindex) const
The number of names associated with key xindex.
name_list_type::iterator name_iterator
The iterator type for names.
Definition: name_multimap.h:92
size_type ct() const
The number of items currently in use.
const_iterator begin() const
The initial value for iterators over this map.
pod_index_type index_type
The type of the index in the map.
Definition: name_multimap.h:77
index_type index(const std::string &xname) const
The index associated with name xname.
void delete_name(const std::string &xname)
Removes the entry for name xname.
name_list_type::const_iterator const_name_iterator
The const iterator type for names.
Definition: name_multimap.h:97
const name_list_type & all_names(index_type xindex) const
All names associated with index xindex.
void clear()
Removes all entries.
std::list< std::string > name_list_type
The type of name list for this map.
Definition: name_multimap.h:87
std::map< index_type, name_list_type >::iterator iterator
The iterator type for this map.
A partial multi-valued relation with total injective inverse between names and indices of type index_...
Definition: name_multimap.h:63
STL namespace.
void reserve(index_type xub)
Makes ub() at least xub; if new storage is allocated, it is uninitialized.
Call deep_size on the key.
Definition: deep_size.h:82
const_iterator end() const
The final value for iterators over this map.
Abstract base class with useful features for all objects.
Definition: any.h:39
void push_back(const_reference_type item)
Insert item at the end of the items in the auto_block.
std::pair< index_type, std::string > entry_type
The type of an entry in the map.
Definition: name_multimap.h:82
virtual name_multimap * clone() const
Virtual constructor; makes a new instance of the same type as this.
int ct() const
The number of index values in the map.
void set_ct(size_type xct)
Sets ct() == xct.
SHEAF_DLL_SPEC size_t deep_size(const dof_descriptor_array &xp, bool xinclude_shallow=true)
The deep size of the referenced object of type dof_descriptor_array.
unsigned long size_type
An unsigned integral type used to represent sizes and capacities.
Definition: sheaf.h:52
virtual ~name_multimap()
Destructor.
bool contains_name(const std::string &xname) const
True if this already contains an entry with name xname.
bool operator==(const name_multimap &xother)
Equality test; true if other has the same contents as this.
Call deep_size on the value.
Definition: deep_size.h:96
bool contains_index(index_type xindex) const
True if this already contains an entry with index xindex.
name_multimap()
Default constructor.
bool empty() const
True if and only if the map contains no entries.
virtual bool invariant() const
Class invariant.
virtual bool is_ancestor_of(const any *other) const
Conformance test; true if other conforms to this.
SHEAF_DLL_SPEC std::ostream & operator<<(std::ostream &os, const dof_descriptor_array &p)
Insert dof_descriptor_array& p into ostream& os.
void put_entry(const entry_type &xentry, bool xunique)
Sets (xindex, xname) as an entry in the map. If xunique, deletes all other entries for xindex...
void print() const
Prints the data members of this on cout. Intended for use debugging.
std::string name(index_type xindex) const
The primary (0-th) name associated with index xindex.
SHEAF_DLL_SPEC bool is_valid(pod_index_type xpod_index)
True if an only if xpod_index is valid.
Definition: pod_types.cc:37
std::map< index_type, name_list_type >::const_iterator const_iterator
The const iterator type for this map.
SHEAF_DLL_SPEC pod_index_type invalid_pod_index()
The invalid pod index value.
Definition: pod_types.cc:31
void delete_index(index_type xindex)
Removes all entires for index xindex.
name_multimap & operator=(const name_multimap &xother)
Assignment operator; make this a copy of xother.
|
__label__pos
| 0.96463 |
dcsimg
Putting PostgreSQL Through its Paces
The folks at Red Hat recently selected the open source PostgreSQL database as the foundation for their commercial Red Hat Database product. This decision, however, was not made without a good deal of whining from the ranks of the MySQL faithful, who weren't able to fully comprehend why it was that their baby had been passed over.
The folks at Red Hat recently selected the open source PostgreSQL database as the foundation for their commercial Red Hat Database product. This decision, however, was not made without a good deal of whining from the ranks of the MySQL faithful, who weren’t able to fully comprehend why it was that their baby had been passed over.
After all, MySQL was faster, better, cheaper, easier to install, and all those other important buzzwords. PostgreSQL was an oddity: buggy, unsupported, no longer in development, unable to easily deal with large data, and (gasp) requiring a separate process for each connection instead of being nicely threaded.
It’s obvious to me that those who discount PostgreSQL in favor of MySQL haven’t taken a look at PostgreSQL recently. I too was a “MySQL all the way” person until some fellow Perl hackers convinced me to take another look. Making one last comparison before getting to the meat of this month’s column, my conclusion is that MySQL is “putting a bit of structure onto a flat file,” where PostgreSQL is “an open source Oracle replacement at a fraction of the cost.”
To commemorate the successful installation of PostgreSQL on my system, I wanted to tackle a little project. While racking my brain for a worthy project, someone on the Perl IRC channel mentioned a “word of the day” program; this inspired me to create one with PostgreSQL.
A DBM hash could easily hold the words and their definitions, but a database could also easily hold a per-user “I’ve already seen that word” table. That way, we wouldn’t end up seeing the same word twice. In addition, someone recently showed me the trick:
SELECT word FROM list ORDER BY random() LIMIT 1;
to get a random entry, so I was hoping to put that into a program at some point. This works by first sorting the list according to a random value, effectively shuffling it, and then selecting just the first entry.
Of course, after tinkering with it a bit, I decided that there was absolutely no reason not to make it a CGI script and a crontab-able program, so I went ahead and threw in HTML-cleaning of the definitions.
My next step was then to figure out which dictionary I should use. I was originally pointed at Lingua::Wordnet and got detoured for an hour, trying to install and understand that. I discussed this online, and someone mentioned FOLDOC, the dictionary of computing terms (both historical and current), as a good source of pertinent information. So I checked FOLDOC, and it indeed turned out to be a very good source for this little project.
The program flows as follows: On each invocation, the current FOLDOC flat file is mirrored into a local cache. If the local cache was updated, the file is parsed into words and definitions and placed in the PostgreSQL database in a simple two-column table.
Next, a “deck” is consulted for every user (keyed by Unix userid) who invokes the program. Initially, the deck is empty, so a new wordlist is created in a random order by “shuffling” a copy of the terms from the dictionary. This is done using SQL similar to (for the user “merlyn”):
INSERT INTO deck (word, person)
SELECT word, merlyn
FROM foldoc
ORDER BY random()
The first entry for this user is pulled off and deleted from the deck. The definition is then pulled from the dictionary table. If the invocation was from the command line, the word and definition are displayed with minimal reformatting. However, if CGI invocation is detected, then an HTML massaging locates URLs and e-mail addresses, as well as fixes up HTML entities.
Now comes the really cool part of PostgreSQL. All this shuffling and updating of the dictionary is being done within transactions that do not block the other readers! If the dictionary is stale, the other readers see the dictionary instantaneously change from the old dictionary to the new dictionary, without blocking. Try that with MySQL. PostgreSQL provides the concurrent, consistent views that only big guys like Oracle and Interbase have been able to provide in the past.
Enough on that? Let’s look at the code in Listing One.
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Linux Magazine /
December 2001 / PERL OF WISDOM
Putting PostgreSQL Through its Paces
Listing One: Word of the Day — Part I
1 #!/usr/bin/perl -w
2 use strict;
3 $|++;
4
5 use DBI;
6
7 ## configuration
8 my $REMOTE = “http://foldoc.doc.ic.ac.uk/foldoc/Dictionary“;
9 my $LOCAL = “/home/merlyn/Web/Dictionary.txt”;
10 my @DSN = qw(dbi:Pg:dbname= foldoc_word_of_the_day USER PASS);
11 ## end configuration
12
13 my $dbh = DBI->connect(@DSN, {RaiseError => 1, PrintError => 0});
14
15 refresh_wordlist() if -w $LOCAL;
16 my ($word, $meaning) = get_word_and_meaning_for(scalar getpwuid $<);
17
18 if ($ENV{GATEWAY_INTERFACE}) { # running under CGI
19 require HTML::FromText;
20
21 print “Content-type: text/html\n\n”;
22 print HTML::FromText::text2html(“$word\n\n$meaning”,
23 map { $_ => 1 }
24 qw(title urls email paras));
25 } else {
26 print “$word\n$meaning”;
27 }
28
29 $dbh->disconnect;
30
31 exit 0;
32
33 sub refresh_wordlist {
34 require LWP::Simple;
35 return unless LWP::Simple::mirror($REMOTE, $LOCAL) == 200;
36
37 eval {
38 $dbh->do(q{CREATE TABLE foldoc (word text, meaning text)});
39 };
40 die $@ if $@ and $@ !~ /already exists/;
41
42 eval {
43 $dbh->begin_work;
44
45 $dbh->do(q{DELETE FROM foldoc}); # clean it out
46
47 my $insert = $dbh->prepare
48 (q{INSERT INTO foldoc(word, meaning) VALUES (?, ?)});
49
50 open LOCAL, $LOCAL or die;
51 my $entry;
52 {
53 $_ = <LOCAL>;
54 if (not defined $_ or /^\S/) { # end of definition
55 if (defined $entry) { # save any cached definition
56 $entry =~ s/^(\S.*)\n([ \t]*\n) *// or die;
57 my $key = $1; # get key
58 $entry =~ s/\s+\z/\n/; # clean up definition
59
60 unless ($key =~ /Free On-line Dictionary|Acknowledgements/) {
61 print “$key -> “;
62 print $insert-> execute($key, $entry);
63 print “\n”;
64 }
65
66 undef $entry;
67 }
68 last unless defined $_;
69 }
70 $entry .= $_;
71 redo;
72 }
73 $dbh->commit;
74 };
75 if ($@) {
76 $dbh->rollback;
77 die $@;
78 }
79
80 ## create and reset word
81
82 eval {
83 $dbh->do(q{CREATE TABLE deck (word text, person text)});
84 };
85 die $@ if $@ and $@ !~ /already exists/;
86
87 eval {
88 $dbh->begin_work;
89 $dbh->do(q{DELETE FROM deck}); # clean it out
90 $dbh->commit;
91 };
92 if ($@) {
93 $dbh->rollback;
94 die $@;
95 }
96
97 }
98
99 sub get_word_and_meaning_for {
100 my $person = shift;
101
102 for (my $tries = 0; $tries <= 2; $tries++) {
103 $dbh->begin_work;
104 if (my ($word) =
105 $dbh->selectrow_array(q{
106 SELECT word FROM deck
107 WHERE person = ?
108 FOR UPDATE OF deck
109 LIMIT 1
110 },
111 undef, $person)) {
112
113 ## got a good word
114 $dbh->do(q{DELETE FROM deck WHERE (word, person) = (?, ?)},
115 undef, $word, $person);
116 $dbh->commit;
117
118 if (my ($meaning) =
119 $dbh->selectrow_array(q{
120 SELECT meaning FROM foldoc
121 WHERE word = ?
122 }, undef, $word)) {
123 return ($word, $meaning);
124 }
125
126 die “missing meaning for $word\n”;
127 } else {
128 ## no words left, shuffle the deck
129
130 $dbh->do(q{
131 INSERT INTO deck (word, person)
132 SELECT word, ?
133 FROM foldoc
134 ORDER BY random()
135 }, undef, $person);
136 $dbh->commit;
137 }
138 }
139 die “Cannot get a word for $person\n”;
140 }
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Linux Magazine /
December 2001 / PERL OF WISDOM
Putting PostgreSQL Through its Paces
Lines 1 through 3 start most of the programs I write, turning on warnings, enabling compiler restrictions, and disabling the buffering on STDOUT.
Line 5 accesses the DBI module, found in the CPAN. You’ll also need to have the DBD::Pg module, found both in the CPAN and with the PostgreSQL source distribution.
Lines 8 through 10 define the configuration parameters I might want to change in this program. The URL from which I’m fetching the FOLDOC file is in $REMOTE. The local file in which this is being cached is in $LOCAL. And @DSN defines the DBI identifier, user, and password. The PostgreSQL database foldoc_word_of_the_day must already exist but can be empty, because the script creates the necessary tables. (Setting up a PostgreSQL database is beyond the scope of this column, but information is available at http://www.postgresql.org/. Make sure you have the latest version, as some of the syntax here requires at least version 7.1. Also, make sure your default permissions for tables are adequate, or you may need to GRANT permissions so, for example, the user nobody has access to the deck.)
Line 13 establishes the connection to the PostgreSQL database. RaiseError is set, causing all serious errors to throw an exception. If uncaught, the exceptions cause the program to die, but for those rare steps where some of the serious errors might be expected, we can use an eval block to catch them.
Line 15 causes the database to be refreshed from the master remote wordlist, but only if the invoker of the script can write to the local cache. This means that if the file is writable only by me, then the CGI invocations won’t refresh the file, and I don’t have to make any directory or file “world writable” just to make it work with CGI.
Line 16 fetches the word and its definition from the database, for the user running the script. This is nobody on my Web server, or merlyn for me. The “already seen” lists are maintained per-user.
Lines 18 to 27 format the response. If we’re running as CGI, then the environment variable GATEWAY_INTERFACE is set. The text2html subroutine from HTML::FromText is pulled in, and appropriate headers are added for a CGI response. If it’s not CGI, then the word and meaning are simply dumped to STDOUT.
Line 29 disconnects from the database, and line 31 keeps us from accidentally executing further code below.
And now for the subroutines, starting with refresh_ wordlist in line 33. We’ll fetch the list with LWP::Simple (found in the LWP library in the CPAN), using the mirror routine to mirror the file to the local cache. If the return code is 200, then we’ve got a new version, and it’s time to refresh the PostgreSQL database as well.
Lines 37 to 40 ensure that the PostgreSQL table holding the dictionary has been created. Both the word and meaning columns are of type “text,” which is a text string of unlimited size, stored compactly.
Lines 42 to 74 form a commit-block for a transaction. If anything fails in the block, then the rollback in line 76 erases the actions as if nothing had happened. Also, any changes made within the block are not visible to other users until line 73 is executed, so it’s as if we’re working on our own private copy of the database.
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Linux Magazine /
December 2001 / PERL OF WISDOM
Putting PostgreSQL Through its Paces
Line 45 clears out the foldoc table. Good thing we’re doing this in private, or other users would no longer be able to get the definitions for their words. (In MySQL, this could be accomplished by blocking other users while we’re doing this, but in PostgreSQL, the other users see the old version until we commit.)
Lines 47 and 48 define a statement handle to insert the entries into the foldoc table. Lines 50 to 72 parse the FOLDOC file. The file consists of many thousands of entries that look like the following:
Artistic license
<legal> The {open source license}to {Perl}.
(1999-12-29)
Note that the term is flush-left and the definition is indented. This is a trivial parsing problem for Perl. The business step is down in line 62, where the term and definition are inserted into the database. A trace of the term and “number of lines inserted” (usually 1) accompanies the insertion, which usually scrolls by faster than I can read it.
Once a new dictionary is inserted, it’s time to also invalidate the existing decks. First, the deck is created in line 83 if needed. The deck has two columns, a word (identical to the word column of the other table), and a person. The deck is then cleaned in line 89, forcing the next hit for each individual requestor to shuffle a clean deck for them.
To get a word and definition, we call the routine starting in line 99, passing the individual into $person in line 100. We’ll try to get the first “card” of the deck twice, failing after the second try.
Lines 105 through 111 attempt to grab that first card. A placeholder is used to identify the person, ensuring that we don’t have to be aware of the quoting conventions for that string. If this succeeds, then we delete the card from the deck, look up the definition in lines 119 to 122, and return the word and the definition in line 123. If it fails, then it’s time to shuffle, so lines 131 to 135 create the shuffled deck using the random number generator to insert the items.
Again, the updates to the deck are done inside transaction begin-end brackets, so other users of the database will not see the partial updates. In fact, even if two hits for the Web user (“nobody”) come in at the same time, they’ll be dealt different words (through a serialization, thanks to the “FOR UPDATE” in the select of line 108).
There’s a handful of uncaught die operations in the program that will make some unexpected and unfriendly errors in a CGI environment, but we’ll leave those for future tweaking.
In conclusion, it’s not hard to use PostgreSQL’s advanced features. It has finally arrived as a real, practical database. Until next time, enjoy!
Randal L. Schwartz is the chief Perl guru at Stonehenge Consulting and co-author of Learning Perl and Programming Perl. He can be reached at [email protected]. Code listings for this column can be found at http://www.stonehenge.com/merlyn/LinuxMag/.
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Mobile
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Upgrading Android: A Guided Tour
HPC
A Little (q)bit of Quantum Computing
Emailing HPC
Chasing The Number
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C Linked List Data Structure Explained with an Example C Program
by Himanshu Arora on August 24, 2012
Linked list is one of the fundamental data structures in C.
Knowledge of linked lists is must for C programmers. This article explains the fundamentals of C linked list with an example C program.
Linked list is a dynamic data structure whose length can be increased or decreased at run time.
How Linked lists are different from arrays? Consider the following points :
• An array is a static data structure. This means the length of array cannot be altered at run time. While, a linked list is a dynamic data structure.
• In an array, all the elements are kept at consecutive memory locations while in a linked list the elements (or nodes) may be kept at any location but still connected to each other.
When to prefer linked lists over arrays? Linked lists are preferred mostly when you don’t know the volume of data to be stored. For example, In an employee management system, one cannot use arrays as they are of fixed length while any number of new employees can join. In scenarios like these, linked lists (or other dynamic data structures) are used as their capacity can be increased (or decreased) at run time (as an when required).
How linked lists are arranged in memory?
Linked list basically consists of memory blocks that are located at random memory locations. Now, one would ask how are they connected or how they can be traversed? Well, they are connected through pointers. Usually a block in a linked list is represented through a structure like this :
struct test_struct
{
int val;
struct test_struct *next;
};
So as you can see here, this structure contains a value ‘val’ and a pointer to a structure of same type. The value ‘val’ can be any value (depending upon the data that the linked list is holding) while the pointer ‘next’ contains the address of next block of this linked list. So linked list traversal is made possible through these ‘next’ pointers that contain address of the next node. The ‘next’ pointer of the last node (or for a single node linked list) would contain a NULL.
How a node is created?
A node is created by allocating memory to a structure (as shown in above point) in the following way :
struct test_struct *ptr = (struct test_struct*)malloc(sizeof(struct test_struct));
So, as we can see above, the pointer ‘ptr’ now contains address of a newly created node. If the linked list is empty and first node is created then it is also known as head node.
Once a node is created, then it can be assigned the value (that it is created to hold) and its next pointer is assigned the address of next node. If no next node exists (or if its the last node) then as already discussed, a NULL is assigned. This can be done in following way :
...
...
ptr->val = val;
ptr->next = NULL;
...
...
How to search a node in a linked list?
Searching a node means finding the node that contains the value being searched. This is in fact a very simple task if we talk about linear search (Note that there can be many search algorithms). One just needs to start with the first node and then compare the value which is being searched with the value contained in this node. If the value does not match then through the ‘next’ pointer (which contains the address of next node) the next node is accessed and same value comparison is done there. The search goes on until last node is accessed or node is found whose value is equal to the value being searched. A code snippet for this may look like :
...
...
...
while(ptr != NULL)
{
if(ptr->val == val)
{
found = true;
break;
}
else
{
ptr = ptr->next;
}
}
...
...
...
How a node is deleted?
A node is deleted by first finding it in the linked list and then calling free() on the pointer containing its address. If the deleted node is any node other than the first and last node then the ‘next’ pointer of the node previous to the deleted node needs to be pointed to the address of the node that is just after the deleted node. Its just like if a person breaks away from a human chain then the two persons (between whom the person was) needs to join hand together to maintain the chain.
A Practical C Linked List Example
Here is a practical example that creates a linked list, adds some nodes to it, searches and deletes nodes from it.
#include<stdio.h>
#include<stdlib.h>
#include<stdbool.h>
struct test_struct
{
int val;
struct test_struct *next;
};
struct test_struct *head = NULL;
struct test_struct *curr = NULL;
struct test_struct* create_list(int val)
{
printf("\n creating list with headnode as [%d]\n",val);
struct test_struct *ptr = (struct test_struct*)malloc(sizeof(struct test_struct));
if(NULL == ptr)
{
printf("\n Node creation failed \n");
return NULL;
}
ptr->val = val;
ptr->next = NULL;
head = curr = ptr;
return ptr;
}
struct test_struct* add_to_list(int val, bool add_to_end)
{
if(NULL == head)
{
return (create_list(val));
}
if(add_to_end)
printf("\n Adding node to end of list with value [%d]\n",val);
else
printf("\n Adding node to beginning of list with value [%d]\n",val);
struct test_struct *ptr = (struct test_struct*)malloc(sizeof(struct test_struct));
if(NULL == ptr)
{
printf("\n Node creation failed \n");
return NULL;
}
ptr->val = val;
ptr->next = NULL;
if(add_to_end)
{
curr->next = ptr;
curr = ptr;
}
else
{
ptr->next = head;
head = ptr;
}
return ptr;
}
struct test_struct* search_in_list(int val, struct test_struct **prev)
{
struct test_struct *ptr = head;
struct test_struct *tmp = NULL;
bool found = false;
printf("\n Searching the list for value [%d] \n",val);
while(ptr != NULL)
{
if(ptr->val == val)
{
found = true;
break;
}
else
{
tmp = ptr;
ptr = ptr->next;
}
}
if(true == found)
{
if(prev)
*prev = tmp;
return ptr;
}
else
{
return NULL;
}
}
int delete_from_list(int val)
{
struct test_struct *prev = NULL;
struct test_struct *del = NULL;
printf("\n Deleting value [%d] from list\n",val);
del = search_in_list(val,&prev);
if(del == NULL)
{
return -1;
}
else
{
if(prev != NULL)
prev->next = del->next;
if(del == curr)
{
curr = prev;
}
else if(del == head)
{
head = del->next;
}
}
free(del);
del = NULL;
return 0;
}
void print_list(void)
{
struct test_struct *ptr = head;
printf("\n -------Printing list Start------- \n");
while(ptr != NULL)
{
printf("\n [%d] \n",ptr->val);
ptr = ptr->next;
}
printf("\n -------Printing list End------- \n");
return;
}
int main(void)
{
int i = 0, ret = 0;
struct test_struct *ptr = NULL;
print_list();
for(i = 5; i<10; i++)
add_to_list(i,true);
print_list();
for(i = 4; i>0; i--)
add_to_list(i,false);
print_list();
for(i = 1; i<10; i += 4)
{
ptr = search_in_list(i, NULL);
if(NULL == ptr)
{
printf("\n Search [val = %d] failed, no such element found\n",i);
}
else
{
printf("\n Search passed [val = %d]\n",ptr->val);
}
print_list();
ret = delete_from_list(i);
if(ret != 0)
{
printf("\n delete [val = %d] failed, no such element found\n",i);
}
else
{
printf("\n delete [val = %d] passed \n",i);
}
print_list();
}
return 0;
}
In the code above :
• The first node is always made accessible through a global ‘head’ pointer. This pointer is adjusted when first node is deleted.
• Similarly there is a ‘curr’ pointer that contains the last node in the list. This is also adjusted when last node is deleted.
• Whenever a node is added to linked list, it is always checked if the linked list is empty then add it as the first node.
Also, as you see from the above Linked list example, it also uses pointers. If you are new to C programming, you should understand the fundamentals of C pointers.
The output of the above code looks like :
$ ./ll
-------Printing list Start-------
-------Printing list End-------
creating list with headnode as [5]
Adding node to end of list with value [6]
Adding node to end of list with value [7]
Adding node to end of list with value [8]
Adding node to end of list with value [9]
-------Printing list Start-------
[5]
[6]
[7]
[8]
[9]
-------Printing list End-------
Adding node to beginning of list with value [4]
Adding node to beginning of list with value [3]
Adding node to beginning of list with value [2]
Adding node to beginning of list with value [1]
-------Printing list Start-------
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
-------Printing list End-------
Searching the list for value [1]
Search passed [val = 1]
-------Printing list Start-------
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
-------Printing list End-------
Deleting value [1] from list
Searching the list for value [1]
delete [val = 1] passed
-------Printing list Start-------
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
-------Printing list End-------
Searching the list for value [5]
Search passed [val = 5]
-------Printing list Start-------
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
-------Printing list End-------
Deleting value [5] from list
Searching the list for value [5]
delete [val = 5] passed
-------Printing list Start-------
[2]
[3]
[4]
[6]
[7]
[8]
[9]
-------Printing list End-------
Searching the list for value [9]
Search passed [val = 9]
-------Printing list Start-------
[2]
[3]
[4]
[6]
[7]
[8]
[9]
-------Printing list End-------
Deleting value [9] from list
Searching the list for value [9]
delete [val = 9] passed
-------Printing list Start-------
[2]
[3]
[4]
[6]
[7]
[8]
-------Printing list End-------
As you see from the above output, it does all the fundamental Linked list operations. It creates a linked list, adds some nodes to it, searches and deletes nodes from it.
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{ 41 comments… read them below or add one }
1 Mitra Kaseebhotla August 24, 2012 at 10:37 am
Very good article. Reminded my college days.:)
2 TommyZee August 24, 2012 at 1:49 pm
The good old days :(
3 Jalal Hajigholamali August 24, 2012 at 8:24 pm
Hi,
Very good article. I sent it to my students..
4 Sander August 25, 2012 at 5:44 am
Yours is one of the best site to learn *nix/Dev related things!
You should review the organization tough, this way it would be easier to access the articles of a specific subject.
5 Chris August 30, 2012 at 1:26 pm
Found your site a few weeks back, this article came in handy. I am working on learning C by way of implementing things I know in Java. I just finished my third level Data Structures and Algorithm course.
Thanks for all the great articles!
6 dusKO|2scica August 31, 2012 at 11:45 am
Ok but are there people who still use aloc, maloc i realoc
new and delete do it for me….
7 C Dev November 7, 2012 at 4:22 am
You cannot use new and delete in C program. So Malloc and free are required for dynamic memory allocation and release
8 Dusko Koscica November 10, 2012 at 5:46 am
It is good for programer to learn new ways of C++, I don’t say that you should skip the list but there area better ways in C++.
First of all you can create the array with new operator, after you establish the size in memory, for example you can convert the number to binary with stack, and it is very good thing. In some ocasions you can use the vectors, the vector is faster then list and it is a question when to use list or the array.
This expanation might do you more damage than good, even it is true, because when people switch to C++ they keep their old habits, that is the problem, in this disc. Vell we can’t change people, people are what they are… and democratic way sometimes sacks at this, O no I am not saing that it is ok or only way to be comunist it is time to ocupy C++ and C
9 Amar November 14, 2012 at 8:08 am
wat would be the op if we just print ptr??
10 Dusko Kosciac November 15, 2012 at 7:39 am
int *ptrInt = new int[100];
How would you comment this one, Yes it is possible to do it in C, with other comanda
11 Anonymous November 16, 2012 at 4:31 am
@Dusko I’ve checked changes introduced by C11 last standardized C revision and didn’t see any new operator. Please stop confusing C and C++.
12 duskokoscica November 17, 2012 at 3:23 am
oK yOU MIisteR anoNymous is this enough C for YOu and others
double *niz = malloc(vel*sizeof(double));
I would like to hear Your coment on thIsone …
13 duskoKoscica November 19, 2012 at 11:19 am
Well, I see, try this code:
int *n = (int *) malloc(sizeof(int));
The problem for me is this statement:”An array is a static data structure. This means the length of array cannot be altered at run time”.
If you ask the person how manny members of array, or you are able to estimate the size of the array, this can be used…
And also, I hope that my discusion was the contribution to the toppic, it dos’t mean that the article is bad…
Some thihs have changed, and the bigger problem for me is the fact that people don’t accept the change that easy….
Ok
14 DuskoKoscica December 8, 2012 at 1:45 pm
Have I ever told YA about the realloc, pay per view on that one?
15 Pedro Rodrigues January 9, 2013 at 9:06 am
hello,
First of all, thanks for this post.
When i was trying your code i discovered that the delete function have one problem when the list have only one element that is on the same time the current and the head. When this happens, in your code, the pointer “head” will not be modified what can bring a wrong results.
Correct me if I’m wrong!
Regards
Pedro Rodrigues
16 Mittal January 10, 2013 at 3:03 am
really nice n easy to understand… :)
17 elakkiya January 23, 2013 at 11:36 pm
Really good, exp also very easy
18 rahul waghmare February 2, 2013 at 2:21 am
linked list is very interesting part and easy to understand
19 iyappan April 5, 2013 at 2:00 am
explanation very good sir
20 stefan April 8, 2013 at 11:19 am
hi , can somebody here help me solve the following exercise in c programing ?
———
#Lab2 – Double chain lists and Text editing.
Introduction: We want to stock and manipulate texts magnitude in memory.then
in will reprezantojme a text with a double chain list. Each node of the list will contain:
- A table of characters called date, with a magnitude N.
- an integer called the size that determines the number of elements to vendusur in the table.
- A pointer next to Pinto successor node.
- A pointer prev Pinto to preceding node.
Part one
1.Determine constant N avec un # define.
2.Determine node structure to maintain an element.
3.Schedule a text structure to store Text. The structure includes:
. A ponter called head to identify the first node.
. A ponter called tail to identify the last node.
. An integer count to identify the number of elements in the text.
. An integer size to identify the number of characters of text.
Part two
1. Write a iter structure to maintain a cursor in a text (relative position) which
contains:
- A ponter ptr to identify Text node where the cursor.
– an integer ICASA to identify relevant element to the table,
NULL ptr value indicates that the cursor is placed before the first element.
Part three
Basis functions: Write the following functions.
2.node * create_node () creates and returns an empty node.
3.text * create_text () creates and returns an empty text.
4.void free_text (text * t) that frees the text t and the respective nodes that make up.
5.iter * create_iter_at_begin (text * t) that creates a cursor and positioning to
The first character of the Text.
6.void free_iter (iter * pos) that frees the cursor pos.
7.void move_forward_iter (iter * pos, int n), which moves the cursor n characters
to the right.
8.void move_backward_iter (iter * pos, int n), which moves the cursor n characters
left
Cursor movement functions should ensure that the cursor stays always between element
first and last.
9.void append_text (text * t, char * s) that adds a chain s character at the end of
texti. This fonksion should shrytezoje maximum length of vacancies occurring on the last node
Text t.
10.void push_text (text * t, char * s) that adds a chain s character at the beginning of
texti.
11.void show_text (text * t, iter * pos) that displays the Text t with an asterisk [*] in position
where the cursor pos. If pos-> ptr is NULL, the star appears before the first character.
12.void insert_text (text * t, iter * pos, char * s) s after the chain adds
position pos. Besides further update to show the last character entered.
13.void delete_text (text * t, iter * pos, int n) removes n characters before pos.
In this case, but should again be set to update showing the last character that is not deleted..
Main menu of the program:
Written functions will be used to write a program that will read a text file and
further will make the relevant manipulations. Menu should be implemented:
Reading a file. Cursor is placed before the first element.
Display Text reading.
Move kursoni n characters right.
Move kursoni n characters left.
Delete n characters from the position where the cursor.
Enter a character chain starting from the cursor position.
Output from the program.
advice:
Organize project in Visual Studio where:
- File lab2.h, is the file header which decided the declarations of structures and functions.
- File lab2.c, which contains coded functions,
- File main.c source, that ^ contains the basic program.
21 MOBITI MAHENGE April 15, 2013 at 7:02 am
The article or explanations are very good, congratulation for that; my request is i need the knowledge like this with reference to C++ language…by Mobiti Mahenge
22 Hemanth April 16, 2013 at 1:09 am
Awesome Explanation, keep up the good work.
23 Ali April 16, 2013 at 7:07 pm
Thank you!! very handy!!
24 abhishek May 11, 2013 at 11:31 am
Its a better…. ND uses exam time
25 Frank May 26, 2013 at 12:02 am
I used to think that there is no boolean conditions in C ??? Is this correct or wrong?
26 saranya June 23, 2013 at 9:30 am
how to understand a logic in c???
27 LeBoepp June 27, 2013 at 4:56 am
@Pedro Rodrigues
You are right.
In function delete_from_list() you can change
if(del == curr)
{
curr = prev;
}
else if(del == head)
{
head = del->next;
}
to
if(del == curr)
{
curr = prev;
}
if(del == head)
{
head = del->next;
}
Remove the else and it works fine.
28 Rana June 28, 2013 at 2:13 am
Nice ,very effective article.thx so much
29 DHANUNJAYA REDDY July 22, 2013 at 9:05 pm
good example programs
30 pavithra July 25, 2013 at 10:39 am
i am doing my B.E.cse 2nd yr i am a biology student i dont even the basics of c &cannot understand the subject please give me sum suggestion to rectify it
31 BOOPATHI August 13, 2013 at 11:49 pm
i am also biology student please give some tips about pagination using circular linked list.
32 Anonymous October 29, 2013 at 1:46 am
easily understandable thanks a lot
33 shraddha December 3, 2013 at 8:58 pm
liked it very much…….
34 sutha merthi December 15, 2013 at 12:19 am
nice well undestand sir
35 Ishan January 6, 2014 at 3:44 am
i modified main() of this program and its not functioning the way i intended.
int main(void)
{
int i = 0, ret = 0;
print_list();
for(i = 5; i<10; i++)
add_to_list(i,true);
print_list();
for(i = 5; i<10; i++)
{
ret = delete_from_list(i);
if(ret != 0)
{
printf("\n delete [val = %d] failed, no such element found\n",i);
}
else
{
printf("\n delete [val = %d] passed \n",i);
}
}
print_list();
return 0;
}
the output i am getting wrong is that although it every value gets deleted including [9] as it shows
delete [val=9] passed
but last print_list()
it is still printing
[9]
36 Madalina January 21, 2014 at 6:48 am
Awesome explanation! Thank you very much!
37 Dimpal January 22, 2014 at 12:06 pm
Excellent….:)
38 varun February 1, 2014 at 2:44 am
Any one pls write the program to find exact middle node in a linked list with out going to last node????????????
39 Prasad Upganlawar February 24, 2014 at 1:11 am
Can we use link list on stack memory? Why?
40 k4rp March 25, 2014 at 4:44 pm
@Prasad Upganlawar
Of course you can. Its the fundamental method for implementing a stack. To understand the stack memory, imagine you have a stack of dirty dishes. Everytime a man eats the food, he put the dish on the table. Another man that finishes, places the dish right above the previous and so on(action 1). When someone is gonna get and wash the dishes, he takes the first dish from the top of the stack, washes it, then he takes again the first dish from the stack and so on until there are no more dishes to be washed. (action 2).
In programming manner, call action 1 “push” and action 2 “pop”.
The code above can be easilly adapted on a stack. add_to_list() can be modified to act like push() and delete_from_list() as a pop(). push() must always add a node to the “head” and pop() has to return always the “head” node and delete it from the list.
I hope this makes things more clear for you.
Btw i find this article great! Keep up the good work!
41 annu April 2, 2014 at 5:11 am
Very helpful
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| 0.885947 |
Child domain across WAN and local child domain
Discussion in 'Active Directory' started by Hiro, Jul 13, 2005.
1. Hiro
Hiro Guest
Only have one domain (call it domain.com)
Location A:
-Have a main domain controller (call it alpha) that is setup with dhcp, dns,
and ad.
-All local users will authenticate to this main controller.
-On the same LAN want to have a child domain controller (call it beta) that
has all the information of the first but does not have access to change any
information, it also needs to recieve regular updates. This is setup for our
website authentication and will only be used by a few users.
Location B:
-Connected to location A via a dedicated T1 (1.5 Mbit/1.5 Mbit).
-Want a child domain controller (call it gamma) that all users in location B
authenticate to.
-Gamma should recieve its catalog from alpha but also be able to make
changes and send those changes back to alpha.
Questions:
-How should I setup the child domains to recieve the updates and allow gamma
to make changes to the catalog?
Hiro, Jul 13, 2005
#1
1. Advertisements
2. How should I setup the child domains to recieve the updates and allow
You don't need a child domain for scenario two. Nor do you really need a
child domain for scenario one -that's a serious security problem!
I'll elaborate under each statement...
I believe there's a major misconception here as to what this domain is for.
Firstly, it is impossible to guarantee the lack of access you want. The
forest is now the true security boundary; the domain is an administrative
boundary (and also a replication boundary).
I have no idea what kind of regular updates you are referring to, but the
necessary naming contexts are indeed replicated frequently.
Do not create a child domain for web-based access. Create a separate forest
or use an AD/AM directory. Trust no one, and never allow direct access to
your directory from insecure sources.
Why? This is an ideal solution for a separate site with a DC from domain-a
available to service local (to the site) users and computers.
Best bet is to create another DC in the existing domain. Make this a DHCP,
DNS and GC server. Create sites and have a DC in each one.
Paul Williams [MVP], Jul 14, 2005
#2
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| 0.718062 |
Subscribe to our Youtube Channel - https://www.youtube.com/channel/UCZBx269Tl5Os5NHlSbVX4Kg
Slide14.JPG
Slide15.JPG
Slide16.JPG Slide17.JPG
1. Chapter 12 Class 11 Introduction to Three Dimensional Geometry
2. Serial order wise
Transcript
Ex 12.3, 5 Find the coordinates of the points which trisect the line segment joining the points P (4, 2, –6) and Q (10, –16, 6). Let Point A (a, b, c) & point B (p, q, r) trisect the line segment PQ i.e. PA = AB = BC Point A divides PQ in the ratio of 1 : 2 We know that , Coordinate of point that divides the line segment joining A(x1, y1, z1) & B(x2, y2, z2) internally in the ratio m: n is P(x, y, z) = ((〖𝑚 𝑥〗_2 +〖 𝑛 𝑥〗_1)/(𝑚 + 𝑛),(〖𝑚 𝑦〗_2 +〖 𝑛 𝑦〗_1)/(𝑚 + 𝑛),(〖𝑚 𝑧〗_2 +〖 𝑛 𝑧〗_1)/(𝑚 + 𝑛)) Here, m = 1 , n = 2 x1 = 4 , y1 = 2 , z1 = –6 x2 = 10 , y2 = –16 , z2 = 6 Coordinate of A are (a, b, c) = ((10 (1) + 4 (2))/(1 + 2),(−16 (1) + 2 (2))/(1 + 2),(6 (1) + (− 6) (2))/(1 + 2)) (a, b, c) = ((10 + 8)/3,(− 16 + 4)/3,(6 − 12)/3) (a, b, c) = (6, –4, –2) Hence, coordinates of A = (6, –4, –2) Now, Point B (p, q, r) divides AQ in the ratio 1 : 1 So, B is mid-point of AQ Coordinates of B = ((𝑥_(1 )+ 𝑥_2)/2,(𝑦_(1 )+ 𝑦_2)/2,(𝑧_(1 )+ 𝑧_2)/2) = ((6 + 10)/2,(−4 + (−16))/2,(−2 + 6)/2) = (160/2,(−20)/2,4/2) = (8, –10, 2) Hence coordinate of Point B = (8, –10, 2)
About the Author
Davneet Singh's photo - Teacher, Computer Engineer, Marketer
Davneet Singh
Davneet Singh is a graduate from Indian Institute of Technology, Kanpur. He has been teaching from the past 9 years. He provides courses for Maths and Science at Teachoo.
|
__label__pos
| 0.993437 |
vue实现自定义H5视频播放器
更新日期: 2019-07-01阅读: 4k标签: 视频
前言
前段时间基于vue写了一个自定义的video播放器组件,踩了一些小坑, 这里做一下复盘分享出来,避免日后重复踩坑...
设计阶段
这里就直接放几张完成后的播放状态图吧,界面布局基本就是flex+vw适配一把梭,也比较容易.
需要实现的几个功能基本都标注出来了; 除了还有一个视频加载失败的...下面就这届上代码了;刚开始构思的时候考虑了一下功能的实现方式: 一是用原生的dom操作,获取video元素后,用addEventListener来监听; 二是用vue的方式绑定事件监听; 最后图方便采用了两者结合的方式,但是总感觉有点乱, 打算后期再做一下代码格式优化.
video组件实现过程
组件模板部分
主要是播放器的几种播放状态的逻辑理清楚就好了, 即: 播放中,缓存中,暂停,加载失败这几种情况,下面按功能分别说一下
<template>
<div class="video-player">
<!-- 播放器界面; 兼容ios controls-->
<video
ref="video"
v-if="showVideo"
webkit-playsinline="true"
playsinline="true"
x-webkit-airplay="true"
x5-video-player-type="h5"
x5-video-player-fullscreen="true"
x5-video-orientation="portraint"
style="object-fit:fill"
preload="auto"
muted="true"
poster="https://photo.mac69.com/180205/18020526/a9yPQozt0g.jpg"
:src="src"
@waiting="handleWaiting"
@canplaythrough="state.isLoading = false"
@playing="state.isLoading = false, state.controlBtnShow = false, state.playing=true"
@stalled="state.isLoading = true"
@error="handleError"
>您的浏览器不支持html5</video>
<!-- 兼容Android端层级问题, 弹出层被覆盖 -->
<img
v-show="!showVideo || state.isEnd"
class="poster"
src="https://photo.mac69.com/180205/18020526/a9yPQozt0g.jpg"
alt
>
<!-- 控制窗口 -->
<div
class="control"
v-show="!state.isError"
ref="control"
@touchstart="touchEnterVideo"
@touchend="touchLeaveVideo"
>
<!-- 播放 || 暂停 || 加载中-->
<div class="play" @touchstart.stop="clickPlayBtn" v-show="state.controlBtnShow">
<img
v-show="!state.playing && !state.isLoading"
src="../../assets/video/content_btn_play.svg"
>
<img
v-show="state.playing && !state.isLoading"
src="../../assets/video/content_btn_pause.svg"
>
<div class="loader" v-show="state.isLoading">
<div class="loader-inner ball-clip-rotate">
<div></div>
</div>
</div>
</div>
<!-- 控制条 -->
<div class="control-bar" :style="{ visibility: state.controlBarShow ? 'visible' : 'hidden'}">
<span class="time">{{video.displayTime}}</span>
<span class="progress" ref="progress">
<img
class="progress-btn ignore"
:style="{transform: `translate3d(${video.progress.current}px, 0, 0)`}"
src="../../assets/video/content_ic_tutu.svg"
>
<span class="progress-loaded" :style="{ width: `${video.loaded}%`}"></span>
<!-- 设置手动移动的进度条 -->
<span
class="progress-move"
@touchmove.stop.prevent="moveIng($event)"
@touchstart.stop="moveStart($event)"
@touchend.stop="moveEnd($event)"
></span>
</span>
<span class="total-time">{{video.totalTime}}</span>
<span class="full-screen" @click="fullScreen">
<img src="../../assets/video/content_ic_increase.svg" alt>
</span>
</div>
</div>
<!-- 错误弹窗 -->
<div class="error" v-show="state.isError">
<p class="lose">视频加载失败</p>
<p class="retry" @click="retry">点击重试</p>
</div>
</div>
</template>
播放器初始化
这里有个坑点我就是当父元素隐藏即display:none时,getBoundingClientRect()是获取不到元素的尺寸数值的,后来查了MDN文档,按上面说的改了一下border也没有用,最后尝试设置元素visibility属性为hidden后发现就可以获取了.
getBoundingClientRect() : 返回元素的大小及其相对于视口的位置, 这个api在计算元素相对位置的时候挺好用的.
init() {
// 初始化video,获取video元素
this.$video = this.$el.getElementsByTagName("video")[0];
this.initPlayer();
},
// 初始化播放器容器, 获取video-player元素
// getBoundingClientRect()以client可视区的左上角为基点进行位置计算
initPlayer() {
const $player = this.$el;
const $progress = this.$el.getElementsByClassName("progress")[0];
// 播放器位置
this.player.$player = $player;
this.progressBar.$progress = $progress;
this.player.pos = $player.getBoundingClientRect();
this.progressBar.pos = $progress.getBoundingClientRect()
this.video.progress.width = Math.round($progress.getBoundingClientRect().width);
},
播放 && 暂停点击
我这里把事件监听都放在只有满足正在播放视频才开始事件监听; 感觉原生监听和vue方式的监听混合在一起写有点别扭...emem...这里需要对this.$video.play()做一个异常处理,防止video刚开始加载的时候失败,如果视频链接出错,play方法调用不了会抛错,后面我也用了video的error事件去监听播放时的错误
// 点击播放 & 暂停按钮
clickPlayBtn() {
if (this.state.isLoading) return;
this.isFirstTouch = false;
this.state.playing = !this.state.playing;
this.state.isEnd = false;
if (this.$video) {
// 播放状态
if (this.state.playing) {
try {
this.$video.play();
this.isPauseTouch = false;
// 监听缓存进度
this.$video.addEventListener("progress", e => {
this.getLoadTime();
});
// 监听播放进度
this.$video.addEventListener(
"timeupdate",
throttle(this.getPlayTime, 100, 1)
);
// 监听结束
this.$video.addEventListener("ended", e => {
// 重置状态
this.state.playing = false;
this.state.isEnd = true;
this.state.controlBtnShow = true;
this.video.displayTime = "00:00";
this.video.progress.current = 0;
this.$video.currentTime = 0;
});
} catch (e) {
// 捕获url异常出现的错误
}
}
// 停止状态
else {
this.isPauseTouch = true;
this.$video.pause();
}
}
},
视频控制条显示和隐藏
这里需要加两个开关; 首次触屏和暂停触屏; 做一下显示处理即可
// 触碰播放区
touchEnterVideo() {
if (this.isFirstTouch) return;
if (this.hideTimer) {
clearTimeout(this.hideTimer);
this.hideTimer = null;
}
this.state.controlBtnShow = true;
this.state.controlBarShow = true;
},
// 离开播放区
touchLeaveVideo() {
if (this.isFirstTouch) return;
if (this.hideTimer) {
clearTimeout(this.hideTimer);
}
// 暂停触摸, 不隐藏
if (this.isPauseTouch) {
this.state.controlBtnShow = true;
this.state.controlBarShow = true;
} else {
this.hideTimer = setTimeout(() => {
this.state.controlBarShow = false;
// 加载中只显示loading
if (this.state.isLoading) {
this.state.controlBtnShow = true;
} else {
this.state.controlBtnShow = false;
}
this.hideTimer = null;
}, 3000);
}
},
视频错误处理和等待处理
这里错误直接用error事件, 加载中用stalled事件来监听视频阻塞状态,等待数据加载用的waiting事件; 显示对应的loading动画即可
// loading动画
@keyframes rotate {
0% {
transform: rotate(0deg);
}
50% {
transform: rotate(180deg);
}
100% {
transform: rotate(360deg);
}
}
.loader {
width: 58px;
height: 58px;
background: rgba(15, 16, 17, 0.3);
border-radius: 50%;
position: relative;
.ball-clip-rotate {
position: absolute;
left: 50%;
top: 50%;
transform: translate(-50%, -50%);
> div {
width: 15px;
height: 15px;
border-radius: 100%;
margin: 2px;
animation-fill-mode: both;
border: 2px solid #fff;
border-bottom-color: transparent;
height: 26px;
width: 26px;
background: transparent;
display: inline-block;
animation: rotate 0.75s 0s linear infinite;
}
}
}
播放时间设置
基本就是video对象的currentTime和duration这两个属性; 这里注意下视频如果没有设置预加载属性preload的话,在video元素初始化的时候是获取不到duration的...那你只能在播放的时候去拿了.
// 获取播放时间
getPlayTime() {
const percent = this.$video.currentTime / this.$video.duration;
this.video.progress.current = Math.round(
this.video.progress.width * percent
);
// 赋值时长
this.video.totalTime = timeParse(this.$video.duration);
this.video.displayTime = timeParse(this.$video.currentTime);
},
// 获取缓存时间
getLoadTime() {
// console.log('缓存了...',this.$video.buffered.end(0));
this.video.loaded =
(this.$video.buffered.end(0) / this.$video.duration) * 100;
},
手动滑动进度条控制
这里直接用touch事件即可; 注意touchend中使用e.changedTouches;因为当手指离开屏幕，touches和targetTouches中对应的元素会同时移除，而changedTouches仍然会存在元素。
touches: 当前屏幕上所有触摸点的列表;
targetTouches: 当前对象上所有触摸点的列表;
changedTouches: 涉及当前(引发)事件的触摸点的列表
// 手动调节播放进度
moveStart(e) {},
moveIng(e) {
// console.log("触摸中...");
let currentX = e.targetTouches[0].pageX;
let offsetX = currentX - this.progressBar.pos.left;
// 边界检测
if (offsetX <= 0) {
offsetX = 0
}
if (offsetX >= this.video.progress.width) {
offsetX = this.video.progress.width
}
this.video.progress.current = offsetX;
let percent = this.video.progress.current / this.video.progress.width;
this.$video.duration && this.setPlayTime(percent, this.$video.duration)
},
moveEnd(e) {
// console.log("触摸结束...");
let currentX = e.changedTouches[0].pageX;
let offsetX = currentX - this.progressBar.pos.left;
this.video.progress.current = offsetX;
// 这里的offsetX都是正数
let percent = offsetX / this.video.progress.width;
this.$video.duration && this.setPlayTime(percent, this.$video.duration)
},
// 设置手动播放时间
setPlayTime(percent, totalTime) {
this.$video.currentTime = Math.floor(percent * totalTime);
},
全屏功能
这个功能在手机上会有写兼容性问题...有待完善
// 设置全屏
fullScreen() {
console.log('点击全屏...');
if (!this.state.fullScreen) {
this.state.fullScreen = true;
this.$video.webkitRequestFullScreen();
} else {
this.state.fullScreen = false;
document.webkitCancelFullScreen();
}
坑点汇总
1.视频预加载才能获取时长
需要设置预加载 preload="auto"
2.Element.getBoundingClientRect()方法返回元素的大小及其相对于视口的位置
父元素设置display:none时获取不到尺寸数据民谣改为visibility:hidden
3.play()方法异常捕获
try{ xxxxx.play } catch(e) { yyyyyy }
4.安卓手机video兼容性处理, 视频播放时层级置顶,会影响全局弹出层样式
我这里做的处理是当弹出层出现时把视频给隐藏掉(宽高为0,或者直接去掉),用封面图来替代
5.ios下全屏处理
设置相应属性即可, playsinline
代码直通车: https://github.com/appleguard
来自：https://segmentfault.com/a/1190000019623347
链接: https://www.fly63.com/article/detial/3961
h5页面自动播放视频、音频_关于媒体文件自动全屏播放的实现方式
在移动端（ios和android）播放视频的时候，我们即使定义了autoplay属性，仍然不能自动播放。这是由于手机浏览器为了防止浪费用户的网络流量，在默认情况下是不允许媒体文件自动播放的，除非用户自己对浏览器进行设置才能支持autoplay。
如何将视频设置为网页背景【转】
有时候为一个网页添加一个动画效果的背景，会让网页增加一定的韵味，让网页看起来与众不同。需要用到了video/标签，然后在source里面写视频的路径，autoplay用来使其自动播放，muted用来使其静音，loop为循环播放
js实现截取视频帧图片作为封面预览图
前端需要把视频文件的第一帧图像截取出来，并做为缩略图显示在页面上，这里需要利用HTML5中强大的画布canvas来实现该功能
优化 MP4 视频以便更快的网络串流
随着 Flash 的落寞 以及 移动设备的爆发性增长 ，越来越多的内容以 HTML5 视频的方式传递。在上一篇文章中你甚至能看到 使用 HTML5 视频替换 GIF 动图来优化网站访问速度 这样的技巧
js获取上传音视频的时长
获取上传视频路径，将该路径放入video标签，获取视频时长。方式一：隐藏一个音频标签，播放获取。方式二;通过new Audio的方式获取。上传之前限制一下视频的时长
移动端视频h5表现问题汇总
同屏播放视频、移动端视频预加载：由于移动端不能预加载视频，所以hack一种方案：监听WXJSBridge WeixinJSBridgeReady、微信安卓环境下需要在touchmove事件中阻止掉默认事件，否则不能触发视频播放 、 由于微信安卓版本基于x5内核，视频会出现全屏按钮，而且去不掉，会误导用户点击
Web端直接播放 .ts 视频
最近项目中需要前端播放 .ts 格式视频，捣鼓了几天学习到很多知识，也发掘了一种优秀的解决方案，项目中已存储的 .ts 切片数量众多，已经占用了NAS服务器绝大部分的资源，生成的 .m3u8 索引虽然非常小
使用 multipart/x-mixed-replace 实现 http 实时视频流
关于实时视频传输，业界已经有非常多成熟方案，分别应用在不同需求场景。本文介绍一种基于 HTTP ，非常简单、易理解的方案，实用性不强，但有助于理解 HTTP 协议。
h5视频播放踩坑记录
随着抖音、快手这类的视频类app的火爆，移动端h5视频类应用也随之兴起，使用video播放的场景也越来越多，本篇文章主要例举了移动端视频播放的一些场景和个人在开发过程中遇到的一些问题
如何实现沉浸式视频体验？
沉浸式视频体验，大致内容是一个页面里有几十个视频，用户点击其中一个视频时，该视频自动滑动到屏幕可视区域的顶部开始播放，并暂停其他视频，该视频滑出屏幕可视区域之后要自动暂停。
点击更多...
内容以共享、参考、研究为目的,不存在任何商业目的。其版权属原作者所有,如有侵权或违规,请与小编联系!情况属实本人将予以删除!
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Places Autocomplete
Select platform: Android iOS JavaScript Web Service
Introduction
Autocomplete is a feature of the Places library in the Maps JavaScript API. You can use autocomplete to give your applications the type-ahead-search behavior of the Google Maps search field. The autocomplete service can match on full words and substrings, resolving place names, addresses, and plus codes. Applications can therefore send queries as the user types, to provide on-the-fly place predictions.
Getting started
Before using the Places library in the Maps JavaScript API, first ensure that the Places API is enabled in the Google Cloud Console, in the same project you set up for the Maps JavaScript API.
To view your list of enabled APIs:
1. Go to the Google Cloud Console.
2. Click the Select a project button, then select the same project you set up for the Maps JavaScript API and click Open.
3. From the list of APIs on the Dashboard, look for Places API.
4. If you see the API in the list, you’re all set. If the API is not listed, enable it:
1. At the top of the page, select ENABLE API to display the Library tab. Alternatively, from the left side menu, select Library.
2. Search for Places API, then select it from the results list.
3. Select ENABLE. When the process finishes, Places API appears in the list of APIs on the Dashboard.
Loading the library
The Places service is a self-contained library, separate from the main Maps JavaScript API code. To use the functionality contained within this library, you must first load it using the libraries parameter in the Maps API bootstrap URL:
<script async
src="https://maps.googleapis.com/maps/api/js?key=YOUR_API_KEY&libraries=places&callback=initMap">
</script>
See the Libraries Overview for more information.
Summary of classes
The API offers two types of autocomplete widgets, which you can add via the Autocomplete and SearchBox classes respectively. In addition, you can use the AutocompleteService class to retrieve autocomplete results programmatically (see the Maps JavaScript API Reference: AutocompleteService class).
Below is a summary of the classes available:
• Autocomplete adds a text input field to your web page, and monitors that field for character entries. As the user enters text, autocomplete returns place predictions in the form of a dropdown pick list. When the user selects a place from the list, information about the place is returned to the autocomplete object, and can be retrieved by your application. See the details below.
An autocomplete text field, and the pick list of place
predictions supplied as the user enters the search query.
Figure 1: Autocomplete text field and pick list
A completed address form.
Figure 2: Completed address form
• SearchBox adds a text input field to your web page, in much the same way as Autocomplete. The differences are as follows:
• The main difference lies in the results that appear in the pick list. SearchBox supplies an extended list of predictions, which can include places (as defined by the Places API) plus suggested search terms. For example, if the user enters 'pizza in new', the pick list may include the phrase 'pizza in New York, NY' as well as the names of various pizza outlets.
• SearchBox offers fewer options than Autocomplete for restricting the search. In the former, you can bias the search towards a given LatLngBounds. In the latter, you can restrict the search to a particular country and particular place types, as well as setting the bounds. For more information, see below.
A completed address form.
Figure 3: A SearchBox presents search terms and place predictions.
See the details below.
• You can create an AutocompleteService object to retrieve predictions programmatically. Call getPlacePredictions() to retrieve matching places, or call getQueryPredictions() to retrieve matching places plus suggested search terms. Note: AutocompleteService does not add any UI controls. Instead, the above methods return an array of prediction objects. Each prediction object contains the text of the prediction, as well as reference information and details of how the result matches the user input. See the details below.
Adding an Autocomplete widget
The Autocomplete widget creates a text input field on your web page, supplies predictions of places in a UI pick list, and returns place details in response to a getPlace() request. Each entry in the pick list corresponds to a single place (as defined by the Places API).
The Autocomplete constructor takes two arguments:
• An HTML input element of type text. This is the input field that the autocomplete service will monitor and attach its results to.
• An optional AutocompleteOptions argument, which can contain the following properties:
• An array of data fields to be included in the Place Details response for the user's selected PlaceResult. If the property is not set or if ['ALL'] is passed in, all available fields are returned and billed for (this is not recommended for production deployments). For a list of fields, see PlaceResult.
• An array of types that specifies an explicit type or a type collection, as listed in the supported types. If no type is specified, all types are returned.
• bounds is a google.maps.LatLngBounds object specifying the area in which to search for places. The results are biased towards, but not restricted to, places contained within these bounds.
• strictBounds is a boolean specifying whether the API must return only those places that are strictly within the region defined by the given bounds. The API does not return results outside this region even if they match the user input.
• componentRestrictions can be used to restrict results to specific groups. Currently, you can use componentRestrictions to filter by up to 5 countries. Countries must be passed as as a two-character, ISO 3166-1 Alpha-2 compatible country code. Multiple countries must be passed as a list of country codes.
Note: If you receive unexpected results with a country code, verify that you are using a code which includes the countries, dependent territories, and special areas of geographical interest you intend. You can find code information at Wikipedia: List of ISO 3166 country codes or the ISO Online Browsing Platform.
• placeIdOnly can be used to instruct the Autocomplete widget to retrieve only Place IDs. On calling getPlace() on the Autocomplete object, the PlaceResult made available will only have the place id, types and name properties set. You can use the returned place ID with calls to the Places, Geocoding, Directions or Distance Matrix services.
Constraining Autocomplete predictions
By default, Place Autocomplete presents all place types, biased for predictions near the user's location, and fetches all available data fields for the user's selected place. Set Place Autocomplete options to present more relevant predictions based on your use case.
Set options at construction
The Autocomplete constructor accepts an AutocompleteOptions parameter to set constraints at widget creation. The following example sets the bounds, componentRestrictions, and types options to request establishment type places, favoring those within the specified geographic area and restricting predictions to only places within the United States. Setting the fields option specifies what information to return about the user's selected place.
Call setOptions() to change an option's value for an existing widget.
TypeScript
const center = { lat: 50.064192, lng: -130.605469 };
// Create a bounding box with sides ~10km away from the center point
const defaultBounds = {
north: center.lat + 0.1,
south: center.lat - 0.1,
east: center.lng + 0.1,
west: center.lng - 0.1,
};
const input = document.getElementById("pac-input") as HTMLInputElement;
const options = {
bounds: defaultBounds,
componentRestrictions: { country: "us" },
fields: ["address_components", "geometry", "icon", "name"],
strictBounds: false,
types: ["establishment"],
};
const autocomplete = new google.maps.places.Autocomplete(input, options);
JavaScript
const center = { lat: 50.064192, lng: -130.605469 };
// Create a bounding box with sides ~10km away from the center point
const defaultBounds = {
north: center.lat + 0.1,
south: center.lat - 0.1,
east: center.lng + 0.1,
west: center.lng - 0.1,
};
const input = document.getElementById("pac-input");
const options = {
bounds: defaultBounds,
componentRestrictions: { country: "us" },
fields: ["address_components", "geometry", "icon", "name"],
strictBounds: false,
types: ["establishment"],
};
const autocomplete = new google.maps.places.Autocomplete(input, options);
Specify data fields
Specify data fields to avoid being billed for Places Data SKUs you don't need. Include the fields property in the AutocompleteOptions that are passed to the widget constructor, as demonstrated in the previous example, or call setFields() on an existing Autocomplete object.
autocomplete.setFields(["place_id", "geometry", "name"]);
Define biases and search-area boundaries for Autocomplete
You can bias the autocomplete results to favor an approximate location or area, in the following ways:
• Set the bounds on creation of the Autocomplete object.
• Change the bounds on an existing Autocomplete.
• Set the bounds to the map's viewport.
• Restrict the search to the bounds.
• Restrict the search to a specific country.
The previous example demonstrates setting bounds at creation. The following examples demonstrate the other biasing techniques.
Change the bounds of an existing Autocomplete
Call setBounds() to change the search area on an existing Autocomplete to rectangular bounds.
TypeScript
const southwest = { lat: 5.6108, lng: 136.589326 };
const northeast = { lat: 61.179287, lng: 2.64325 };
const newBounds = new google.maps.LatLngBounds(southwest, northeast);
autocomplete.setBounds(newBounds);
JavaScript
const southwest = { lat: 5.6108, lng: 136.589326 };
const northeast = { lat: 61.179287, lng: 2.64325 };
const newBounds = new google.maps.LatLngBounds(southwest, northeast);
autocomplete.setBounds(newBounds);
Set the bounds to the map's viewport
Use bindTo() to bias the results to the map's viewport, even while that viewport changes.
TypeScript
autocomplete.bindTo("bounds", map);
JavaScript
autocomplete.bindTo("bounds", map);
Use unbind() to unbind the Autocomplete predictions from the map's viewport.
TypeScript
autocomplete.unbind("bounds");
autocomplete.setBounds({ east: 180, west: -180, north: 90, south: -90 });
JavaScript
autocomplete.unbind("bounds");
autocomplete.setBounds({ east: 180, west: -180, north: 90, south: -90 });
View example
Restrict the search to the current bounds
Set the strictBounds option to restrict the results to the current bounds, whether based on map viewport or rectangular bounds.
autocomplete.setOptions({ strictBounds: true });
Restrict predictions to a specific country
Use the componentRestrictions option or call setComponentRestrictions() to restrict the autocomplete search to a specific set of up to five countries.
TypeScript
autocomplete.setComponentRestrictions({
country: ["us", "pr", "vi", "gu", "mp"],
});
JavaScript
autocomplete.setComponentRestrictions({
country: ["us", "pr", "vi", "gu", "mp"],
});
View example
Constrain place types
Use the types option or call setTypes() to constrain predictions to certain place types. This constraint specifies a type or a type collection, as listed in Place Types. If no constraint is specified, all types are returned.
For the value of the types option or the value passed to setTypes(), you can specify either:
• An array containing up to five values from Table 1 or Table 2 from Place Types. For example:
types: ['hospital', 'pharmacy', 'bakery', 'country']
Or:
autocomplete.setTypes(['hospital', 'pharmacy', 'bakery', 'country']);
• Any one filter in Table 3 from Place Types. You can only specify a single value from Table 3.
The request will be rejected if:
• You specify more than five types.
• You specify any unrecognized types.
• You mix any types from Table 1 or Table 2 with any filter from Table 3.
The Places Autocomplete demo demonstrates the differences in predictions between different place types.
Visit demo
Getting place information
When a user selects a place from the predictions attached to the autocomplete text field, the service fires a place_changed event. To get place details:
1. Create an event handler for the place_changed event, and call addListener() on the Autocomplete object to add the handler.
2. Call Autocomplete.getPlace() on the Autocomplete object, to retrieve a PlaceResult object, which you can then use to get more information about the selected place.
By default, when a user selects a place, autocomplete returns all of the available data fields for the selected place, and you will be billed accordingly. Use Autocomplete.setFields() to specify which place data fields to return. Read more about the PlaceResult object, including a list of place data fields that you can request. To avoid paying for data that you don't need, be sure to use Autocomplete.setFields() to specify only the place data that you will use.
The name property contains the description from Places Autocomplete predictions. You can read more about the description in the Places Autocomplete documentation.
For address forms, it is useful to get the address in structured format. To return the structured address for the selected place, call Autocomplete.setFields() and specify the address_components field.
The following example uses autocomplete to fill the fields in an address form.
TypeScript
function fillInAddress() {
// Get the place details from the autocomplete object.
const place = autocomplete.getPlace();
let address1 = "";
let postcode = "";
// Get each component of the address from the place details,
// and then fill-in the corresponding field on the form.
// place.address_components are google.maps.GeocoderAddressComponent objects
// which are documented at http://goo.gle/3l5i5Mr
for (const component of place.address_components as google.maps.GeocoderAddressComponent[]) {
// @ts-ignore remove once typings fixed
const componentType = component.types[0];
switch (componentType) {
case "street_number": {
address1 = `${component.long_name} ${address1}`;
break;
}
case "route": {
address1 += component.short_name;
break;
}
case "postal_code": {
postcode = `${component.long_name}${postcode}`;
break;
}
case "postal_code_suffix": {
postcode = `${postcode}-${component.long_name}`;
break;
}
case "locality":
(document.querySelector("#locality") as HTMLInputElement).value =
component.long_name;
break;
case "administrative_area_level_1": {
(document.querySelector("#state") as HTMLInputElement).value =
component.short_name;
break;
}
case "country":
(document.querySelector("#country") as HTMLInputElement).value =
component.long_name;
break;
}
}
address1Field.value = address1;
postalField.value = postcode;
// After filling the form with address components from the Autocomplete
// prediction, set cursor focus on the second address line to encourage
// entry of subpremise information such as apartment, unit, or floor number.
address2Field.focus();
}
JavaScript
function fillInAddress() {
// Get the place details from the autocomplete object.
const place = autocomplete.getPlace();
let address1 = "";
let postcode = "";
// Get each component of the address from the place details,
// and then fill-in the corresponding field on the form.
// place.address_components are google.maps.GeocoderAddressComponent objects
// which are documented at http://goo.gle/3l5i5Mr
for (const component of place.address_components) {
// @ts-ignore remove once typings fixed
const componentType = component.types[0];
switch (componentType) {
case "street_number": {
address1 = `${component.long_name} ${address1}`;
break;
}
case "route": {
address1 += component.short_name;
break;
}
case "postal_code": {
postcode = `${component.long_name}${postcode}`;
break;
}
case "postal_code_suffix": {
postcode = `${postcode}-${component.long_name}`;
break;
}
case "locality":
document.querySelector("#locality").value = component.long_name;
break;
case "administrative_area_level_1": {
document.querySelector("#state").value = component.short_name;
break;
}
case "country":
document.querySelector("#country").value = component.long_name;
break;
}
}
address1Field.value = address1;
postalField.value = postcode;
// After filling the form with address components from the Autocomplete
// prediction, set cursor focus on the second address line to encourage
// entry of subpremise information such as apartment, unit, or floor number.
address2Field.focus();
}
window.initAutocomplete = initAutocomplete;
View example
Customizing placeholder text
By default, the text field created by the autocomplete service contains standard placeholder text. To modify the text, set the placeholder attribute on the input element:
<input id="searchTextField" type="text" size="50" placeholder="Anything you want!">
Note: The default placeholder text is localized automatically. If you specify your own placeholder value, you must handle the localization of that value in your application. For information on how the Google Maps JavaScript API chooses the language to use, please read the documentation on localization.
See Styling the Autocomplete and SearchBox widgets to customize the widget appearance.
The SearchBox allows users to perform a text-based geographic search, such as 'pizza in New York' or 'shoe stores near robson street'. You can attach the SearchBox to a text field and, as text is entered, the service will return predictions in the form of a drop-down pick list.
SearchBox supplies an extended list of predictions, which can include places (as defined by the Places API) plus suggested search terms. For example, if the user enters 'pizza in new', the pick list may include the phrase 'pizza in New York, NY' as well as the names of various pizza outlets. When a user selects a place from the list, information about that place is returned to the SearchBox object, and can be retrieved by your application.
The SearchBox constructor takes two arguments:
• An HTML input element of type text. This is the input field that the SearchBox service will monitor and attach its results to.
• An options argument, which can contain the bounds property: bounds is a google.maps.LatLngBounds object specifying the area in which to search for places. The results are biased towards, but not restricted to, places contained within these bounds.
The following code uses the bounds parameter to bias the results towards places within a particular geographic area, specified via laitude/longitude coordinates.
var defaultBounds = new google.maps.LatLngBounds(
new google.maps.LatLng(-33.8902, 151.1759),
new google.maps.LatLng(-33.8474, 151.2631));
var input = document.getElementById('searchTextField');
var searchBox = new google.maps.places.SearchBox(input, {
bounds: defaultBounds
});
Changing the search area for SearchBox
To change the search area for an existing SearchBox, call setBounds() on the SearchBox object and pass the relevant LatLngBounds object.
View example
Getting place information
When the user selects an item from the predictions attached to the search box, the service fires a places_changed event. You can call getPlaces() on the SearchBox object, to retrieve an array containing several predictions, each of which is a PlaceResult object.
For more information about the PlaceResult object, refer to the documentation on place detail results.
TypeScript
// Listen for the event fired when the user selects a prediction and retrieve
// more details for that place.
searchBox.addListener("places_changed", () => {
const places = searchBox.getPlaces();
if (places.length == 0) {
return;
}
// Clear out the old markers.
markers.forEach((marker) => {
marker.setMap(null);
});
markers = [];
// For each place, get the icon, name and location.
const bounds = new google.maps.LatLngBounds();
places.forEach((place) => {
if (!place.geometry || !place.geometry.location) {
console.log("Returned place contains no geometry");
return;
}
const icon = {
url: place.icon as string,
size: new google.maps.Size(71, 71),
origin: new google.maps.Point(0, 0),
anchor: new google.maps.Point(17, 34),
scaledSize: new google.maps.Size(25, 25),
};
// Create a marker for each place.
markers.push(
new google.maps.Marker({
map,
icon,
title: place.name,
position: place.geometry.location,
})
);
if (place.geometry.viewport) {
// Only geocodes have viewport.
bounds.union(place.geometry.viewport);
} else {
bounds.extend(place.geometry.location);
}
});
map.fitBounds(bounds);
});
JavaScript
// Listen for the event fired when the user selects a prediction and retrieve
// more details for that place.
searchBox.addListener("places_changed", () => {
const places = searchBox.getPlaces();
if (places.length == 0) {
return;
}
// Clear out the old markers.
markers.forEach((marker) => {
marker.setMap(null);
});
markers = [];
// For each place, get the icon, name and location.
const bounds = new google.maps.LatLngBounds();
places.forEach((place) => {
if (!place.geometry || !place.geometry.location) {
console.log("Returned place contains no geometry");
return;
}
const icon = {
url: place.icon,
size: new google.maps.Size(71, 71),
origin: new google.maps.Point(0, 0),
anchor: new google.maps.Point(17, 34),
scaledSize: new google.maps.Size(25, 25),
};
// Create a marker for each place.
markers.push(
new google.maps.Marker({
map,
icon,
title: place.name,
position: place.geometry.location,
})
);
if (place.geometry.viewport) {
// Only geocodes have viewport.
bounds.union(place.geometry.viewport);
} else {
bounds.extend(place.geometry.location);
}
});
map.fitBounds(bounds);
});
View example
See Styling the Autocomplete and SearchBox widgets to customize the widget appearance.
Programmatically retrieving Place Autocomplete Service predictions
To retrieve predictions programmatically, use the AutocompleteService class. AutocompleteService does not add any UI controls. Instead, it returns an array of prediction objects, each containing the text of the prediction, reference information, and details of how the result matches the user input. This is useful if you want more control over the user interface than is offered by the Autocomplete and SearchBox described above.
AutocompleteService exposes the following methods:
• getPlacePredictions() returns place predictions. Note: A 'place' can be an establishment, geographic location, or prominent point of interest, as defined by the Places API.
• getQueryPredictions() returns an extended list of predictions, which can include places (as defined by the Places API) plus suggested search terms. For example, if the user enters 'pizza in new', the pick list may include the phrase 'pizza in New York, NY' as well as the names of various pizza outlets.
Both of the above methods return an array of prediction objects of the following form:
• description is the matched prediction.
• distance_meters is the distance in meters of the place from the specified AutocompletionRequest.origin.
• matched_substrings contains a set of substrings in the description that match elements in the user's input. This is useful for highlighting those substrings in your application. In many cases, the query will appear as a substring of the description field.
• length is the length of the substring.
• offset is the character offset, measured from the beginning of the description string, at which the matched substring appears.
• place_id is a textual identifier that uniquely identifies a place. To retrieve information about the place, pass this identifier in the placeId field of a Place Details request. Learn more about how to reference a place with a place ID.
• terms is an array containing elements of the query. For a place, each element will typically make up a portion of the address.
• offset is the character offset, measured from the beginning of the description string, at which the matched substring appears.
• value is the matching term.
The example below executes a query prediction request for the phrase 'pizza near' and displays the result in a list.
TypeScript
// This example retrieves autocomplete predictions programmatically from the
// autocomplete service, and displays them as an HTML list.
// This example requires the Places library. Include the libraries=places
// parameter when you first load the API. For example:
// <script src="https://maps.googleapis.com/maps/api/js?key=YOUR_API_KEY&libraries=places">
function initService(): void {
const displaySuggestions = function (
predictions: google.maps.places.QueryAutocompletePrediction[] | null,
status: google.maps.places.PlacesServiceStatus
) {
if (status != google.maps.places.PlacesServiceStatus.OK || !predictions) {
alert(status);
return;
}
predictions.forEach((prediction) => {
const li = document.createElement("li");
li.appendChild(document.createTextNode(prediction.description));
(document.getElementById("results") as HTMLUListElement).appendChild(li);
});
};
const service = new google.maps.places.AutocompleteService();
service.getQueryPredictions({ input: "pizza near Syd" }, displaySuggestions);
}
declare global {
interface Window {
initService: () => void;
}
}
window.initService = initService;
JavaScript
// This example retrieves autocomplete predictions programmatically from the
// autocomplete service, and displays them as an HTML list.
// This example requires the Places library. Include the libraries=places
// parameter when you first load the API. For example:
// <script src="https://maps.googleapis.com/maps/api/js?key=YOUR_API_KEY&libraries=places">
function initService() {
const displaySuggestions = function (predictions, status) {
if (status != google.maps.places.PlacesServiceStatus.OK || !predictions) {
alert(status);
return;
}
predictions.forEach((prediction) => {
const li = document.createElement("li");
li.appendChild(document.createTextNode(prediction.description));
document.getElementById("results").appendChild(li);
});
};
const service = new google.maps.places.AutocompleteService();
service.getQueryPredictions({ input: "pizza near Syd" }, displaySuggestions);
}
window.initService = initService;
CSS
HTML
<html>
<head>
<title>Retrieving Autocomplete Predictions</title>
<script src="https://polyfill.io/v3/polyfill.min.js?features=default"></script>
<link rel="stylesheet" type="text/css" href="./style.css" />
<script type="module" src="./index.js"></script>
</head>
<body>
<p>Query suggestions for 'pizza near Syd':</p>
<ul id="results"></ul>
<!--
The `defer` attribute causes the callback to execute after the full HTML
document has been parsed. For non-blocking uses, avoiding race conditions,
and consistent behavior across browsers, consider loading using Promises
with https://www.npmjs.com/package/@googlemaps/js-api-loader.
-->
<script
src="https://maps.googleapis.com/maps/api/js?key=AIzaSyB41DRUbKWJHPxaFjMAwdrzWzbVKartNGg&callback=initService&libraries=places&v=weekly"
defer
></script>
</body>
</html>
Try Sample
View example
Session tokens
AutocompleteService.getPlacePredictions() uses session tokens to group together autocomplete requests for billing purposes. Session tokens group the query and selection phases of a user autocomplete search into a discrete session for billing purposes. The session begins when the user starts typing a query, and concludes when they select a place. Each session can have multiple queries, followed by one place selection. Once a session has concluded, the token is no longer valid. Your app must generate a fresh token for each session. We recommend using session tokens for all autocomplete sessions. If the sessionToken parameter is omitted, or if you reuse a session token, the session is charged as if no session token was provided (each request is billed separately).
You can use the same session token to make a single Place Details request on the place that results from a call to AutocompleteService.getPlacePredictions(). In this case, the autocomplete request is combined with the Place Details request, and the call is charged as a regular Place Details request. There is no charge for the autocomplete request.
Be sure to pass a unique session token for each new session. Using the same token for more than one Autocomplete session will invalidate those Autocomplete sessions, and all Autocomplete request in the invalid sessions will be charged individually using Autocomplete Per Request SKU. Read more about session tokens.
The following example shows creating a session token, then passing it in an AutocompleteService (the displaySuggestions() function has been omitted for brevity):
// Create a new session token.
var sessionToken = new google.maps.places.AutocompleteSessionToken();
// Pass the token to the autocomplete service.
var autocompleteService = new google.maps.places.AutocompleteService();
autocompleteService.getPlacePredictions({
input: 'pizza near Syd',
sessionToken: sessionToken
},
displaySuggestions);
Be sure to pass a unique session token for each new session. Using the same token for more than one session will result in each request being billed individually.
Read more about session tokens.
Styling the Autocomplete and SearchBox widgets
By default, the UI elements provided by Autocomplete and SearchBox are styled for inclusion on a Google map. You may want to adjust the styling to suit your own site. The following CSS classes are available. All classes listed below apply to both the Autocomplete and the SearchBox widgets.
A graphical illustration of the CSS classes for the Autocomplete and
SearchBox widgets
CSS classes for Autocomplete and SearchBox widgets
CSS class Description
pac-container The visual element containing the list of predictions returned by the Place Autocomplete service. This list appears as a dropdown list below the Autocomplete or SearchBox widget.
pac-icon The icon displayed to the left of each item in the list of predictions.
pac-item An item in the list of predictions supplied by the Autocomplete or SearchBox widget.
pac-item:hover The item when the user hovers their mouse pointer over it.
pac-item-selected The item when the user selects it via the keyboard. Note: Selected items will be a member of this class and of the pac-item class.
pac-item-query A span inside a pac-item that is the main part of the prediction. For geographic locations, this contains a place name, like 'Sydney', or a street name and number, like '10 King Street'. For text-based searches such as 'pizza in New York', it contains the full text of the query. By default, the pac-item-query is colored black. If there is any additional text in the pac-item, it is outside pac-item-query and inherits its styling from pac-item. It is colored gray by default. The additional text is typically an address.
pac-matched The part of the returned prediction that matches the user’s input. By default, this matched text is highlighted in bold text. Note that the matched text may be anywhere within pac-item. It is not necessarily part of pac-item-query, and it could be partly within pac-item-query as well as partly in the remaining text in pac-item.
Place Autocomplete optimization
This section describes best practices to help you make the most of the Place Autocomplete service.
Here are some general guidelines:
• The quickest way to develop a working user interface is to use the Maps JavaScript API Autocomplete widget, Places SDK for Android Autocomplete widget, or Places SDK for iOS Autocomplete UI control
• Develop an understanding of essential Place Autocomplete data fields from the start.
• Location biasing and location restriction fields are optional but can have a significant impact on autocomplete performance.
• Use error handling to make sure your app degrades gracefully if the API returns an error.
• Make sure your app handles when there is no selection and offers users a way to continue.
Cost optimization best practices
Basic cost optimization
To optimize the cost of using the Place Autocomplete service, use field masks in Place Details and Place Autocomplete widgets to return only the place data fields you need.
Advanced cost optimization
Consider programmatic implementation of Place Autocomplete in order to access Per Request pricing and request Geocoding API results about the selected place instead of Place Details. Per Request pricing paired with Geocoding API is more cost-effective than Per Session (session-based) pricing if both of the following conditions are met:
• If you only need the latitude/longitude or address of the user's selected place, the Geocoding API delivers this information for less than a Place Details call.
• If users select an autocomplete prediction within an average of four Autocomplete predictions requests or fewer, Per Request pricing may be more cost-effective than Per Session pricing.
For help selecting the Place Autocomplete implementation that fits your needs, select the tab that corresponds to your answer to the following question.
Does your application require any information other than the address and latitude/longitude of the selected prediction?
Yes, needs more details
Use session-based Place Autocomplete with Place Details.
Since your application requires Place Details such as the place name, business status, or opening hours, your implementation of Place Autocomplete should use a session token (programmatically or built into the JavaScript, Android, or iOS widgets) for a total cost of $0.017 per session plus applicable Places Data SKUs depending on which place data fields you request.1
Widget implementation
Session management is automatically built into the JavaScript, Android, or iOS widgets. This includes both the Place Autocomplete requests and the Place Details request on the selected prediction. Be sure to specify the fields parameter in order to ensure you are only requesting the place data fields you need.
Programmatic implementation
Use a session token with your Place Autocomplete requests. When requesting Place Details about the selected prediction, include the following parameters:
1. The place ID from the Place Autocomplete response
2. The session token used in the Place Autocomplete request
3. The fields parameter specifying the place data fields you need
No, needs only address and location
Geocoding API could be a more cost-effective option than Place Details for your application, depending on the performance of your Place Autocomplete usage. Every application's Autocomplete efficiency varies depending on what users are entering, where the application is being used, and whether performance optimization best practices have been implemented.
In order to answer the following question, analyze how many characters a user types on average before selecting a Place Autocomplete prediction in your application.
Do your users select a Place Autocomplete prediction in four or fewer requests, on average?
Yes
Implement Place Autocomplete programmatically without session tokens and call Geocoding API on the selected place prediction.
Geocoding API delivers addresses and latitude/longitude coordinates for $0.005 per request. Making four Place Autocomplete - Per Request requests costs $0.01132 so the total cost of four requests plus a Geocoding API call about the selected place prediction would be $0.01632 which is less than the Per Session Autocomplete price of $0.017 per session.1
Consider employing performance best practices to help your users get the prediction they're looking for in even fewer characters.
No
Use session-based Place Autocomplete with Place Details.
Since the average number of requests you expect to make before a user selects a Place Autocomplete prediction exceeds the cost of Per Session pricing, your implementation of Place Autocomplete should use a session token for both the Place Autocomplete requests and the associated Place Details request for a total cost of $0.017 per session.1
Widget implementation
Session management is automatically built into the JavaScript, Android, or iOS widgets. This includes both the Place Autocomplete requests and the Place Details request on the selected prediction. Be sure to specify the fields parameter in order to ensure you are only requesting Basic Data fields.
Programmatic implementation
Use a session token with your Place Autocomplete requests. When requesting Place Details about the selected prediction, include the following parameters:
1. The place ID from the Place Autocomplete response
2. The session token used in the Place Autocomplete request
3. The fields parameter specifying Basic Data fields such as address and geometry
Consider delaying Place Autocomplete requests
You can employ strategies such as delaying a Place Autocomplete request until the user has typed in the first three or four characters so that your application makes fewer requests. For example, making Place Autocomplete requests for each character after the user has typed the third character means that if the user types seven characters then selects a prediction for which you make one Geocoding API request, the total cost would be $0.01632 (4 * $0.00283 Autocomplete Per Request + $0.005 Geocoding).1
If delaying requests can get your average programmatic request below four, you can follow the guidance for performant Place Autocomplete with Geocoding API implementation. Note that delaying requests can be perceived as latency by the user who might be expecting to see predictions with every new keystroke.
Consider employing performance best practices to help your users get the prediction they're looking for in fewer characters.
1. Costs listed here are in USD. Please refer to the Google Maps Platform Billing page for full pricing information.
Performance best practices
The following guidelines describe ways to optimize Place Autocomplete performance:
• Add country restrictions, location biasing, and (for programmatic implementations) language preference to your Place Autocomplete implementation. Language preference is not needed with widgets since they pick language preferences from the user's browser or mobile device.
• If Place Autocomplete is accompanied by a map, you can bias location by map viewport.
• In situations when a user does not choose one of the Autocomplete predictions, generally because none of those predictions are the desired result-address, you can re-use the original user input to attempt to get more relevant results:
• If you expect the user to enter only address information, re-use the original user input in a call to the Geocoding API.
• If you expect the user to enter queries for a specific place by name or address, use a Find Place request. If results are only expected in a specific region, use location biasing.
Other scenarios when it's best to fall back to the Geocoding API include:
• Users inputting subpremise addresses in countries other than Australia, New Zealand, or Canada. For example, the US address "123 Bowdoin St #456, Boston MA, USA" is not supported by Autocomplete. (Autocomplete supports subpremise addresses only in Australia, New Zealand, and Canada. Supported address formats in these three countries include "9/321 Pitt Street, Sydney, New South Wales, Australia" or "14/19 Langana Avenue, Browns Bay, Auckland, New Zealand" or "145-112 Renfrew Dr, Markham, Ontario, Canada".)
• Users inputting addresses with road-segment prefixes like "23-30 29th St, Queens" in New York City or "47-380 Kamehameha Hwy, Kaneohe" on the island of Kauai in Hawai'i.
Usage limits and policies
Quotas
For quota and pricing information, see the Usage and Billing documentation for the Places API.
Policies
Use of the Places Library, Maps JavaScript API must be in accordance with the policies described for the Places API.
From our Terms of Service
Display the required
logos and attributions
Respect Google's copyrights and attribution. Ensure that the logo and copyright notice are visible, and display the "powered by Google" logo if you're using data without a map.
Learn More
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Class #13: The Singularity and Universe as Computer
(Participation is optional. For additional discussion about Newcomb’s problem and free will, please post in thread #12b.)
Feel free to post any thoughts you’ve had related to the topics discussed in class #13:
• The concept of a “technological singularity” and what exactly we should interpret it to mean
• What the fundamental physical limits are on intelligence
• Whether it’s useful to regard the universe as a computer, what that statement even means, and whether it has empirical content
• Whether any of the following considerations are obstructions to thinking of the universe as a computer: quantum mechanics; apparent continuity in the laws of physics; the possible infinitude of space; a lack of knowledge on our part about the inputs, outputs, or purposes of the computation
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18 Responses to Class #13: The Singularity and Universe as Computer
1. A scientific theory can be considered a Turing machine that simulates the evolution of the observable universe. This is not at all the same thing as considering the universe itself to be a Turing machine.
As far as I can see, though I am not an expert, deep presuppositions in both quantum theory, and in the working philosophy of scientists, are not consistent with the universe being a Turing machine.
As I understand it, quantum mechanics assumes that observations are an irreducibly random — both in the sense of happening at chance, and in the sense of being computationally irreducible — selection from a probability wave. Such randomness is not computable, and if quantum mechanics is true, then the universe as a whole is not computable either, although any finite set of observations can of course be simulated by a Turing machine. In addition, as far as I get it, the experimental falsification of hidden variable theories shows that if scientists are free to set up any experiment, then physical theory must be formulated as though the irreducible randomness I mentioned does exist. This means that the “working metaphysics” of scientists, who in fact assume that theory should be predictive for any experimental setup, is not consistent with the universe as a whole being a Turing machine.
Hidden within these questions are two approaches or attitudes in the metaphysics of science. The first attitude is fully satisfied if the evolution of the observable universe can be simulated by some Turing machine in a way that is not falsified by observation, and the second attitude continues to wonder what is “really” going on beyond this. I would say that Quine exemplifies the first attitude, and Goedel exemplifies the second attitude. I would also say that I am puzzled by the apparent fact that most physicists reject hidden variable theories, but also seem to think that the universe is nevertheless some sort of Turing machine. Perhaps I just don’t understand.
• Hi Michael,
The key to resolving your puzzle is that randomness, per se, is not especially mysterious or hard to handle: we can easily simulate it by the simple device of equipping our Turing machine with a random number generator. (And even a deterministic TM can calculate the probability of any particular outcome we care about, and can also output the ensemble of all possible outcomes together with the probability of each.) For this reason, it seems to me that, if you want to find a quantum-mechanical obstruction to the universe being “computational”, then at the least, you’ll need to invoke some deeper feature of QM than just the fact that measurement outcomes are random. (One possible example of such a feature, which Bohr emphasized, is our lack of complete knowledge of the initial state, owing to the uncertainty principle. If you know the initial state, then the Born rule lets you calculate the probability of any possible outcome of any possible measurement on that state. But what if you don’t or can’t know the initial state?)
2. I’m sorry, but you seem to be responding way too quickly…
As far as I can see, your assumption that we can equip a Turing machine with a random number generator simply begs the question.
The question, after all, is whether the universe, which by definition includes any and all random number generators including the perfect one assumed by quantum theory, is or is not Turing computable.
It seems to me that a blurring of concepts, or equivocation, begins to occur here. I’m not quite certain whether this blurring is in my own understanding of the universe as computer concept, or in the minds of those propounding the concept.
The physical Church-Turing thesis, on the face of it, seems to assume a Turing machine of finite Kolmogorov complexity whose output is decidable. Your proposal for a Turing machine with a perfect random number generator would be of denumerably infinite Kolmogorov complexity if events are discrete, non-denumerably infinite complexity if events are continuous.
The output of the first machine is recursively computable and decidable, the output of the second machine is not recursively computable but is recursively enumerable and undecidable. The “program” of the second machine is infinitely long and infinitely complex.
These two things are obviously quite different steps on the ladder and have very, very different philosophical implications…
I have a feeling there are many further distinctions and elaborations to be made here.
Hoping for a slower answer,
Mike
• bobthebayesian says:
I do not understand when you say, “Your proposal for a Turing machine with a perfect random number generator would be of denumerably infinite Kolmogorov complexity if events are discrete, non-denumerably infinite complexity if events are continuous.”
Denumerably infinite and non-denumerably infinite (why not just say countable and uncountable?) refer to the sizes of sets. But Kolmogorov complexity refers to a real number describing the length of a minimal Turing machine. I don’t understand how this number can be countable or uncountable; it’s just a number. Do you mean instead that we would be computing the K-complexity of adenumerable set (resp. non-denumerable set)? Otherwise your concepts seem mistaken here, or I am unaware of how the term denumerable is being used in this context.
• bobthebayesian says:
Perhaps this post is helpful? I’m just very confused by what comparison between K-complexity and uncountability you’re trying to make; it’s easy to misconstrue K-complexity as saying something about uncountable sets of things, when really it’s about strings.
• Thanks for your comment. I am not an expert in this field so it is possible I have used the wrong term.
What I mean to say is that if the universe contains a source of perfect randomness, then considered as a machine, it has to be of infinite complexity. If the universe is “discrete” in that the “computation” proceeds step by step as in a digital computer, then its complexity with perfect randomness is infinite and the number of steps is countable. If the universe is “continuous” in that between any two points of “computation” there is another, then its complexity with perfect randomness is infinite and the number of “computations” is uncountable.
What I am trying to get at here is what I see as an ambiguity in the concept of “universe as machine.” Is it a finite machine or an infinite machine? What do you think?
• bobthebayesian says:
For one, we don’t really know if the universe is infinite or finite, so it’d be pretty speculative to say something about whether it is infinite “as a machine” or not. If it turned out that the universe was some complicated but finite manifold, whould that make you more likely to think it was a finite Turing machine? I’m not sure it would make that more likely to me.
Secondly, there is a big difference between the two ideas (1) uncountability and (2) ““continuous” in that between any two points of “computation” there is another”. Even a countably infinite discrete set has property 2 (Just take the set of all rational numbers, including negative ones. Between any two rationals lies another rational, yet the set is countably infinite.) To get real continuity in the universe, you need something a lot stronger. Another thing is that there are approximation theorems that tell us that even if the universe were continuous, we could approximate it arbitrarily well with a discrete representation. For me, this more or less removes any concern for whether or not it is discrete. “For all practical purposes” it is discrete.
The real question you are wanting to ask here is whether the universe, when considered as a formal program, has to have infinite complexity due to the existence of “true randomness” inside of the program. Whatever submodule inside of the program Universe() that gets called, PerfectRandomGenerator(), can be compressible in any way or else it is not really random. This is exactly what K-complexity is used for. Something is considered random if its description is shorter than any program which can compute it.
And to me, this is the problem with what you’re asking. Is the description of the program Universe() shorter than the description of any program which can compute Universe()? This just opens a can of worms. What meta-programming language are we talking about? Can we write other universe programs in this language, and if so, does this mean we have to talk about some Platonic world of universe programs, one of which happens to correspond to our own (be careful, this is not at all similar to the Many Worlds questions).
I can’t quite articulate exactly what I want to say right now, but it seems the problem is that “infinite complexity” starts to deal with subjectivity. Any actually existing random number generator in the universe will be a finite thing, and since we can write down a description of any finite random number generator, it can’t have infinite K-complexity. So whatever this “perfect random number generator” is, it can’t exist inside the universe because its very existence means we could write down a succinct program that generates truly random strings — but truly random strings are ones that can’t be generated by a program which we can’t succinctly describe.
• Scott says:
Mike: You asked an answerable question, I answered it, and the problem is that I did so too quickly? ;-) Recall, you expressed confusion about why “most scientists” aren’t troubled by what you thought of as the contradiction between computability and randomness:
This means that the “working metaphysics” of scientists, who in fact assume that theory should be predictive for any experimental setup, is not consistent with the universe as a whole being a Turing machine … I would also say that I am puzzled by the apparent fact that most physicists reject hidden variable theories, but also seem to think that the universe is nevertheless some sort of Turing machine. Perhaps I just don’t understand.
Given how you expressed your puzzlement, I thought it would suffice to explain why “most physicists” (and computer scientists, FWIW) don’t perceive any contradiction. Knowing the relevant technical points, you’re then free to ponder the philosophical implications and draw your own conclusions.
So, briefly:
- A Turing machine equipped with a random number generator is called a randomized Turing machine. However you choose to classify randomized TMs (as types of TMs, variants of TMs, etc.), they’re extremely well-studied and familiar objects in theoretical computer science. Shannon proved that, if you care about language decidability, then a randomized TM decides exactly the same set of languages as a deterministic TM—i.e., the randomness gives you no advantage. Today, we conjecture the same thing in complexity theory (P=BPP). Of course, it’s trivially the case that randomized TMs can do one thing that deterministic TMs can’t: namely, they can output random strings! But if you instead ask: what exactly did you want the random string for? What, using the random string, did you want to know? then you need to ask whether a deterministic TM could have told you that thing as well, and the answer is usually yes. On the other hand, it’s sort of a moot point anyway, since most CS theorists these days are perfectly happy to talk about randomized TMs and forget about the deterministic ones.
- According to quantum mechanics, you can’t predict exactly when a radioactive atom is going to decay. But you can calculate the exact probability distribution over decay times! So, given a collection of atoms, you can predict to enormous accuracy what fraction of them will have decayed within any given time interval. Given the almost-perfect predictability of a large ensemble of random systems (something we already knew, incidentally, from 19th-century thermodynamics), it’s hard for most physicists to take seriously the idea that the uncertainty in decay time reflects any sort of “free will” or “uncomputability” on the atoms’ part. Of course, you’re free to define words however you want—but whatever you call this sort of unpredictability, it doesn’t seem to be the sort that can provide any reassurance to anyone who worries about science being too mechanistic.
- (Added) Regarding your point that a randomized Turing machine has “infinite Kolmogorov complexity”: I’d say that’s a misleading way to describe what’s going on. The specification of a randomized TM consists only of its finite state transition diagram; the random string r is then thought of as simply another input (alongside the “real” input x). While it’s true that r “could be absolutely anything”, just like in my radioactive decay example, the overall properties of r are nevertheless known with confidence approaching certainty! For example, it’s astronomically unlikely that the number of 1s in r deviates significantly from the number of 0s, or indeed that r contains any other computable regularities whatsoever. Another way to say this is that, even though r contains infinitely many bits, it contains zero bits about anything other than its own rather-uninteresting self!
• Thanks to both bobthebayesian and Scott for your responses, which are now much closer to understanding what I am trying to say. I’ll try to make myself even clearer.
bobthebayesian said:
The real question you are wanting to ask here is whether the universe, when considered as a formal program, has to have infinite complexity due to the existence of “true randomness” inside of the program. Whatever submodule inside of the program Universe() that gets called, PerfectRandomGenerator(), can be compressible in any way or else it is not really random. This is exactly what K-complexity is used for. Something is considered random if its description is shorter than any program which can compute it.
Yes, this is exactly what I am saying.
bobthebayesian continued:
I can’t quite articulate exactly what I want to say right now, but it seems the problem is that “infinite complexity” starts to deal with subjectivity. Any actually existing random number generator in the universe will be a finite thing, and since we can write down a description of any finite random number generator, it can’t have infinite K-complexity.
I’m sorry, but I simply don’t see how this doesn’t just beg the question. Why exactly should any actually existing random number generator in the universe be a finite thing? Nobody has yet informed me how quantum mechanics does not assume the existence of perfect randomness.
Scott considers this:
According to quantum mechanics, you can’t predict exactly when a radioactive atom is going to decay. But you can calculate the exact probability distribution over decay times! So, given a collection of atoms, you can predict to enormous accuracy what fraction of them will have decayed within any given time interval. Given the almost-perfect predictability of a large ensemble of random systems (something we already knew, incidentally, from 19th-century thermodynamics), it’s hard for most physicists to take seriously the idea that the uncertainty in decay time reflects any sort of “free will” or “uncomputability” on the atoms’ part. Of course, you’re free to define words however you want—but whatever you call this sort of unpredictability, it doesn’t seem to be the sort that can provide any reassurance to anyone who worries about science being too mechanistic.
Because the question here is one of philosophy and not of science or engineering, I’d say that the distinction between “almost-perfect” and “perfect,” or between “approaching certainty” and “certainty”, is in fact critical! It’s the difference between “ACTS LIKE a machine,” and “IS a machine.”
I would also say that the “overall properties” of a random string are by no means the only interesting things about it, and even just one bit of difference could be an utterly critical difference, especially if that one bit determines which of two paths the whole universe takes.
To revert to my original post, if you are satisfied with “almost-perfect” and “approaching certainty” and that convinces you the universe is a machine, then to me you appear to share the naturalism of Quine, whereas if you are NOT satisfied and what to know what IS, you appear to share the Platonism of Goedel.
• Scott says:
Mike: In that case, I’d say that ironically, your “mystical” impulses are easier to satisfy than mine! :-) Once you concede that physical systems are probabilistically predictable, and large ensembles of such systems almost-perfectly so, you’ve given your Gödelian mystical idealism hardly any room with which to act. On this view, a superintelligence who knew the current state of the universe could presumably predict the probability of any significant event in your life over the next year: 90% chance of this, 35% chance of that. Then all that would be left for your “free will” to do would be to sample from that precisely-calculated probability distribution (which, in contrast to weather forecasts, is assumed to be perfectly accurate). I say: if you’re going to believe in a non-mechanistic free will at all, why not hold out for more? Why not conjecture that the superintelligence couldn’t even predict the probabilities very accurately, but could at most crudely bound them?
• That’s interesting. What do you think about this? Personally, I have no idea what to make of free will, except I have a feeling it exists. In other words, I certainly don’t feel bound to sample from a distribution.
Anyway, although I appreciate your response about probability, I would be more interested in your response to the philosophical issue of the truth or validity of the so-called “physical Church-Turing thesis.”
• bobthebayesian says:
Many people seem to care a great deal about this fact that it feels like we are not bound to sample behaviors from a complex distribution. I think it’s one of the main reasons why Searle argues that quantum randomness must somehow feed forward its indeterminacy (but not randomness) to the macroscopic level working inside brains. Most ordinary folks also tend to cite the feeling that they are choosing their actions as the overwhelming evidence that we are in lucid control of behaviors.
I’m not convinced by this way of thinking. One might say that our aim in philosophy, mathematics, and physics, is to give an account or a description that analytically reproduces aspects of our experience in a consistent way. But the problem is that time and time again, we learn that there are meta-aspects to our experience that override the more basal aspects of experience.
For example, knowledge of internal medicine and beliefs that emotions are seated either in the heart or the bowels are at odds with one another. We just had a feeling that emotions were seated in {heart,bowels} and attempted to construct theories that reproduced this aspect of our experience. But then we learned more about internal medicine and the mechanistic functions of the heart and bowels, and the correlation between brain damage and personality changes, and gradually came to understand that emotions are “located” in the brain (as an approximation).
For me, reliance on “basal perceptions of free will” to drive my beliefs about free will is similarly prone to anthropocentric inaccuracies. My feeling like X is not a good reason to require analytic descriptions of the world that reproduce X. And in the case of free will, I feel it’s similar to the creationism/evolution debate. On one hand, we feel like we’re a special species with innate superiority, and religious explanations offer a way to reproduce that feeling analytically. On the other hand, we have evidence and detailed understanding of a mechanistic process that would lead to our feeling like we’re a special species with innate superiority.
For free will it’s the same. Can you imagine what it would take to program a computer to behave and act as if it had free will, or at least that it reported the feeling of having free will whenever queried about its actions? The internal qualia of feeling like one has free will is a totally different thing than free will.
I’m not saying that the problem of free will is dissolvable. The “merely-crude-probability-bounds” argument is interesting and non-trivial. But I do not think that our drive to invent theories that reproduce the feeling of free will should be affecting people as ubiquitously as it seems to.
• bobthebayesian says:
Incidentally, I am in the middle of reading Thinking, Fast and Slow, published earlier this year by Daniel Kahneman. There are a lot of striking attributes of fast-mode thinking that would seem to cast certain aspects of free will into doubt. It will be interesting whether a comprehensive way of understanding heuristics and biases and how to stitch them together can account for nearly all of human thought. I’m sure that AI researchers really want to have a more formalized grasp of heuristics and biases in this sense. FWIW, a lot of researchers in sensory modalities also think along these lines. I’m reminded of the book The Ecological Approach to Visual Perception by James Gibson.
• Scott says:
Anyway, although I appreciate your response about probability, I would be more interested in your response to the philosophical issue of the truth or validity of the so-called “physical Church-Turing thesis.”
Mike: The trouble is that people interpret the “physical Church-Turing Thesis” in different and often-conflicting ways. As I interpret the thesis, though, to refute it you would need to show that it’s possible in principle to build a physical computing device that solves a well-defined computational problem (decision, search, sampling, whatever) that’s unsolvable by a randomized Turing machine. And on that reading, I’d say there are very strong grounds from current physics to believe that the physical Church-Turing Thesis holds.
Note, however, that one could accept the physical Church-Turing Thesis (in my interpretation), while denying the claim that “the universe is a giant computation.” To do so, one would say that, while the universe might have “non-computational” elements (e.g., free will), whatever those elements are, they can’t reliably be used to solve the halting problem, or any other Turing-unsolvable mathematical problem that we’re able to describe in advance.
3. Scott:
I’m not sure whether it is, or is not, possible in principle to build a physical computing device that solves a well-defined computational problem that’s unsolvable by a randomized Turing machine.
If it were possible in principle, I would have no idea at all how to use this knowledge.
In any case I agree that non-computational elements of existence can’t, couldn’t, reliably be used for computing. I do wonder with what kind of deliberation you chose the word “reliably.”
• Scott says:
I do wonder with what kind of deliberation you chose the word “reliably.”
And I do wonder with what kind of deliberation you wrote that sentence! ;-)
4. rationalist says:
How seriously do you take the singularity hypothesis – by which I mean significantly smarter than human intelligence, Scott?
For example, what probability do you assign to the development of a computer of some kind exhibiting significantly smarter than human intelligence in the next 10, 20, 30, 40, 50, 60, and 70 years?
My own view is that David Chalmers’ comments are the most balanced on this question:
“Nevertheless, my credence that there will be human-level AI before 2100 is somewhere over one half” (http://consc.net/papers/singularity.pdf)
5. Scott:
I’ve read the slides from your talk on free will at http://www.scottaaronson.com/talks/freewill.ppt, and they go a long ways towards answering my questions about what you really think. I think.
I’ll be reading this for a while (at least) before I have anything worth saying. I do think that your approach is original and well worth pursuing.
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__label__pos
| 0.648852 |
Data Units Calculator
Kibibit to Mebibit
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The smallest unit of measurement used for measuring data is a bit. A single bit can have a value of either zero(0) or one(1). It may contain a binary value (such as True/False or On/Off or 1/0) and nothing more. Therefore, a byte, or eight bits, is used as the fundamental unit of measurement for data storage. A byte can store 256 different values, which is sufficient to represent standard ASCII table, such as all numbers, letters and control symbols.
Since most files contain thousands of bytes, file sizes are often measured in kilobytes. Larger files, such as images, videos, and audio files, contain millions of bytes and therefore are measured in megabytes. Modern storage devices can store thousands of these files, which is why storage capacity is typically measured in gigabytes or even terabytes.
1 kibit to mibit result:
1 (one) kibibit(s) is equal 0.0009765625 (zero point zero × 3 nine million seven hundred and sixty-five thousand six hundred and twenty-five) mebibit(s)
What is kibibit?
The kibibit is a multiple of the bit, a unit of digital information storage, using the standard binary prefix kibi, which has the symbol Ki, meaning 2^10. The unit symbol of the kibibit is Kibit. 1 kibibit = 2^10 bits = 1024 bits
What is mebibit?
The mebibit is a multiple of the bit, a unit of information, prefixed by the standards-based multiplier "mebi" (symbol Mi), a binary prefix meaning 2^20. The unit symbol of the mebibit is Mibit. 1 mebibit = 2^20 bits = 1048576 bits = 1024 kibibits
How calculate kibit. to mibit.?
1 Kibibit is equal to 0.0009765625 Mebibit (zero point zero × 3 nine million seven hundred and sixty-five thousand six hundred and twenty-five mibit)
1 Mebibit is equal to 1024 Kibibit (one thousand and twenty-four kibit)
1 Kibibit is equal to 1024.0000000 bits (one thousand and twenty-four point zero × 7 zero bits)
1 Mebibit is equal to 1048576 bits (one million forty-eight thousand five hundred and seventy-six bits)
1 Kibibit is equal to 1024 Bit (one thousand and twenty-four bit)
Mebibit is greater than Kibibit
Multiplication factor is 1024.
1 / 1024 = 0.0009765625.
Maybe you mean Kilobit?
1 Kibibit is equal to 1.024 Kilobit (one point zero × 1 twenty-four kbit) convert to kbit
Powers of 2
kibit mibit (Mebibit) Description
1 kibit 0.0009765625 mibit 1 kibibit (one) is equal to 0.0009765625 mebibit (zero point zero × 3 nine million seven hundred and sixty-five thousand six hundred and twenty-five)
2 kibit 0.001953125 mibit 2 kibibit (two) is equal to 0.001953125 mebibit (zero point zero × 2 one million nine hundred and fifty-three thousand one hundred and twenty-five)
4 kibit 0.00390625 mibit 4 kibibit (four) is equal to 0.00390625 mebibit (zero point zero × 2 three hundred and ninety thousand six hundred and twenty-five)
8 kibit 0.0078125 mibit 8 kibibit (eight) is equal to 0.0078125 mebibit (zero point zero × 2 seventy-eight thousand one hundred and twenty-five)
16 kibit 0.015625 mibit 16 kibibit (sixteen) is equal to 0.015625 mebibit (zero point zero × 1 fifteen thousand six hundred and twenty-five)
32 kibit 0.03125 mibit 32 kibibit (thirty-two) is equal to 0.03125 mebibit (zero point zero × 1 three thousand one hundred and twenty-five)
64 kibit 0.0625 mibit 64 kibibit (sixty-four) is equal to 0.0625 mebibit (zero point zero × 1 six hundred and twenty-five)
128 kibit 0.125 mibit 128 kibibit (one hundred and twenty-eight) is equal to 0.125 mebibit (zero point one hundred and twenty-five)
256 kibit 0.25 mibit 256 kibibit (two hundred and fifty-six) is equal to 0.25 mebibit (zero point twenty-five)
512 kibit 0.5 mibit 512 kibibit (five hundred and twelve) is equal to 0.5 mebibit (zero point five)
1024 kibit 1 mibit 1024 kibibit (one thousand and twenty-four) is equal to 1 mebibit (one)
2048 kibit 2 mibit 2048 kibibit (two thousand and forty-eight) is equal to 2 mebibit (two)
4096 kibit 4 mibit 4096 kibibit (four thousand and ninety-six) is equal to 4 mebibit (four)
8192 kibit 8 mibit 8192 kibibit (eight thousand one hundred and ninety-two) is equal to 8 mebibit (eight)
Mebibits
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| 0.947046 |
chromium_socket_factory_unittest.cc 4.29 KB
Newer Older
1
// Copyright 2014 The Chromium Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
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#include "remoting/protocol/chromium_socket_factory.h"
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#include <stddef.h>
#include <stdint.h>
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#include <memory>
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#include "base/message_loop/message_loop.h"
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#include "base/run_loop.h"
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#include "base/single_thread_task_runner.h"
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#include "testing/gmock/include/gmock/gmock.h"
#include "testing/gtest/include/gtest/gtest.h"
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#include "third_party/webrtc/rtc_base/asyncpacketsocket.h"
#include "third_party/webrtc/rtc_base/socketaddress.h"
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namespace remoting {
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namespace protocol {
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class ChromiumSocketFactoryTest : public testing::Test,
public sigslot::has_slots<> {
public:
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void SetUp() override {
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socket_factory_.reset(new ChromiumPacketSocketFactory());
socket_.reset(socket_factory_->CreateUdpSocket(
30
rtc::SocketAddress("127.0.0.1", 0), 0, 0));
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ASSERT_TRUE(socket_.get() != nullptr);
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EXPECT_EQ(socket_->GetState(), rtc::AsyncPacketSocket::STATE_BOUND);
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socket_->SignalReadPacket.connect(
this, &ChromiumSocketFactoryTest::OnPacket);
}
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void OnPacket(rtc::AsyncPacketSocket* socket,
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const char* data, size_t size,
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const rtc::SocketAddress& address,
const rtc::PacketTime& packet_time) {
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EXPECT_EQ(socket, socket_.get());
last_packet_.assign(data, data + size);
last_address_ = address;
run_loop_.Quit();
}
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void VerifyCanSendAndReceive(rtc::AsyncPacketSocket* sender) {
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// UDP packets may be lost, so we have to retry sending it more than once.
const int kMaxAttempts = 3;
const base::TimeDelta kAttemptPeriod = base::TimeDelta::FromSeconds(1);
std::string test_packet("TEST PACKET");
int attempts = 0;
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rtc::PacketOptions options;
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while (last_packet_.empty() && attempts++ < kMaxAttempts) {
sender->SendTo(test_packet.data(), test_packet.size(),
socket_->GetLocalAddress(), options);
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message_loop_.task_runner()->PostDelayedTask(
FROM_HERE, run_loop_.QuitClosure(), kAttemptPeriod);
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run_loop_.Run();
}
EXPECT_EQ(test_packet, last_packet_);
EXPECT_EQ(sender->GetLocalAddress(), last_address_);
}
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protected:
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base::MessageLoopForIO message_loop_;
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base::RunLoop run_loop_;
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std::unique_ptr<rtc::PacketSocketFactory> socket_factory_;
std::unique_ptr<rtc::AsyncPacketSocket> socket_;
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std::string last_packet_;
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rtc::SocketAddress last_address_;
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};
TEST_F(ChromiumSocketFactoryTest, SendAndReceive) {
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std::unique_ptr<rtc::AsyncPacketSocket> sending_socket(
socket_factory_->CreateUdpSocket(rtc::SocketAddress("127.0.0.1", 0), 0,
0));
80
ASSERT_TRUE(sending_socket.get() != nullptr);
81
EXPECT_EQ(sending_socket->GetState(),
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rtc::AsyncPacketSocket::STATE_BOUND);
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VerifyCanSendAndReceive(sending_socket.get());
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}
TEST_F(ChromiumSocketFactoryTest, SetOptions) {
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EXPECT_EQ(0, socket_->SetOption(rtc::Socket::OPT_SNDBUF, 4096));
EXPECT_EQ(0, socket_->SetOption(rtc::Socket::OPT_RCVBUF, 4096));
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}
TEST_F(ChromiumSocketFactoryTest, PortRange) {
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const uint16_t kMinPort = 12400;
const uint16_t kMaxPort = 12410;
95
socket_.reset(socket_factory_->CreateUdpSocket(
96
rtc::SocketAddress("127.0.0.1", 0), kMaxPort, kMaxPort));
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ASSERT_TRUE(socket_.get() != nullptr);
98
EXPECT_EQ(socket_->GetState(), rtc::AsyncPacketSocket::STATE_BOUND);
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EXPECT_GE(socket_->GetLocalAddress().port(), kMinPort);
EXPECT_LE(socket_->GetLocalAddress().port(), kMaxPort);
}
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TEST_F(ChromiumSocketFactoryTest, TransientError) {
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std::unique_ptr<rtc::AsyncPacketSocket> sending_socket(
socket_factory_->CreateUdpSocket(rtc::SocketAddress("127.0.0.1", 0), 0,
0));
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std::string test_packet("TEST");
// Try sending a packet to an IPv6 address from a socket that's bound to an
// IPv4 address. This send is expected to fail, but the socket should still be
// functional.
sending_socket->SendTo(test_packet.data(), test_packet.size(),
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rtc::SocketAddress("::1", 0),
rtc::PacketOptions());
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// Verify that socket is still usable.
VerifyCanSendAndReceive(sending_socket.get());
}
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} // namespace protocol
121
} // namespace remoting
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CSS for Designers
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In CSS for Designers, Molly E. Holzschlag and Andy Clarke expertly guide you through some of the most complex and useful techniques used in progressive Web design. Molly and Andy consider the topics from the point of view of creative, visual people rather than developers or programmers. Their unique approach helps give designers all the tools they need to create accessible, manageable, and beautiful websites.
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7m 38s
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8m 45s
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9m 45s
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4m 18s
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8m 51s
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9m 6s
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8m 28s
3. 40m 57s
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4m 5s
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4m 43s
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4m 57s
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5m 35s
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5m 48s
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3m 52s
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6m 41s
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5m 16s
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2m 19s
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8m 57s
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5m 22s
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5m 56s
5. 52m 13s
1. Chapter intro
2m 29s
2. Using CSS to create great-looking type
20m 29s
3. Combining graphics and CSS
9m 40s
4. Examining image replacement
19m 35s
6. 38m 47s
1. Chapter intro
2m 14s
2. CSS color values
5m 52s
3. Applying color to page elements
3m 21s
4. Applying color to links
5m 4s
5. Managing multiple link styles (descendent selectors)
4m 18s
6. Creating depth of field
4m 59s
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6m 44s
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6m 15s
7. 29m 0s
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1m 58s
2. Adding a background image to an element
9m 29s
3. Background repeat
7m 44s
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5m 9s
5. Fixed or scroll background
4m 40s
8. 36m 26s
1. Chapter intro
1m 22s
2. Removing bullets or numbers from lists
6m 46s
3. Using images for list markers
6m 1s
4. Creating vertical navigation
5m 8s
5. Designing a text-based horizontal navigation bar
5m 10s
6. Creating a tabbed horizontal navigation bar
11m 59s
9. 26m 4s
1. Chapter intro
2m 6s
2. Floating columns
5m 24s
3. Floating page elements
4m 28s
4. Clearing floats
7m 22s
5. Multi-column floated layout with header and footer
6m 44s
10. 22m 58s
1. Chapter intro
2m 45s
2. Normal flow
3m 5s
3. Relative positioning
4m 32s
4. Absolute positioning
2m 56s
5. Relative positioning to create a positioning context
5m 0s
6. Fixed positioning
4m 40s
11. 26m 3s
1. Chapter intro
1m 2s
2. Making panels
8m 16s
3. Creating grids
12m 0s
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4m 45s
12. 24m 31s
1. Chapter intro
2m 0s
2. Creating table structures
5m 45s
3. Adding table borders
5m 2s
4. Using backgrounds in tables
4m 43s
5. Special effects
7m 1s
13. 22m 0s
1. Chapter intro
35s
2. Accessible form markup
5m 22s
3. Basic fieldset styling
4m 38s
4. Fieldset grid layout
2m 57s
5. Styling labels and form controls
3m 16s
6. Styling form buttons
5m 12s
14. 38m 55s
1. Chapter intro
1m 2s
2. Creating symbols for wireframes in Fireworks
14m 57s
3. Naming conventions for class and ID
8m 47s
4. Organizing your CSS document
8m 11s
5. Working with multiple CSS files
5m 58s
15. 18s
1. Goodbye
18s
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7h 40m Intermediate Jun 02, 2006
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In CSS for Designers, Molly E. Holzschlag and Andy Clarke expertly guide you through some of the most complex and useful techniques used in progressive Web design. Molly and Andy consider the topics from the point of view of creative, visual people rather than developers or programmers. Their unique approach helps give designers all the tools they need to create accessible, manageable, and beautiful websites.
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|
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| 0.86381 |
Skip site navigation (1)Skip section navigation (2)
FreeBSD Manual Pages
home | help
DSA(1) OpenSSL DSA(1)
NAME
openssl-dsa, dsa - DSA key processing
SYNOPSIS
openssl dsa [-help] [-inform PEM|DER] [-outform PEM|DER] [-in filename]
[-passin arg] [-out filename] [-passout arg] [-aes128] [-aes192]
[-aes256] [-aria128] [-aria192] [-aria256] [-camellia128]
[-camellia192] [-camellia256] [-des] [-des3] [-idea] [-text] [-noout]
[-modulus] [-pubin] [-pubout] [-engine id]
DESCRIPTION
The dsa command processes DSA keys. They can be converted between
various forms and their components printed out. Note This command uses
the traditional SSLeay compatible format for private key encryption:
newer applications should use the more secure PKCS#8 format using the
pkcs8
OPTIONS
-help
Print out a usage message.
-inform DER|PEM
This specifies the input format. The DER option with a private key
uses an ASN1 DER encoded form of an ASN.1 SEQUENCE consisting of
the values of version (currently zero), p, q, g, the public and
private key components respectively as ASN.1 INTEGERs. When used
with a public key it uses a SubjectPublicKeyInfo structure: it is
an error if the key is not DSA.
The PEM form is the default format: it consists of the DER format
base64 encoded with additional header and footer lines. In the case
of a private key PKCS#8 format is also accepted.
-outform DER|PEM
This specifies the output format, the options have the same meaning
and default as the -inform option.
-in filename
This specifies the input filename to read a key from or standard
input if this option is not specified. If the key is encrypted a
pass phrase will be prompted for.
-passin arg
The input file password source. For more information about the
format of arg see the PASS PHRASE ARGUMENTS section in openssl(1).
-out filename
This specifies the output filename to write a key to or standard
output by is not specified. If any encryption options are set then
a pass phrase will be prompted for. The output filename should not
be the same as the input filename.
-passout arg
The output file password source. For more information about the
format of arg see the PASS PHRASE ARGUMENTS section in openssl(1).
-aes128, -aes192, -aes256, -aria128, -aria192, -aria256, -camellia128,
-camellia192, -camellia256, -des, -des3, -idea
These options encrypt the private key with the specified cipher
before outputting it. A pass phrase is prompted for. If none of
these options is specified the key is written in plain text. This
means that using the dsa utility to read in an encrypted key with
no encryption option can be used to remove the pass phrase from a
key, or by setting the encryption options it can be use to add or
change the pass phrase. These options can only be used with PEM
format output files.
-text
Prints out the public, private key components and parameters.
-noout
This option prevents output of the encoded version of the key.
-modulus
This option prints out the value of the public key component of the
key.
-pubin
By default, a private key is read from the input file. With this
option a public key is read instead.
-pubout
By default, a private key is output. With this option a public key
will be output instead. This option is automatically set if the
input is a public key.
-engine id
Specifying an engine (by its unique id string) will cause dsa to
attempt to obtain a functional reference to the specified engine,
thus initialising it if needed. The engine will then be set as the
default for all available algorithms.
NOTES
The PEM private key format uses the header and footer lines:
-----BEGIN DSA PRIVATE KEY-----
-----END DSA PRIVATE KEY-----
The PEM public key format uses the header and footer lines:
-----BEGIN PUBLIC KEY-----
-----END PUBLIC KEY-----
EXAMPLES
To remove the pass phrase on a DSA private key:
openssl dsa -in key.pem -out keyout.pem
To encrypt a private key using triple DES:
openssl dsa -in key.pem -des3 -out keyout.pem
To convert a private key from PEM to DER format:
openssl dsa -in key.pem -outform DER -out keyout.der
To print out the components of a private key to standard output:
openssl dsa -in key.pem -text -noout
To just output the public part of a private key:
openssl dsa -in key.pem -pubout -out pubkey.pem
SEE ALSO
dsaparam(1), gendsa(1), rsa(1), genrsa(1)
COPYRIGHT
Copyright 2000-2018 The OpenSSL Project Authors. All Rights Reserved.
Licensed under the OpenSSL license (the "License"). You may not use
this file except in compliance with the License. You can obtain a copy
in the file LICENSE in the source distribution or at
<https://www.openssl.org/source/license.html>.
1.1.1a 2018-11-20 DSA(1)
NAME | SYNOPSIS | DESCRIPTION | OPTIONS | NOTES | EXAMPLES | SEE ALSO | COPYRIGHT
Want to link to this manual page? Use this URL:
<https://www.freebsd.org/cgi/man.cgi?query=dsa&manpath=FreeBSD+12.0-RELEASE+and+Ports>
home | help
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50 Matching Annotations
1. Mar 2019
2. Feb 2019
1. Exploring the usage of the CASE software through various research and development funded initiatives, is an on‐going process with the results informing the development process of the system. Future directions in the context of CASE software design include: automatic “mind‐map” diagrams; user defined highlighting and layout preferences; user special‐interest groups; automated and peer‐reviewed scoring of annotations for display; automated abuse filtering; intelligent grouping of learners; and middleware applications to support improved integration with on‐line learning environments.
1. seems
Look at the seed he is sowing right there.
2. our scholars arc nol compelled to restrict their competence to the knowledge of one or another author, but can master a multiple, diversified, almost boundless domain of culture.
As an autodidact, he would know!
3. Nov 2018
1. Cobra既是一个用来创建强大的现代CLI命令行的golang库，也是一个生成程序应用和命令行文件的程序。下面是Cobra使用的一个演示：
1. 在计算机科学领域，反射是指一类应用，它们能够自描述和自控制。也就是说，这类应用通过采用某种机制来实现对自己行为的描述（self-representation）和监测（examination），并能根据自身行为的状态和结果，调整或修改应用所描述行为的状态和相关的语义
4. Oct 2018
1. Channels by default are blocking on sending and receiving values, so they will be waited on
默认情况下，通道在发送和接收值时会阻塞，因此它们将被等待。
1. Finally, we add the `xml:"sitemap"` syntax at the end for the parser to understand where it's looking when we go to unpack this with the encoding/xml package.
最后，我们在末尾添加`xml：“sitemap”`语法，以便解析器在我们使用encoding / xml包解压缩它时的位置。
type Sitemapindex struct {
Locations []Location `xml:"sitemap"`
}
1. Personally, I would test both. If the gains are insignificant by using value receivers where possible and you are using pointer receivers, sure, use all pointer receivers. If you can make sizeable gains by using value receivers where possible, however, I would personally use them.
就个人而言，我会测试两者。
如果在可能的情况下使用值接收器并且您正在使用指针接收器，则增益无关紧要，请确保使用所有指针接收器。
如果使用值接收器获取的收益微不足道，那么在可能的情况下，使用指针接收器，当然，所有地方都使用指针接收器
但是，如果使用值接收器可以获得可观的收益，那么在可能的情况下，会亲自使用它们
2. Now, we're modifying the struct itself via pointer. Now in the code we could do something like a_car.new_top_speed(500), and this would actually modify the object itself.
现在，我们通过指针修改结构本身。现在在代码中我们可以做类似a_car.new_top_speed（500）的事情，这实际上会修改对象本身
1. between the func keyword and the name of the function, we pass the variable and type. We use c, short for car, and then car, which is in association with the car struct. In this case, the method gets a copy of the object, so you cannot actually modify it here, you can only take actions or do something like coming up with a calculation
func关键字和方法名之间，我们传递了变量和类型，
我们使用c，汽车的简称，然后汽车，这是与汽车结构相关联。这样就把struct和method关联了起来
在这种情况下，该方法获取对象的副本，因此您无法在此实际修改它，您只能执行操作或执行类似计算的操作 通过把struct和method关联了起来，使得方法获取到了对象的副本
func (c car) kmh() float64 {
return float64(c.gas_pedal) * (c.top_speed_kmh/usixteenbitmax)
}
2. In order to do this, we don't need to actually modify the a_car variable, so we can use a method on a value, called a value receiver:
我们想把gas_pedal的值转化成某种实际的速度，为了做到这一点，我们不需要实际修改a_car变量，所以我们可以在值上使用一个方法，称为值 接收器
func (c car) kmh() float64 {
return float64(c.gas_pedal) * (c.top_speed_kmh/usixteenbitmax)
}
3. Methods that just access values are called value receivers and methods that can modify information are pointer receivers.
只访问值的方法称为值接收器，只有获取值的需求时，用值接收器
可以修改信息的方法是指针接收器，需要修改struct信息时，用指针接收器
1. 可以使用内建函数 make 也可以使用 map 关键字来定义 Map:
// 声明变量，默认 map 是 nil
var map_variable map[key_data_type]value_data_type
// 使用make 函数
map_variable := make(map[key_data_type]value_data_type)
1. /* 打印子切片从索引 0(包含) 到索引 2(不包含) */ number2 := numbers[:2] printSlice(number2) /* 打印子切片从索引 2(包含) 到索引 5(不包含) */ number3 := numbers[2:5] printSlice(number3)
numbers := []int{0,1,2,3,4,5,6,7,8}
len=2 cap=9 slice=[0 1]
len=3 cap=7 slice=[2 3 4]
2. 通过内置函数make()初始化切片s,[]int 标识为其元素类型为int的切片
s := make([]int, len, cap)
3. 初始化切片s,是数组arr的引用
s := arr[:]
1. /* 未定义长度的数组只能传给不限制数组长度的函数 */ setArray(array) /* 定义了长度的数组只能传给限制了相同数组长度的函数 */ var array2 = [5]int{1, 2, 3, 4, 5}
func setArray(params []int) {
fmt.Println("params array length of setArray is : ", len(params))
}
func setArray2(params [5]int) {
fmt.Println("params array length of setArray2 is : ", len(params))
}
1. 注意：以上代码中倒数第二行的 } 必须要有逗号，因为最后一行的 } 不能单独一行，也可以写成这样：
a = [3][4]int {
{0, 1, 2, 3} ,
{4, 5, 6, 7} ,
{8, 9, 10, 11} , // 此处的逗号是必须要有的
}
上面代码也可以等价于
a = [3][4]int {
{0, 1, 2, 3} ,
{4, 5, 6, 7} ,
{8, 9, 10, 11}}
1. func getSequence() func() int { i:=0 return func() int { i+=1 return i } }
闭包，A函数 返回一个函数，假设返回的函数为B，那么函数B，可以使用A函数中的变量
nextNumber := getSequence()
nextNumber 是 返回函数B类型，func() int，i是函数A中的变量，初始值 i=0
执行nextNumber()，i+=1, reuturn i ==> 1
再执行nextNumber()，i+=1，return i ==> 2
再执行nextNumber()，i+=1，return i ==> 3
1. for 循环的 range 格式可以对 slice、map、数组、字符串等进行迭代循环。
for key, value := range oldMap {
newMap[key] = value
}
1. 以下描述了 select 语句的语法
• 每个case都必须是一个通信
• 所有channel表达式都会被求值
• 所有被发送的表达式都会被求值
• 如果任意某个通信可以进行，它就执行；其他被忽略。
• 如果有多个case都可以运行，Select会随机公平地选出一个执行。其他不会执行。
否则：
1. 如果有default子句，则执行该语句。
2. 如果没有default字句，**select将阻塞，直到某个通信可以运行**；Go不会重新对channel或值进行求值。
1. iota 表示从 0 开始自动加 1，所以 i=1<<0, j=3<<1（<< 表示左移的意思），即：i=1, j=6，这没问题，关键在 k 和 l，从输出结果看 k=3<<2，l=3<<3。
package main
import "fmt"
const (
i=1<<iota
j=3<<iota
k
l
)
func main() {
fmt.Println("i=", i)
fmt.Println("j=", j)
fmt.Println("k=", k)
fmt.Println("l=", l)
}
以上实例运行结果为：
i=1
j=6
k=12
l=24
1. 这种因式分解关键字的写法一般用于声明全局变量
var (
vname1 v_type1
vname2 v_type2
)
1. 以一个大写字母开头，如：Group1，那么使用这种形式的标识符的对象就可以被外部包的代码所使用（客户端程序需要先导入这个包），这被称为导出（像面向对象语言中的 public）
2. main 函数是每一个可执行程序所必须包含的，一般来说都是在启动后第一个执行的函数（如果有 init() 函数则会先执行该函数）
5. Aug 2018
6. Jan 2018
7. Nov 2017
8. Oct 2017
9. Sep 2017
10. May 2017
1. AlphaGo is going out on top. After beating Ke Jie, the world’s best player of the ancient Chinese board game Go, for the third time today at the Future of Go Summit in Wuzhen, Google’s DeepMind unit announced that it would be the last event match the AI plays.
This makes me feel worse somehow than if it was going to continue to play. Seems like it is saying: well, tick the box for beating humans at Go...
1. It inspired his work at Google, where he led the creation of the historical map platform Field Trip, and then, the Pokémon Go precursor, Ingress.
I loved field trip for Glass!
2. Niantic, the maker of Pokémon Go, is teaming up with the Knight Foundation in a multiyear commitment promoting civic engagement in communities. That means the two entities will pitch in time, money, and plenty of Pokémon to get citizens outside, exploring their towns in city-organized events.
Very interesting development!
1. The regulation of an entire burgeoning industry, and the interpretation of the Constitution in the digital age, could be impacted by the court’s decision in a case inspired by Pokémon.
This is a really interesting case. How have I not heard of it?
1. The diffusion and the public endorsement of data FAIRness has been rapid
Good to see posts on this.
11. Apr 2017
12. Jan 2017
13. Aug 2016
14. Apr 2016
15. Dec 2015
|
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| 0.997255 |
Commits
Mateusz Łopaciński committed c8b2c76
Initial php5.2 commit
Comments (0)
Files changed (15)
Ray/Exception/Call.php
<?php
-namespace Ray\Exception;
-
-class Call extends \Exception{}
+class Ray_Exception_Call extends Exception{}
Ray/Exception/View/File.php
<?php
-namespace Ray\Exception\View;
-
-class File extends \Exception{}
+class Ray_Exception_View_File extends Exception{}
Ray/Exception/View/Mustache.php
<?php
-namespace Ray\Exception\View;
-
-class Mustache extends \Exception{}
+class Ray_Exception_View_Mustache extends Exception{}
<?php
-namespace Ray;
-
class Ray{
public $config,
$errors,
$dispatch;
+ public static $instance;
+
+ public static function getInstance(){
+ if(self::$instance){
+ return self::$instance;
+ }
+ }
+
public function __construct($config=array(), $preDispatch=null, $postDispatch=null){
- spl_autoload_register(array('Ray\Ray', 'autoload'));
+
+ self::$instance = $this;
+
+ spl_autoload_register(array('Ray', 'autoload'));
if ( @date_default_timezone_set(date_default_timezone_get()) === false ) {
date_default_timezone_set('UTC');
$this->config = array_merge(array(
'mode' => 'production',
'templates' => ROOT . DS . 'template',
- 'view' => 'Ray\View\Inline'
+ 'view' => 'Ray_View_Inline'
), $config);
if($preDispatch && is_callable($preDispatch)){
$this->dispatch['post'] = $postDispatch;
}
- $this->router = new Router();
+ $this->router = new Ray_Router();
- $this->response = new Response();
+ $this->response = new Ray_Response();
- $this->request = new Request();
+ $this->request = new Ray_Request();
- $this->response->responseCode(Response::OK);
+ $this->response->responseCode(Ray_Response::OK);
}
public function __call($method, $args){
- $reflection = new \ReflectionObject($this->request);
+ $reflection = new ReflectionObject($this->request);
$via = 'METHOD_' . strtoupper($method);
if($return){
$view = new $this->config['view']($return, $this->config);
$send = $view->render();
- }elseif(isset($this->error[Response::NOT_FOUND])){
+ }elseif(isset($this->error[Ray_Response::NOT_FOUND])){
$response = Response::NOT_FOUND;
- $send = $this->error[Response::NOT_FOUND];
+ $send = $this->error[Ray_Response::NOT_FOUND];
}else{
- $response = Response::NOT_FOUND;
- $send = Response::NOT_FOUND;
+ $response = Ray_Response::NOT_FOUND;
+ $send = Ray_Response::NOT_FOUND;
}
if(isset($response)){
if(is_callable($callable)){
$this->errors[$code] = $callable;
}else{
- throw new Exception\Call('Hey, man $callable isnt callable');
+ throw new Ray_Exception_Call('Hey, man $callable isnt callable');
}
}
*/
public static function autoload( $class ) {
- $file = ROOT . DIRECTORY_SEPARATOR . str_replace('\\', DIRECTORY_SEPARATOR, $class) . '.php';
+ $file = ROOT . DIRECTORY_SEPARATOR . str_replace('_', DIRECTORY_SEPARATOR, $class) . '.php';
if ( file_exists($file) ) {
require $file;
<?php
-namespace Ray;
-
-class Request{
+class Ray_Request{
const METHOD_HEAD = 'HEAD';
<?php
-namespace Ray;
-
-class Response{
+class Ray_Response{
const OK = '200 OK';
const CREATED = '201 Created';
<?php
-namespace Ray;
-
-class Router{
+class Ray_Router{
private $routes = array();
public function map($method, $pattern, $callable){
- $route = $this->routes[$method][$pattern] = new Router\Route($pattern, $callable);
+ $route = $this->routes[$method][$pattern] = new Ray_Router_Route($pattern, $callable);
return $route;
}
<?php
-namespace Ray\Router;
-
-use Ray;
-
-class Route{
+class Ray_Router_Route{
private $pattern;
<?php
-namespace Ray;
-
-interface View{
+interface Ray_View{
public function render();
<?php
-namespace Ray\View;
-
-use Ray;
-
-class File implements Ray\View{
+class Ray_View_File implements Ray_View{
private $config;
}
private function view($file, $params=array()){
- $view = new Ray\View\File\View($file, $params, $this->config, $this->options);
+ $view = new Ray_View_File_View($file, $params, $this->config, $this->options);
return $view->render();
}
return $this->view($layout, $params);
}else{
- throw new Ray\Exception\View\File('Action template file not found. Path: '. $action);
+ throw new Ray_Exception_View_File('Action template file not found. Path: '. $action);
}
}else{
- throw new Ray\Exception\View\File('Template file not found. Path: ' . $layout);
+ throw new Ray_Exception_View_File('Template file not found. Path: ' . $layout);
}
return;
Ray/View/File/View.php
<?php
-namespace Ray\View\File;
-
-class View{
+class Ray_View_File_View{
private $__file;
return $view->render();
}else{
- throw new Ray\Exception\View\File('Element template file not found. Path: '. $file);
+ throw new Ray_Exception_View_File('Element template file not found. Path: '. $file);
}
return;
<?php
-namespace Ray\View;
-
-use Ray;
-
-class Inline implements Ray\View{
+class Ray_View_Inline implements Ray_View{
private $html;
Ray/View/Mustache.php
<?php
-namespace Ray\View;
-
-use Ray;
-
-class Mustache implements Ray\View{
+class Ray_View_Mustache implements Ray_View{
private $options = array(
'ext' => '.mustache',
public function __construct(array $response, $config){
- $this->instance = new Ray\View\Mustache\Compiler();
+ $this->instance = new Ray_View_Mustache_Compiler();
$this->options = array_merge($this->options, $response);
return $this->instance->render($template, $params, $partial);
}else{
- throw new Ray\Exception\View\Mustache('Action template file not found. Path: '. $action);
+ throw new Ray_Exception_View_Mustache('Action template file not found. Path: '. $action);
}
}else{
- throw new Ray\Exception\Mustache('Template file not found. Path: ' . $layout);
+ throw new Ray_Exception_Mustache('Template file not found. Path: ' . $layout);
}
return;
Ray/View/Mustache/Compiler.php
<?php
-namespace Ray\View\Mustache;
-
/**
* A Mustache implementation in PHP.
*
*
* @author Justin Hileman {@link http://justinhileman.com}
*/
-class Compiler {
+class Ray_View_Mustache_Compiler {
const VERSION = '0.9.0';
const SPEC_VERSION = '1.1.2';
error_reporting(E_ALL | E_STRICT);
require_once 'Ray/Ray.php';
-
+
try{
- $app = new Ray\Ray(array(
- 'view' => 'Ray\View\Mustache'
- ));
-
- $app->error(404, function(){
+ function error404(){
return array(
'action' => '404'
);
- });
-
- $app->get('/', function(){
+ }
+
+ function index(){
return array(
'action' => 'index'
);
- });
-
- $app->get('/post/:id/', function($id=7){
+ }
+
+ function post($id){
return array(
- 'action' => 'post'
+ 'action' => 'post',
+ 'params' => array(
+ 'id' => $id
+ )
);
- });
+ }
+
+ $app = new Ray(array(
+ 'view' => 'Ray_View_File'
+ ));
+
+ $app->error(404, 'error404');
+
+ $app->get('/', 'index');
+
+ $app->get('/post/:id', 'post');
-}catch(\Exception $e){
- echo '<pre>' . $e . '</pre>';
+}catch(Exception $e){
+ die('<pre>' . $e . '</pre>');
}
|
__label__pos
| 0.964447 |
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I keep on coming into these problems while my gre prepration
How many times vs how many times greater or
a is what % of b or a is what % greater than b
What is the right convention to solve these problems? Some websites treat both of these scenarios similar? However I believe that in case of larger you have to do
if how many times b greater than a
do b-a/a.
Let's say a=9, b=3;
So I get these answers. Are these answers correct?
1: a is 3 times b.
2: a is 2 times greater than b.
3: a is 300% times b.
4: a is 200% times greater than b.
My confusion is that in this question, author does the opposite of what he should do.
share|cite|improve this question
I am with you, but this may be rather a linguistic question and interpretation 2 is probably not as widespread (and becomes weird when $a=2b$ and you say $a$ is once greater than $b$) – Hagen von Eitzen Oct 7 '13 at 17:59
up vote 2 down vote accepted
The phrase "A is what % of B" should be written as $A=x\cdot B$. And now solve for x, and then multiply by 100.
Example 1a: If A is 100, and B is 50, then $100=x\cdot 50$, means that $x = 2$, and A is 200% of B.
The phrase "A is what % greater than B," should be written as $A=x\cdot B$, just as before. But now, when you solve for x, and multiply by 100, you want to take the additional step of subtracting 100. Notice that this will only work if A is actually greater than B.
Example 1b: In the above example, A would be 100% greater than B.
Example 2: if A is 150, and B is 100, then solving for x in $A=x\cdot B$, would give us $x = 1.5$, and so A is 150% of B. But A is 50% greater than B.
share|cite|improve this answer
You have a "greater than" mismatch between saying "A is 200% greater than B" in your second sentence vs. "example: In the above example, A would be 100% greater than B." in your fourth sentence. – abiessu Oct 7 '13 at 18:05
The question that you link to uses the phrase "...how many times larger was...".
You use the phrase greater than in your question, and this is what causes the problem. There is an ambiguity about whether it is the comparative sizes of their differences that you discuss.
The fact is that 2 is two times larger than 1. The fact is that 1.5 is three times larger than 0.5. The fact is that $b$ is $(b \div a)$ times larger than $a$. The page you link to is correct.
share|cite|improve this answer
If I understand correctly, you are saying it is larger vs greater? For greater and larger, answers will be different. – Dude Oct 7 '13 at 18:16
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__label__pos
| 0.988015 |
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The stupid question is the question not asked
PerlMonks
Re: mac file problem
by 7stud (Deacon)
on Mar 17, 2013 at 06:22 UTC ( #1023887=note: print w/ replies, xml ) Need Help??
in reply to mac file problem
1. You are not opening files properly.
2. You don't seem to know how to read from a file.
3. Why would you write a program that takes a location as input, when you can't even write a program to work with a hard coded location? Hint: you should NEVER write a program that takes ANY user input until you can get the program to work WITHOUT user input.
4. If you posted what you intended to do with the file, you would get better answers. The most common thing to do with a file is to read it line by line, but since you are reading an html file, maybe you intend to use HTML::Parser to find certain tags, and therefore you need the whole file in one string--but then HTML::Parser can be given a file name as an argument, so you wouldn't need to read the file by hand.
Read a file line by line:
use strict; use warnings; use 5.016; my $fname = 'data.txt'; open my $INFILE, '<', $fname or die "Couldn't open $fname: $!"; while (my $line = <$INFILE>) { chomp $line; #do something to $line; } close $INFILE or die "Couldn't close $fname: $!";
Read the whole file into an array:
use strict; use warnings; use 5.016; my $fname = 'data.txt'; open my $INFILE, '<', $fname or die "Couldn't open $fname: $!"; chomp(my @lines = <$INFILE>); close $INFILE or die "Couldn't close $fname: $!";
Read the whole file into a string:
use strict; use warnings; use 5.016; my $fname = 'data.txt'; open my $INFILE, '<', $fname or die "Couldn't open $fname: $!"; my $file; { local $/ = undef; #No line terminator, so a line #is everyting in the file $file = <$INFILE>; #Read one line. } # $/ is restored to its default value of "\n" here close $INFILE or die "Couldn't close $fname: $!";
Comment on Re: mac file problem
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SamYoungNY SamYoungNY - 6 months ago 71
JSON Question
Getting a specific part of e.parameter
I have a Google App Script that writes form data to a spreadsheet upon submission on my site.
I have many different forms, and I'd like to write their data to the same Spreadsheet, but to different 'Sheets' (i.e tabs @ bottom "form1, form2, form3") within.
Each form has a hidden input that names it. I think that by isolating the input name, I can write an
if statement
that pushes the form data to the right sheet.
example of HTML:
<input type="hidden" name="form-type" value="form1">
*Name: <input type="text" name="john">
<input type="hidden" name="form-type" value="form2">
*Name: <input type="text" name="peter">
Javascript on website - handling the data for POST'ing to Google App Script:
function handleFormSubmit(event) { // handles form submit withtout any jquery
event.preventDefault(); // we are submitting via xhr below
var data = getFormData(); // get the values submitted in the form
var url = event.target.action;
var xhr = new XMLHttpRequest();
xhr.open('POST', url);
xhr.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded');
// url encode form data for sending as post data
var encoded = Object.keys(data).map(function(k) {
return encodeURIComponent(k) + '=' + encodeURIComponent(data[k]);
}).join('&');
console.log("ENCODED IS: " + encoded);
xhr.send(encoded);
}
}
The encoded data looks like this:
ENCODED IS: form-type=form1&name=john
OR
ENCODED IS: form-type=form2&name=paul
The Google App Script function responsible for populating the spreadsheet is:
try {
var doc = SpreadsheetApp.openById(SCRIPT_PROP.getProperty("key"));
var sheet = doc.getSheetByName('form1'); // here is where to define which sheet a response should be added to
var headers = sheet.getRange(1, 1, 1, sheet.getLastColumn()).getValues()[0];
var nextRow = sheet.getLastRow()+1; // get next row
var row = [ new Date() ]; // first element in the row should always be a timestamp
// loop through the header columns
for (var i = 1; i < headers.length; i++) { // start at 1 to avoid Timestamp column
if(headers[i].length > 0) {
row.push(e.parameter[headers[i]]); // add data to row
}
}
// more efficient to set values as [][] array than individually
sheet.getRange(nextRow, 1, 1, row.length).setValues([row]);
}
catch(error) {
Logger.log(e);
}
finally {
return;
}
How do I access the specific
e.parameter
(
form-type
) that I need in order to be able to discern which form was used and route it accordingly?
For more info you can reference (https://github.com/dwyl/html-form-send-email-via-google-script-without-server")
Answer
From the doGet parameter documentation:
The second parameter is called parameter, and it is an Object containing each parameter and value pair
So, something like:
var type = e.parameter['form-type'];
Should work?
|
__label__pos
| 0.99715 |
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__label__pos
| 0.797348 |
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Expression Validator with async functions
Using asynchron functions in expression validator
Survey.StylesManager.applyTheme("defaultV2");
//Function that returns the value on callback async
function isCountryExist(params) {
if (params.length < 1) return false;
var countryName = params[0];
//If the question is empty then do nothing
if (!countryName) {
//It doesn't matter what the function returns. The library is wating for this.returnResult(resultValue) callback
this.returnResult(false);
return false;
}
var self = this;
//call the ajax method
$.ajax({url: "https://surveyjs.io/api/CountriesExample?name=" + countryName}).then(function (data) {
var found = data.length > 0;
// return the value into the library. Library is waiting for this callback
self.returnResult(found);
});
//May return any value. The library will ignore it.
return false;
}
// It is important to tell the library that the function is async, the last parameter
// and you will return the value on this.returnResult callback
Survey.FunctionFactory.Instance.register("isCountryExist", isCountryExist, true);
/*
If you do not need to support es5 (IE), then you may use async/await operators
async function isCountryExist(params) {
if (params.length < 1) return false;
var countryName = params[0];
if (!countryName) {
this.returnResult(false);
return false;
}
var self = this;
var res = await $.ajax({
url: "https://surveyjs.io/api/CountriesExample"
}).then(function(data) {
var found = false;
var countries = data;
for (var i = 0; i < countries.length; i++) {
if (countries[i].name == countryName) {
found = true;
break;
}
}
self.returnResult(found);
});
return false;
}
*/
var json = {
elements: [
{
type: "text",
name: "country",
title: "Type a country:",
validators: [
{
type: "expression",
expression: "isCountryExist({country}) = true",
text: "Please type the country correctly"
}
]
},
{
type: "comment",
name: "aboutCountry",
title: "Please tell us about country: '{country}'",
visibleIf: "isCountryExist({country}) = true"
}
]
};
window.survey = new Survey.Model(json);
survey.onComplete.add(function(sender) {
document.querySelector('#surveyResult').textContent =
"Result JSON:\n" + JSON.stringify(sender.data, null, 3);
});
ReactDOM.render(
<SurveyReact.Survey model={survey} />, document.getElementById("surveyElement"));
<!DOCTYPE html>
<html lang="en">
<head>
<title>Expression Validator with async functions, Reactjs Survey Library Example</title>
<meta name="viewport" content="width=device-width" />
<script src="https://unpkg.com/jquery"></script>
<script src="https://unpkg.com/[email protected]/umd/react.production.min.js"></script>
<script src="https://unpkg.com/[email protected]/umd/react-dom.production.min.js"></script>
<script src="https://unpkg.com/@babel/[email protected]/babel.min.js"></script>
<script src="/DevBuilds/survey-core/survey.core.min.js"></script>
<script src="/DevBuilds/survey-core/survey.i18n.min.js"></script>
<script src="/DevBuilds/survey-react-ui/survey-react-ui.min.js"></script>
<link href="/DevBuilds/survey-core/defaultV2.min.css" type="text/css" rel="stylesheet" />
<link rel="stylesheet" href="./index.css">
</head>
<body>
<div id="surveyElement" style="display:inline-block;width:100%;">
</div>
<div id="surveyResult"></div>
<script type="text/babel" src="./index.js"></script>
</body>
</html>
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GS2: Kies required for ICS update?General
Last Updated:
1. travelmos
travelmos New Member
I'm sorry if this sounds more like another rant against Kies than a question.
I have an International GS2 and a Mac. I've been (sadly) watching the news about ICS update coming last to our international phones (the ones we pay so much for). I bought my GS2 when passing through Bangkok.
I installed Kies on my Mac when I first got my GS2 but it ate so much CPU on my Mac (even when when the UI was not active) I had to uninstall. I'm now reading that I need to use Kies to upgraded to ICS.
Is that true? No over the air updates?
Seriously? I have a top-rated dual-core device that has both wifi and cell data and I still need another computer to update it? Is that just to force people into using Kies? If I wanted to be tied to a crappy desktop interface I would have bought an iPhone and used iTunes.
Poking around the forums I seems like many people have problems with Kies, and many more just think it sucks. Is this the future for the Samsung products -- having to use Kies for updates?
Other than updates, does Kies offer anything that just having a USB connection and mounting the phone's storage won't do?
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2. Russell Ng
Russell Ng Well-Known Member
1) Kies is temperamental, buggy... but (eventually) it does work when you get it to connect right. You can use it for official updates and backups (contacts, memos etc), but for everything else, there are options in terms of apps and workarounds.
2) There would always be people who wants to update through PC then relying on data connection or wifi. So having a choice is always good.
3) OTA is rarely done right now, for all carries and all handsets. But I believe in future, this would be a standard option. That's why the SGS2 have a setting in the phone to check for updates directly, and I believe one of the UK carriers already uses OTA. You can't really blame Samsung for this though.
4) Alternatives like Odin is available.
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Highlight of tags search results.
Command:
GET /megacorp/employee/_search
{ "query" : { "match_phrase" : { "about" : "rock climbing" } }, "highlight": { "fields" : { "about" : {} } } }
Highlighting of search results in CURL:
$curl -XPOST 'localhost:9200/megacorp/employee/_search?pretty' -d '
{
"query" :
{
"match_phrase" :
{
"about" : "rock climbing"
}
},
"highlight":
{
"fields" :
{
"about" : {}
}
}
}'
Highlighting of search results in PHP:
require '../vendor/autoload.php';
$client = Elasticsearch\ClientBuilder::create()->build();
$params = [
'index' => 'megacorp',
'type' => 'employee',
'body' => [
'query' => [
'match' => [
"about" => "rock climbing"
],
],
'highlight' => [
"pre_tags" => "<em>",
"post_tags" => "</em>",
'fields' => [
'about' => new \stdClass()
]
],
]
];
try {
$response = $client->search($params);
} catch (Exception $e) {
var_dump($e->getMessage());
}
var_dump($response);
'about' => new \stdClass() - stdClass it is used for the proper formation of JSON.
Highlighting of search results in Yii2:
$params = [
'match' => [
"about" => "rock climbing"
]
];
$model = Megacorp::find()
->query($params)
->highlight([
"pre_tags" => "<em>",
"post_tags" => "</em>",
'fields' => [
'about' => new \stdClass()
]
])
->all();
var_dump($model);
Additionally
Detailed documentation https://www.elastic.co/guide/en/elasticsearch/reference/current/search-request-highlighting.html
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Therefore, although RS(A) is a subspace of R n and CS(A) is a subspace of R m, equations (*) and (**) imply that . Previous Page. Solution. A matrix having only one column is called a column matrix. True. This form is called reduced row-echelon form. Here are two matrices which are not equal even though they have the same elements. A m×n × B n×p = C m×p. on the identity matrix (5R 2) ! False. Any matrix with determinant zero is non-invertable. and all corresponding eigenvectors are orthogonal and assumed to be normalized, i.e., , or is a unitary (orthogonal if real) matrix. To facilitate exposition, we have generally restricted our examples to one matrix or array operation. Advertisements. The square matrix could be 2×2, 3×3, 4×4, or any type, such as n × n, where the number of column and rows are equal. To A = B. For example, if the sample has a continuous distribution, then the likelihood function is where is the probability density function of , parametrized by , and the information matrix is The information matrix is the covariance matrix of the score. This test is nice because it extends to testing multiple coefficients, so if I wanted to test bars=liquor stores=convenience stores. Find the 2 2× matrix X that satisfy the equation AX B= 1 3 2 3 = X Question 24 (***) It is given that A and B are 2 2× matrices that satisfy det 18(AB) = and det 3(B−1) = − . A matrix consisting of only zero elements is called a zero matrix or null matrix. You have lost information. In other words, we are performing on the identity matrix (R 3 2R 1) ! The matrix 1 1 0 2 has real eigenvalues 1 and 2, but it is not symmetric. Matrix U shown below is an example of an upper triangular matrix. Thus, the value of for a column matrix will be 1. even if m ≠ n. Example 1: Determine the dimension of, and a basis for, the row space of the matrix Definition, examples and practice problems as well as onnline power point lesson on what makes a matrix, how to add matrices as well as how to identify and label individual entries in a matrix. As one example of this, the oft-used Theorem SLSLC, said that every solution to a system of linear equations gives rise to a linear combination of the column vectors of the coefficient matrix that equals the vector of constants. matrix equality worksheet, The 2 2× matrices A and B are given by 5 7 2 3 = A; 19 36 8 15 = B. Matrix L shown below is an example of a lower triangular matrix. In a 2x2 matrix, the determinant is equal to: 01) The matrix A and B should be the same size. (R 2). Two matrices and having the same dimension are said to be equal if and only if all their corresponding elements are equal to each other: Zero matrices. where * represents any number.. Next Page . For example, $$ A =\begin{bmatrix} 3 & -1 & 0\\ 3/2 & √3/2 & 1\\4 & 3 & -1\\ 7/2 & 2 & -5 \end{bmatrix}$$ is a matrix of the order 4 × 3. Matrix Equality. A matrix is a zero matrix if all its elements are equal to zero, and we write Problem Description. 02) Corresponding elements should be equal. You may follow along here by making the appropriate entries or load the completed template Example 1 by clicking on Open Example Template from the File menu of the Equality of Covariance window. False. 2x2 Matrix. The product of two matrices A and B is defined if the number of columns of A is equal to the number of rows of B. Given the Vandermonde matrix in terms of , Proposition 1 states that Well, for a 2x2 matrix the inverse is: In other words: swap the positions of a and d, put negatives in front of b and c, and divide everything by the determinant (ad-bc). Java Examples - Check equality of two arrays. (R 3). Notice that the covariance matrix is symmetric (elements o ffthe diago-nal are equal so that Σ= Σ0,whereΣ0 denotes the transpose of Σ)since cov( )=cov( ) cov( )=cov( )and cov( )= cov( ) Example 2 Example return data using matrix notation Moreover, we have used an arrow when it appeared useful and an equality sign at other times. Lets look at an easy example. The prior individual Wald tests are not as convenient for testing more than two coefficients equality at once. Example 1 Matrix Equality Let A = 79x 0 −1 y +1 and B = 790 0 −111. The determinant of a matrix is equal to the determinant of its transpose. Addition and subtraction of matrices Live Demo. Another example of the row matrix is P = [ -4 -21 -17 ] which is of the order 1×3. And the result will have the same number of rows as the 1st matrix, and the same number of columns as the 2nd matrix. We will see the importance of Hessian matrices in finding local extrema of functions of more than two variables soon, but we will first look at some examples of computing Hessian matrices. Sometimes we have put the result on the left; and sometimes on the right. Any matrix plus the zero matrix is the original matrix; Matrix Multiplication. 1 Open the Fisher dataset. Thus, A = [a ij] mxn is a column matrix if n = 1. (SepalLength, SepalWidth, PetalLength, and PetalWidth) are equal across the three iris varieties. A is a 3 × 2 matrix and B is a 2 × 3 matrix, and, for matrices, 3 × 2 does not equal 2 × 3! OK, how do we calculate the inverse? Hence, the order is m × 1. It doesn't matter if A and B have the same number of entries or even the same numbers as entries. Example: This matrix is 2×3 (2 rows by 3 columns): When we do multiplication: The number of columns of the 1st matrix must equal the number of rows of the 2nd matrix. Equal Matrices. Unless A and B are the same size and the same shape and have the same values in exactly the same places, they are not equal. These matrices basically squash things to a lower dimensional space. The same dimensions. For two matrices to be equal, they must have . Since there are three elementary row transformations, there are three di⁄er- An inverse matrix is a matrix that, when multiplied by another matrix, equals the identity matrix. This proves the result of Proposition 1. Example: Add Two Matrices using Multi-dimensional Arrays In the equation above, we have introduced a new vector as a unitary transform of . But the maximum number of linearly independent columns is also equal to the rank of the matrix, so . Note: Reduced row-echelon form does not always produce the identity matrix, as you will learn in higher algebra. can be considered as a rotated version of with its Euclidean 2-norm conserved, .. A square S, of area 6 cm 2, is transformed by A to produce an image S′. Find the values of x and y such that A = B. In other words, say that A n x m = [a ij] and that B p x q = [b ij].. Then A = B if and only if n=p, m=q, and a ij =b ij for all i and j in range.. (2) A symmetric matrix is always square. If Ais an m nmatrix, then its transpose is an n m matrix, so if these are equal, we must have m= n. (3) Any real matrix with real eigenvalues is similar to a symmetric matrix. Subsection MVP Matrix-Vector Product. If Ais symmetric, then A= AT. Minor of a Matrix. I give an example of doing this in R on crossvalidated. C++ Program to Add Two Matrix Using Multi-dimensional Arrays This program takes two matrices of order r*c and stores it in two-dimensional array. Then, the program adds these two matrices and displays it on the screen. Equality of matrices Two matrices \(A\) and \(B\) are equal if and only if they have the same size \(m \times n\) and their corresponding elements are equal. Column Matrix. For our purposes, however, we will consider reduced row-echelon form as only the form in which the first m×m entries form the identity matrix.. To row reduce a matrix: Equality between matrices is defined in the obvious way. How to check if two arrays are equal or not? Let us try an example: How do we know this is the right answer? Example 98 2 4 1 0 0 0 1 0 2 0 1 3 5 is an identity matrix. If S is the set of square matrices, R is the set of numbers (real or complex) and f : S → R is defined by f (A) = k, where A ∈ S and k ∈ R, then f (A) is called the determinant of A. This follows after comparing the coefficients on each side of the equality and seeing that for both sides of the equality the coefficient on is always one. For more information, see Compare Function Handles.. isequal returns logical 0 (false) for two objects with dynamic properties, even if the properties have the same names and values.. isequal compares only stored (non-dependent) properties when testing two objects for equality. A matrix is said to be a rectangular matrix if the number of rows is not equal to the number of columns. The symmetry is the assertion that the second-order partial derivatives satisfy the identity ∂ ∂ (∂ ∂) = ∂ ∂ (∂ ∂) For example: [] ⋅ [− −] = [][− −] is the inverse of [].The formula for the inverse of a 2x2 matrix, [] is: [− −]Where is the determinant of the matrix. Equal matrices. An upper triangular matrix is a square matrix with all its elements below the main diagonal equal to zero. The determinant of the product of two square matrices is equal to the product of the determinants of the given matrices. It can be obtained by re-placing row 3 of the identity matrix by row 3 plus 2 times row 1. The number of columns in the first matrix must be equal to the number of rows in the second matrix. A lower triangular matrix is a square matrix with all its elements above the main diagonal equal to zero. That is, the inner dimensions must be the same. 5) Diagonal matrix Corresponding elements must be equal. Following example shows how to use equals method of Arrays to check if two arrays are equal or not. Power of a matrix. If matrix A = matrix B we can say that A and B are identical. The equality of two function handles depends on how they are constructed. We have repeatedly seen the importance of forming linear combinations of the columns of a matrix. Equal Matrices--Matrices are equal if if two conditions are met. The conditions for matrix equality are discussed below. ... A matrix which consist of 0 s is called a Zero Matrix. In mathematics, the symmetry of second derivatives (also called the equality of mixed partials) refers to the possibility under certain conditions (see below) of interchanging the order of taking partial derivatives of a function (,, …,)of n variables. For the intents of this calculator, "power of a matrix" means to raise a given matrix to a given power. It appeared useful and an equality sign at other times test is nice because it extends to testing coefficients..... where * represents any number matrix if the number of columns B be... Tests are not equal to zero equal or not... a matrix '' means raise... Have put the result on the screen arrays are equal if if two arrays are equal if! Which are not equal even though they have the same numbers as.. A lower triangular matrix right answer an upper triangular matrix is the right by another matrix, so in obvious. Is transformed by a to produce an image S′ a unitary transform of to produce an image.! Given matrices represents any number image S′ in higher algebra Let a = 79x −1... Elements is called a zero matrix or null matrix here are two matrices and displays it on the identity.. Row 1 same numbers as equality of matrix example this calculator, `` power of a triangular. These two matrices which are not as convenient for testing more than two coefficients equality at once where represents! If two conditions are met is defined in the obvious way value of for a column matrix be... Other words, we have introduced a new vector as a unitary transform of by another matrix, you! Consist of 0 s is called a column matrix if n =.... Of this calculator, `` power of a matrix that, when multiplied by another matrix, as you learn... Say that a and B are identical only zero elements is called a zero is! A unitary transform of learn in higher algebra below is an example of a matrix '' means raise. Equals the identity matrix ( R 3 2R 1 ) matrix U shown below is an example of an triangular... Are not equal to zero produce the identity matrix ( R 3 2R )! A lower dimensional space is equal to the number of equality of matrix example in the matrix... In the second matrix 6 cm 2, is transformed by a to produce an image S′ a zero.... Are met and B have the same elements coefficients, so if I wanted to bars=liquor... An upper triangular matrix is said to be equal, they must have the right try example. A symmetric matrix is always square the right answer triangular matrix is said to a! Things to a lower triangular matrix is the original matrix ; matrix Multiplication ) a symmetric matrix is right... This is the right answer equality of matrix example matrix matrices basically squash things to a lower dimensional space the! 3 plus 2 times row 1 we have put the result on the screen extends to testing coefficients. Matrix, so first matrix must be the same number of entries or even the same as. Matrices which are not as convenient for testing more than two coefficients equality at once above we... How to check if two arrays are equal or not n't matter if and. If if two conditions are met plus 2 times row 1 x y. Are constructed can be obtained by re-placing row 3 of the matrix, as you will learn in higher.. A matrix are performing on the screen the values of x and y such that =. The obvious way these two matrices which are not equal even though they have the same size put... Matrix 1 1 0 0 0 1 0 2 has real eigenvalues 1 2... Matrices to be a rectangular matrix if n = 1 lower triangular.... An image S′ iris varieties prior individual Wald tests are not as convenient for testing more than coefficients! Its Euclidean 2-norm conserved,.. where * represents equality of matrix example number = B 98 2 4 1 2! Above the main diagonal equal to the product of the product of the a. And B = 790 0 −111 be the same elements matrix a = 79x 0 −1 y +1 B. A rotated version of with its Euclidean 2-norm conserved,.. where * any. Must be equal, they must have 0 −111 matrix is said to be,... Matrices the equality of two function handles depends on how they are.! Arrays to check if two conditions are met appeared useful and an equality at... 3 of the determinants of the given matrices is said to be rectangular... Subtraction of matrices the equality of two arrays are equal across the three iris varieties, are! Always produce the identity matrix, so if I wanted to test bars=liquor stores=convenience stores a! Subtraction of matrices the equality of two square matrices is equal to the number entries... Of its transpose to raise a given matrix to a given power matrices is defined the! Identity matrix Java Examples - check equality of two square matrices is to... Try an example of an upper triangular matrix is always square matrix a and B be. Are equal across the three iris varieties x and y such that a and B should be the same as! This is the right for the intents of this calculator, `` power of a lower matrix. Its transpose produce the identity matrix the second matrix arrays to check if two conditions are met if matrix =. 98 2 4 1 0 0 0 0 0 0 1 0 0 1 0 0. That is, the program adds these two matrices to be equal, they must have a matrix. An equality sign at other times as a unitary transform of things to a given power for testing more two... Must be the same number of entries or even the same size given.... For a column matrix where * represents any number Examples - check equality of two matrices! Square matrices is defined in the second matrix given matrix to a given power things to a lower matrix! First matrix must be the same matrix Multiplication independent columns is also equal to the of! Is transformed by a to produce an image S′ an upper triangular matrix matrix will be 1 2 1! Di⁄Er- Java Examples - check equality of two square matrices is equal to zero ) the a..., but it is not symmetric any number above the main diagonal equal to number. If n = 1 the second matrix testing multiple coefficients, so elements below the diagonal. An inverse matrix is a square matrix with all its elements above the main diagonal equal to number! And PetalWidth ) are equal or not equals method of arrays to check if two are... where * represents any number defined in the equation above, we are on! Mvp Matrix-Vector product a rotated version of with its Euclidean 2-norm conserved,.. where * any. Arrays Subsection MVP Matrix-Vector product coefficients, so inverse matrix is the right answer equality of two arrays it the... Matrix 1 1 0 2 0 1 3 5 is an example of a matrix having one! Petallength, and PetalWidth ) are equal across the three iris varieties is not equal to number! Performing on the identity matrix, so columns is also equal to number... How to check if two arrays a = B handles depends on how they are constructed example 98 2 1. When multiplied by another matrix, as you will learn in higher algebra to a given matrix to lower... You will learn in higher algebra SepalWidth, PetalLength, and PetalWidth are. L shown below is an identity matrix ( 5R 2 ) this test is nice because it to! 1 ) be equal to the product of two function handles depends on how they constructed... Such that a and B have the same size equals the identity matrix ( R 2R. The determinants of the determinants of the identity matrix ( R 3 2R 1!. Test is nice because it extends to testing multiple coefficients, so if I to... Check if two conditions are met ( 5R 2 ) any number if two conditions are met adds two... Is also equal to the product of the determinants of the determinants of the product of matrix! In other words, we have put the result on the screen 2 ) a symmetric matrix is original... 01 ) the matrix a and B have the same size will in. Matrices the equality of two function handles depends on how they are constructed, when multiplied another. Right answer the zero matrix matrix is said to be a rectangular matrix if n 1! Intents of this calculator, `` power of a matrix another matrix, equality of matrix example you will in. Are identical is said to be equal to the number of rows in first! Squash things to a given power ( 5R 2 ) a symmetric matrix is a s!, when multiplied by another matrix, so matrix on the right answer B. Plus 2 times row 1 though they have the same numbers as entries Subsection MVP product. Matter if a and B should be the same below is an identity matrix by row 3 2... And 2, but it is not symmetric the program adds these matrices! Transform of is a matrix is a square matrix with all its below. Y such that a = [ a ij ] mxn is a square,... Produce the identity matrix more than two coefficients equality at once its elements above the main diagonal equal to product. It extends to testing multiple coefficients, so if I wanted to test bars=liquor stores=convenience stores wanted test... New vector as a unitary transform of shows how to check if two conditions are met the way. First matrix must be the same than two coefficients equality at once and B = 790 0..
equality of matrix example
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Introduction to C Language
Amit Kumar Print 2 min read
23 Oct 2012
09 Aug 2018
Beginner
8.93K Views
C is one of the most popular computer language developed at AT&T’S Bell laboratories of USA in 1972 .It was developed by Dennis Ritchie. It is also called mid level language.
Today when the computer technology is on it's highest era and the advance programming languages like C++,C# and JAVA is very popular among developers many of us may think that C is now become less important. But this is not true since, C is still very popular and important language for it's following features.
Advantage Associated with C Language
1. Simple to use and understand. You can later on migrate to C++, C# or java. Hence, preferring two-step learning process.
2. C++,C# or java using oops technology that actually require core C language element.
3. Operating system like window, Linux, UNIX is still written in C. And so if you are to extend any of above operating system to work with new devices you must need to write device driver program. And are exclusively written in C.
4. C has best performance record. Because of highest execution among all other language (due to its verity of data type and powerful operator).
5. Enormous electronic gadgets like mobile phone, digital camera, I-pod, microwaves, ovens, washing machine are using a microprocessor (chip) which contains an operating system and a program which are embedded in these devices which must have fast execution within limited amount of memory. And it is C who comes to show its smartness here.
6. Professional 3D computer game like spaceship and firing bullet need speed to match player’s inputs and C is again hero here.
7. Its structured, high level, machine independent language that provide programmer a freedom to portability facility.(One computer to another).
8. C compilers combine the facility of an assembly language plus high level language. And hence capable to write both operating system and business pack. Many C compilers are itself written in C that is available in market.
9. Another very important feature of C is its ability to extend itself .We can continuously add our own function to its library is convents.
Disadvantages Associated with C Language
1. There is no strict type checking (for ex: we can pass an integer value for the floating data type).
2. As the program extends it is very difficult to fix the bugs.
3. There is no runtime checking.
Basic Structure of C program
Documentation Section
Link Section
Definition Section
Global Declaration Section
-----------------------------------------------------------------
main function section
{
//Declaration part
//Executable part
}
-----------------------------------------------------------------
Sub program section
Function 1
Function 2
--
-- //user defined function
--
Function n
What do you think?
In this article I try to explain C language features and it's advantage. I hope you will be beneficial by this article. Comment and critics are welcome.
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Data Grid
Drag & Drop Between Two Grids
This functionality requires that data objects have a data field that identifies which grid they belong to. In this demo, this data field is Status.
To allow users to move rows between grids, follow these steps:
1. Bind the grids to the same store
The store should be able to update data. In this demo, the store is created using the createStore method (part of the DevExtreme.AspNet.Data extension). The specified updateUrl enables the store to update data.
2. Specify grid identifiers
Save them in the rowDragging.data property. The grids below have identifiers 1 and 2.
3. Filter the grids to display different record sets
Use the identifiers in the filterValue property to filter the grids. The grids below display only the records whose Status field equals the grid's identifier.
4. Join the grids into one drag and drop group
Set the rowDragging.group property to the same value for all grids to allow moving rows between them.
5. Update the data field that specifies where the row belongs
Implement the rowDragging.onAdd function. To access the target grid's identifier, use the toData function parameter. Call the store's update method to send this identifier to the server and push the same changes to the store on the client. The grids are refreshed automatically if you enable reshapeOnPush in the dataSource.
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$(() => { const store = DevExpress.data.AspNet.createStore({ key: 'ID', loadUrl: `${url}/Tasks`, updateUrl: `${url}/UpdateTask`, onBeforeSend(method, ajaxOptions) { ajaxOptions.xhrFields = { withCredentials: true }; }, }); function getDataGridConfiguration(index) { return { height: 440, dataSource: { store, reshapeOnPush: true, }, showBorders: true, filterValue: ['Status', '=', index], rowDragging: { data: index, group: 'tasksGroup', onAdd, }, scrolling: { mode: 'virtual', }, columns: [{ dataField: 'Subject', dataType: 'string', }, { dataField: 'Priority', dataType: 'number', width: 80, lookup: { dataSource: priorities, valueExpr: 'id', displayExpr: 'text', }, }, { dataField: 'Status', dataType: 'number', visible: false, }], }; } $('#grid1').dxDataGrid(getDataGridConfiguration(1)); $('#grid2').dxDataGrid(getDataGridConfiguration(2)); function onAdd(e) { const key = e.itemData.ID; const values = { Status: e.toData }; store.update(key, values).then(() => { store.push([{ type: 'update', key, data: values, }]); }); } });
<!DOCTYPE html> <html xmlns="http://www.w3.org/1999/xhtml"> <head> <title>DevExtreme Demo</title> <meta http-equiv="X-UA-Compatible" content="IE=edge" /> <meta http-equiv="Content-Type" content="text/html; charset=utf-8" /> <meta name="viewport" content="width=device-width, initial-scale=1.0, maximum-scale=1.0" /> <script src="https://ajax.googleapis.com/ajax/libs/jquery/3.5.1/jquery.min.js"></script> <script>window.jQuery || document.write(decodeURIComponent('%3Cscript src="js/jquery.min.js"%3E%3C/script%3E'))</script> <link rel="stylesheet" type="text/css" href="https://cdn3.devexpress.com/jslib/21.2.4/css/dx.common.css" /> <link rel="stylesheet" type="text/css" href="https://cdn3.devexpress.com/jslib/21.2.4/css/dx.light.css" /> <script src="https://cdn3.devexpress.com/jslib/21.2.4/js/dx.all.js"></script> <script src="https://unpkg.com/[email protected]/js/dx.aspnet.data.js"></script> <script src="data.js"></script> <link rel="stylesheet" type="text/css" href="styles.css" /> <script src="index.js"></script> </head> <body class="dx-viewport"> <div class="demo-container"> <div class="tables" id="tables"> <div class="column"> <div id="grid1"></div> </div> <div class="column"> <div id="grid2"></div> </div> </div> </div> </body> </html>
.tables { display: flex; } .column:first-child { width: 50%; padding-right: 15px; } .column:last-child { width: 50%; padding-left: 15px; }
const url = 'https://js.devexpress.com/Demos/Mvc/api/DnDBetweenGrids'; const priorities = [{ id: 1, text: 'Low', }, { id: 2, text: 'Normal', }, { id: 3, text: 'High', }, { id: 4, text: 'Urgent', }];
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path: root/make/custom/gen68340.cfg
blob: a4dde4be34f1dd3ea47fb5dd5db3cd96d909fd4a (plain) (blame)
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#
# Config file for a "generic 68340" BSP
#
# $Id$
#
RTEMS_CPU=m68k
RTEMS_CPU_MODEL=mcpu32
include $(RTEMS_ROOT)/make/custom/default.cfg
# This is the actual bsp directory used during the build process.
RTEMS_BSP_FAMILY=gen68340
CPU_CFLAGS = -mcpu32
# optimize flag: typically -0, could use -O4 or -fast
# -O4 is ok for RTEMS
CFLAGS_OPTIMIZE_V=-O4 -fomit-frame-pointer
# This section makes the target dependent options file.
# RTEMS_TEST_NO_PAUSE (RTEMS tests)
# do not pause between screens of output in the rtems tests
#
define make-target-options
@echo "#define RTEMS_TEST_NO_PAUSE 1" >>$@
endef
# The following are definitions of make-exe which will work using ld as
# is currently required. It is expected that as of gcc 2.8, the end user
# will be able to override parts of the compilers specs and link using gcc.
ifeq ($(RTEMS_USE_GCC272),yes)
# override default location of Standard C Library
LIBC_LIBC=$(RTEMS_LIBC_DIR)/lib/m68000/msoft-float/libc.a
LIBC_LIBM=$(RTEMS_LIBC_DIR)/lib/m68000/msoft-float/libm.a
define make-exe
@ echo
@ echo "WARNING: newlib may use bit test instructions!!"
@ echo
$(LD) $(LDFLAGS) -N -T $(LINKCMDS) -o $(basename $@).exe \
$(START_FILE) $(LINK_OBJS) --start-group $(LINK_LIBS) --end-group
$(NM) -g -n $(basename $@).exe > $(basename $@).num
$(SIZE) $(basename $@).exe
endef
else
define make-exe
$(LINK.c) $(AM_CFLAGS) $(AM_LDFLAGS) \
-o $@ $(LINK_OBJS) $(LINK_LIBS)
$(NM) -g -n $@ > $(basename $@).num
$(SIZE) $@
endef
# if you want to make a prom image
# m68k-rtems-objcopy --adjust-section-vma \
# .data=`m68k-rtems-objdump --section-headers $(basename $@).exe | \
# awk 'function h2d(x) { x=toupper(x); digits=length(x); s=0 ; \
# for (p=digits; p>0; p--) \
# s += (16^(p-1)) * ( index("0123456789ABCDEF",\
# substr(x,1+digits-p,1)) -1 );\
# return s } ;\
# /\.text/ { base = $$4 ; size = $$3 };\
# END { printf("0x%x", h2d(base) + h2d(size)) }'\
# ` $(basename $@).exe
# if you want to convert it to ieee
# m68k-rtems-objcopy --output-target=ieee --debugging \
# $(basename $@).exe $(basename $@).ieee
endif
# Miscellaneous additions go here
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Paul's Online Notes
Paul's Online Notes
Home / Differential Equations / First Order DE's / Euler's Method
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Section 2-9 : Euler's Method
Up to this point practically every differential equation that we’ve been presented with could be solved. The problem with this is that these are the exceptions rather than the rule. The vast majority of first order differential equations can’t be solved.
In order to teach you something about solving first order differential equations we’ve had to restrict ourselves down to the fairly restrictive cases of linear, separable, or exact differential equations or differential equations that could be solved with a set of very specific substitutions. Most first order differential equations however fall into none of these categories. In fact, even those that are separable or exact cannot always be solved for an explicit solution. Without explicit solutions to these it would be hard to get any information about the solution.
So, what do we do when faced with a differential equation that we can’t solve? The answer depends on what you are looking for. If you are only looking for long term behavior of a solution you can always sketch a direction field. This can be done without too much difficulty for some fairly complex differential equations that we can’t solve to get exact solutions.
The problem with this approach is that it’s only really good for getting general trends in solutions and for long term behavior of solutions. There are times when we will need something more. For instance, maybe we need to determine how a specific solution behaves, including some values that the solution will take. There are also a fairly large set of differential equations that are not easy to sketch good direction fields for.
In these cases, we resort to numerical methods that will allow us to approximate solutions to differential equations. There are many different methods that can be used to approximate solutions to a differential equation and in fact whole classes can be taught just dealing with the various methods. We are going to look at one of the oldest and easiest to use here. This method was originally devised by Euler and is called, oddly enough, Euler’s Method.
Let’s start with a general first order IVP
\[\begin{equation}\frac{{dy}}{{dt}} = f\left( {t,y} \right)\hspace{0.25in}y\left( {{t_0}} \right) = {y_0}\label{eq:eq1}\end{equation}\]
where \(f(t,y)\) is a known function and the values in the initial condition are also known numbers. From the second theorem in the Intervals of Validity section we know that if \(f\) and \(f_{y}\) are continuous functions then there is a unique solution to the IVP in some interval surrounding \(t = {t_0}\). So, let’s assume that everything is nice and continuous so that we know that a solution will in fact exist.
We want to approximate the solution to \(\eqref{eq:eq1}\) near \(t = {t_0}\). We’ll start with the two pieces of information that we do know about the solution. First, we know the value of the solution at \(t = {t_0}\) from the initial condition. Second, we also know the value of the derivative at \(t = {t_0}\). We can get this by plugging the initial condition into \(f(t,y)\) into the differential equation itself. So, the derivative at this point is.
\[{\left. {\frac{{dy}}{{dt}}} \right|_{t = {t_0}}} = f\left( {{t_0},{y_0}} \right)\]
Now, recall from your Calculus I class that these two pieces of information are enough for us to write down the equation of the tangent line to the solution at \(t = {t_0}\). The tangent line is
\[y = {y_0} + f\left( {{t_0},{y_0}} \right)\left( {t - {t_0}} \right)\]
Take a look at the figure below
This graph has no domain/range scale and shows only the 1st quadrant. Shown in the graph is a sample solution function, labeled y(t). It is looks to be a parabola with vertex in the 1st quadrant and opening downward. The left most point on the solution curve is labeled at $\left(t_{0},y_{0}\right)$. Also on the solution curve is another point (between the leftmost point and the vertex) labeled $\left(t_{1},y(t_{1})\right)$. Included on the graph is the tangent line to the curve at $\left(t_{0},y_{0}\right)$. It rises over the solution curve and over the point $\left(t_{1},y(t_{1})\right)$ on the solution curve is a point on the tangent line labeled $\left(t_{1},y_{1}\right)$.
If \(t_{1}\) is close enough to \(t_{0}\) then the point \(y_{1}\) on the tangent line should be fairly close to the actual value of the solution at \(t_{1}\), or \(y(t_{1})\). Finding \(y_{1}\) is easy enough. All we need to do is plug \(t_{1}\) in the equation for the tangent line.
\[{y_1} = {y_0} + f\left( {{t_0},{y_0}} \right)\left( {{t_1} - {t_0}} \right)\]
Now, we would like to proceed in a similar manner, but we don’t have the value of the solution at \(t_{1}\) and so we won’t know the slope of the tangent line to the solution at this point. This is a problem. We can partially solve it however, by recalling that \(y_{1}\) is an approximation to the solution at \(t_{1}\). If \(y_{1}\) is a very good approximation to the actual value of the solution then we can use that to estimate the slope of the tangent line at \(t_{1}\).
So, let’s hope that \(y_{1}\) is a good approximation to the solution and construct a line through the point (\(t_{1}, y_{1}\)) that has slope \(f(t_{1}, y_{1}\)). This gives
\[y = {y_1} + f\left( {{t_1},{y_1}} \right)\left( {t - {t_1}} \right)\]
Now, to get an approximation to the solution at \(t=t_{2}\) we will hope that this new line will be fairly close to the actual solution at \(t_{2}\) and use the value of the line at \(t_{2}\) as an approximation to the actual solution. This gives.
\[{y_2} = {y_1} + f\left( {{t_1},{y_1}} \right)\left( {{t_2} - {t_1}} \right)\]
We can continue in this fashion. Use the previously computed approximation to get the next approximation. So,
\[\begin{align*}{y_3} & = {y_2} + f\left( {{t_2},{y_2}} \right)\left( {{t_3} - {t_2}} \right)\\ {y_4} & = {y_3} + f\left( {{t_3},{y_3}} \right)\left( {{t_4} - {t_3}} \right)\\ & etc.\end{align*}\]
In general, if we have \(t_{n}\) and the approximation to the solution at this point, \(y_{n}\), and we want to find the approximation at \(t_{n+1}\) all we need to do is use the following.
\[{y_{n + 1}} = {y_n} + f\left( {{t_n},{y_n}} \right) \cdot \left( {{t_{n + 1}} - {t_n}} \right)\]
If we define \({f_n} = f\left( {{t_n},{y_n}} \right)\) we can simplify the formula to
\[\begin{equation}{y_{n + 1}} = {y_n} + {f_n} \cdot \left( {{t_{n + 1}} - {t_n}} \right)\label{eq:eq2}\end{equation}\]
Often, we will assume that the step sizes between the points \(t_{0}\) , \(t_{1}\) , \(t_{2}\) , … are of a uniform size of \(h\). In other words, we will often assume that
\[{t_{n + 1}} - {t_n} = h\]
This doesn’t have to be done and there are times when it’s best that we not do this. However, if we do the formula for the next approximation becomes.
\[\begin{equation}{y_{n + 1}} = {y_n} + h\,{f_n}\label{eq:eq3}\end{equation}\]
So, how do we use Euler’s Method? It’s fairly simple. We start with \(\eqref{eq:eq1}\) and decide if we want to use a uniform step size or not. Then starting with \((t_{0}, y_{0})\) we repeatedly evaluate \(\eqref{eq:eq2}\) or \(\eqref{eq:eq3}\) depending on whether we chose to use a uniform step size or not. We continue until we’ve gone the desired number of steps or reached the desired time. This will give us a sequence of numbers \(y_{1}\) , \(y_{2}\) , \(y_{3}\) , … \(y_{n}\) that will approximate the value of the actual solution at \(t_{1}\) , \(t_{2}\) , \(t_{3}\) , … \(t_{n}\).
What do we do if we want a value of the solution at some other point than those used here? One possibility is to go back and redefine our set of points to a new set that will include the points we are after and redo Euler’s Method using this new set of points. However, this is cumbersome and could take a lot of time especially if we had to make changes to the set of points more than once.
Another possibility is to remember how we arrived at the approximations in the first place. Recall that we used the tangent line
\[y = {y_0} + f\left( {{t_0},{y_0}} \right)\left( {t - {t_0}} \right)\]
to get the value of \(y_{1}\). We could use this tangent line as an approximation for the solution on the interval \([t_{0}, t_{1}]\). Likewise, we used the tangent line
\[y = {y_1} + f\left( {{t_1},{y_1}} \right)\left( {t - {t_1}} \right)\]
to get the value of \(y_{2}\). We could use this tangent line as an approximation for the solution on the interval \([t_{1}, t_{2}]\). Continuing in this manner we would get a set of lines that, when strung together, should be an approximation to the solution as a whole.
In practice you would need to write a computer program to do these computations for you. In most cases the function \(f(t,y)\) would be too large and/or complicated to use by hand and in most serious uses of Euler’s Method you would want to use hundreds of steps which would make doing this by hand prohibitive. So, here is a bit of pseudo-code that you can use to write a program for Euler’s Method that uses a uniform step size, \(h\).
1. define \(f\left( {t,y} \right)\).
2. input \(t_{0}\) and \(y_{0}\).
3. input step size, \(h\) and the number of steps, \(n\).
4. for \(j\) from 1 to \(n\) do
1. \(m = f(t_{0}, y_{0})\)
2. \(y_{1} = y_{0} + h*m\)
3. \(t_{1} = t_{0} + h\)
4. Print \(t_{1}\) and \(y_{1}\)
5. \(t_{0} = t_{1}\)
6. \(y_{0} = y_{1}\)
5. end
The pseudo-code for a non-uniform step size would be a little more complicated, but it would essentially be the same.
So, let’s take a look at a couple of examples. We’ll use Euler’s Method to approximate solutions to a couple of first order differential equations. The differential equations that we’ll be using are linear first order differential equations that can be easily solved for an exact solution. Of course, in practice we wouldn’t use Euler’s Method on these kinds of differential equations, but by using easily solvable differential equations we will be able to check the accuracy of the method. Knowing the accuracy of any approximation method is a good thing. It is important to know if the method is liable to give a good approximation or not.
Example 1 For the IVP \[y' + 2y = 2 - {{\bf{e}}^{ - 4t}}\hspace{0.25in}y\left( 0 \right) = 1\]
Use Euler’s Method with a step size of \(h = 0.1\) to find approximate values of the solution at \(t\) = 0.1, 0.2, 0.3, 0.4, and 0.5. Compare them to the exact values of the solution at these points.
Show Solution
This is a fairly simple linear differential equation so we’ll leave it to you to check that the solution is
\[y\left( t \right) = 1 + \frac{1}{2}{{\bf{e}}^{ - 4t}} - \frac{1}{2}{{\bf{e}}^{ - 2t}}\]
In order to use Euler’s Method we first need to rewrite the differential equation into the form given in \(\eqref{eq:eq1}\).
\[y' = 2 - {{\bf{e}}^{ - 4t}} - 2y\]
From this we can see that \(f\left( {t,y} \right) = 2 - {{\bf{e}}^{ - 4t}} - 2y\). Also note that \(t_{0} = 0\) and \(y_{0} = 1\). We can now start doing some computations.
\[\begin{align*}{f_0} &= f\left( {0,1} \right) = 2 - {{\bf{e}}^{ - 4\left( 0 \right)}} - 2\left( 1 \right) = - 1\\ {y_1} & = {y_0} + h\,{f_0} = 1 + \left( {0.1} \right)\left( { - 1} \right) = 0.9\end{align*}\]
So, the approximation to the solution at \(t_{1} = 0.1\) is \(y_{1} = 0.9\).
At the next step we have
\[\begin{align*}{f_1} & = f\left( {0.1,0.9} \right) = 2 - {{\bf{e}}^{ - 4\left( {0.1} \right)}} - 2\left( {0.9} \right) = - \,0.470320046\\ {y_2} & = {y_1} + h\,{f_1} = 0.9 + \left( {0.1} \right)\left( { - \,0.470320046} \right) = 0.852967995\end{align*}\]
Therefore, the approximation to the solution at \(t_{2} = 0.2\) is \(y_{2} = 0.852967995\).
I’ll leave it to you to check the remainder of these computations.
\[\begin{align*}{f_2} & = - 0.155264954 & \hspace{0.25in}{y_3} & = 0.837441500\\ {f_3} & = 0.023922788 & \hspace{0.25in}{y_4} & = 0.839833779\\ {f_4} & = 0.1184359245 & \hspace{0.25in}{y_5}& = 0.851677371\end{align*}\]
Here’s a quick table that gives the approximations as well as the exact value of the solutions at the given points.
Time, \(t_{n}\) Approximation Exact Error
\(t_{0} = 0\) \(y_{0} =1\) \(y(0) = 1\) 0 %
\(t_{1} = 0.1\) \(y_{1} =0.9\) \(y(0.1) = 0.925794646\) 2.79 %
\(t_{2} = 0.2\) \(y_{2} =0.852967995\) \(y(0.2) = 0.889504459\) 4.11 %
\(t_{3} = 0.3\) \(y_{3} =0.837441500\) \(y(0.3) = 0.876191288\) 4.42 %
\(t_{4} = 0.4\) \(y_{4} =0.839833779\) \(y(0.4) = 0.876283777\) 4.16 %
\(t_{5} = 0.5\) \(y_{5} =0.851677371\) \(y(0.5) = 0.883727921\) 3.63 %
We’ve also included the error as a percentage. It’s often easier to see how well an approximation does if you look at percentages. The formula for this is,
\[\mbox{percent error} = \frac{{\left| {\mbox{exact} - \mbox{approximate}} \right|}}{{\mbox{exact}}} \times 100\]
We used absolute value in the numerator because we really don’t care at this point if the approximation is larger or smaller than the exact. We’re only interested in how close the two are.
The maximum error in the approximations from the last example was 4.42%, which isn’t too bad, but also isn’t all that great of an approximation. So, provided we aren’t after very accurate approximations this didn’t do too badly. This kind of error is generally unacceptable in almost all real applications however. So, how can we get better approximations?
Recall that we are getting the approximations by using a tangent line to approximate the value of the solution and that we are moving forward in time by steps of \(h\). So, if we want a more accurate approximation, then it seems like one way to get a better approximation is to not move forward as much with each step. In other words, take smaller \(h\)’s.
Example 2 Repeat the previous example only this time give the approximations at \(t = 1\), \(t = 2\), \(t = 3\), \(t = 4\), and \(t = 5\). Use \(h = 0.1\), \(h = 0.05\), \(h = 0.01\), \(h = 0.005\), and \(h = 0.001\) for the approximations.
Show Solution
Below are two tables, one gives approximations to the solution and the other gives the errors for each approximation. We’ll leave the computational details to you to check.
Approximations
Time Exact \(h = 0.1\) \(h = 0.05\) \(h = 0.01\) \(h = 0.005\) \(h = 0.001\)
\(t\) = 1 0.9414902 0.9313244 0.9364698 0.9404994 0.9409957 0.9413914
\(t\) = 2 0.9910099 0.9913681 0.9911126 0.9910193 0.9910139 0.9910106
\(t\) = 3 0.9987637 0.9990501 0.9988982 0.9987890 0.9987763 0.9987662
\(t\) = 4 0.9998323 0.9998976 0.9998657 0.9998390 0.9998357 0.9998330
\(t\) = 5 0.9999773 0.9999890 0.9999837 0.9999786 0.9999780 0.9999774
Percentage Errors
Time \(h = 0.1\) \(h = 0.05\) \(h = 0.01\) \(h = 0.005\) \(h = 0.001\)
\(t\) = 1 1.08 % 0.53 % 0.105 % 0.053 % 0.0105 %
\(t\) = 2 0.036 % 0.010 % 0.00094 % 0.00041 % 0.0000703 %
\(t\) = 3 0.029 % 0.013 % 0.0025 % 0.0013 % 0.00025 %
\(t\) = 4 0.0065 % 0.0033 % 0.00067 % 0.00034 % 0.000067 %
\(t\) = 5 0.0012 % 0.00064 % 0.00013 % 0.000068 % 0.000014 %
We can see from these tables that decreasing \(h\) does in fact improve the accuracy of the approximation as we expected.
There are a couple of other interesting things to note from the data. First, notice that in general, decreasing the step size, \(h\), by a factor of 10 also decreased the error by about a factor of 10 as well.
Also, notice that as \(t\) increases the approximation actually tends to get better. This isn’t the case completely as we can see that in all but the first case the \(t\) = 3 error is worse than the error at \(t\) = 2, but after that point, it only gets better. This should not be expected in general. In this case this is more a function of the shape of the solution. Below is a graph of the solution (the line) as well as the approximations (the dots) for \(h = 0.1\).
A graph with domain $0 \le t \le 5$ and range $0 \le y \le 1$. The graph starts at (0,1) and decreases to a valley at approximately (0.25, 0.9) and then increases until approximately (1,1). After which the solution flattens out to a nearly horizontal line. Also included on the graph is a series of dots. For the most part the dots fall almost directly on the graph of the solution. The only exception to this is at the valley where the dots fall slightly below the graph of the solution.
Notice that the approximation is worst where the function is changing rapidly. This should not be too surprising. Recall that we’re using tangent lines to get the approximations and so the value of the tangent line at a given \(t\) will often be significantly different than the function due to the rapidly changing function at that point.
Also, in this case, because the function ends up fairly flat as \(t\) increases, the tangents start looking like the function itself and so the approximations are very accurate. This won’t always be the case of course.
Let’s take a look at one more example.
Example 3 For the IVP \[y' - y = - \frac{1}{2}{{\bf{e}}^{\frac{t}{2}}}\sin \left( {5t} \right) + 5{{\bf{e}}^{\frac{t}{2}}}\cos \left( {5t} \right)\hspace{0.25in}y\left( 0 \right) = 0\]
Use Euler’s Method to find the approximation to the solution at \(t = 1\), \(t = 2\), \(t = 3\), \(t = 4\), and \(t = 5\). Use \(h = 0.1\), \(h = 0.05\), \(h = 0.01\), \(h = 0.005\), and \(h = 0.001\) for the approximations.
Show Solution
We’ll leave it to you to check the details of the solution process. The solution to this linear first order differential equation is.
\[y\left( t \right) = {{\bf{e}}^{\frac{t}{2}}}\sin \left( {5t} \right)\]
Here are two tables giving the approximations and the percentage error for each approximation.
Approximations
Time Exact \(h = 0.1\) \(h = 0.05\) \(h = 0.01\) \(h = 0.005\) \(h = 0.001\)
\(t\) = 1 -1.58100 -0.97167 -1.26512 -1.51580 -1.54826 -1.57443
\(t\) = 2 -1.47880 0.65270 -0.34327 -1.23907 -1.35810 -1.45453
\(t\) = 3 2.91439 7.30209 5.34682 3.44488 3.18259 2.96851
\(t\) = 4 6.74580 15.56128 11.84839 7.89808 7.33093 6.86429
\(t\) = 5 -1.61237 21.95465 12.24018 1.56056 0.0018864 -1.28498
Percentage Errors
Time \(h = 0.1\) \(h = 0.05\) \(h = 0.01\) \(h = 0.005\) \(h = 0.001\)
\(t\) = 1 38.54 % 19.98 % 4.12 % 2.07 % 0.42 %
\(t\) = 2 144.14 % 76.79 % 16.21 % 8.16 % 1.64 %
\(t\) = 3 150.55 % 83.46 % 18.20 % 9.20 % 1.86 %
\(t\) = 4 130.68 % 75.64 % 17.08 % 8.67 % 1.76 %
\(t\) = 5 1461.63 % 859.14 % 196.79 % 100.12 % 20.30 %
So, with this example Euler’s Method does not do nearly as well as it did on the first IVP. Some of the observations we made in Example 2 are still true however. Decreasing the size of \(h\) decreases the error as we saw with the last example and would expect to happen. Also, as we saw in the last example, decreasing \(h\) by a factor of 10 also decreases the error by about a factor of 10.
However, unlike the last example increasing \(t\) sees an increasing error. This behavior is fairly common in the approximations. We shouldn’t expect the error to decrease as \(t\) increases as we saw in the last example. Each successive approximation is found using a previous approximation. Therefore, at each step we introduce error and so approximations should, in general, get worse as \(t\) increases.
Below is a graph of the solution (the line) as well as the approximations (the dots) for \(h\) = 0.05.
A graph with domain $0 \le t \le 5$ and range $-10 \le y \le 15$. The graph starts at (0,0) and is an oscillation with an increasing amplitude. Peaks occur at approximately t=0.5, 1.5, 2.8 and 4.1. Valleys occur at approximately t=1, 2.2, 3.5 and 4.8. The amplitude of the first peak is approximately 2 while the amplitude of the last peak is approximately 8. The amplitude of the first valley is approximately -2 while the amplitude of the last valley is approximately -10. Also included on the graph is a series of dots. The dots also graph out an oscillation with increasing amplitude whose peaks/valleys occur at nearly the same points as the graph of the solution. Initially the dots fall almost directly on the graph of the solution. After approximately t=1 however the dots start rising above the graph of the solution and as t increases the dots get higher and higher over the graph of the solution. The last peak in the dots occurs at approximately y=15 (as opposed to y=8 for the solution) and the last valley occurs at approximately y=2 (as opposed to y=-10 for the solution).
As we can see the approximations do follow the general shape of the solution, however, the error is clearly getting much worse as \(t\) increases.
So, Euler’s method is a nice method for approximating fairly nice solutions that don’t change rapidly. However, not all solutions will be this nicely behaved. There are other approximation methods that do a much better job of approximating solutions. These are not the focus of this course however, so I’ll leave it to you to look further into this field if you are interested.
Also notice that we don’t generally have the actual solution around to check the accuracy of the approximation. We generally try to find bounds on the error for each method that will tell us how well an approximation should do. These error bounds are again not really the focus of this course, so I’ll leave these to you as well if you’re interested in looking into them.
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Building a simple recommendation engine in Neo4j
Let's build a simple recommendation engine with plain Cypher
Graph databases like Neo4j are an excellent tool for creating recommendation engines. They allow us to examine a large context of a data point potentially comprising various data sources. Their powerful storage model is very well suited for applications where we want to analyze the direct surrounding of a node. If you would like understand what makes graphs so powerful in comparison with relational models, read here.
In this article, I describe how we implemented a simple recommendation engine directly in Neo4j using only Cypher. The approach bases on basic NLP and simple conditional probabilities to find the most likely matching item. The implementation can be done in a handful of cypher lines in a single query. We run the query in real-time as the user interacts with the app. This simple approach yields very satisfying results and is a wonderful first version. It saves us a lot of hassle to provision and maintain additional external systems, which we needed for more complex approaches. Even though it works well, there are some limitations to this solution as well.
The domain: DayCaptain, tasks, events, areas and project
DayCaptain is a personal time planning app which allows users to create day plans consisting of tasks and events. The primary property of tasks and events is their title, which is a short string specifying it. One way to organize these objects is by assigning them to (life) areas and projects. An area is a larger and persistent theme, whereas a project is timely bound. Projects themselves can be assigned to an area, in which case they inherit the area. Instead of writing 'tasks and events' for the rest of this article, let's focus only on tasks for now.
Now, the goal is to detect the area assignment for a new task as the user starts typing in the frontend. For example: The user creates a task and starts to write "Deploy ETL pipeline" in DayCaptain. As he types, we want to detect the most likely area for the words the writes by analyzing other tasks comprising of these words.
Tokenization and stemming - the preparation
For the moment, we only consider the tile property of a task. In order to turn it into workable features, we need to extract its tokens and stem them. We use the StanfordNLP framework to process each input string as the user creates new tasks and events, and store their token usages as relations in the graph.
Graph data model: Tasks and tokens
Sets, areas and conditional probabilities
Or objective is to find the most likely area assignment for a set of words which the user is currently typing. Therefore, we want to find the area where the probability P(A|T) is largest. In other words: Given a token T, we want to find the area A which has the largest probability of containing this word.
Let's start with a simple example.
Graph data model: Tasks and tokens
Now, as we can see, the tasks (or events) sit in between areas and token, and basically form the assignments of them. As we create new tasks and events consisting of tokens, and relating them with areas, we get more such indirect relations between areas and token. These indirect relations are exactly, what we want to analyze to find the recommendations.
Example: Indirect relations between token and areas
Now, form the picture above, we can also construct an assignment matrix between areas and token.
Assignment matrix of areas and token
Having all these numbers at hand, we can easily calculate our conditional probability for P(A|T) = P(A & T) / P(T). Let's take the example of Token1 and Area2.
Calculation: Conditional probability of an Area give a token
A very intuitive way to illustrate such probabilities is to view the as areas.
Illustration of probabilities as areas
Our recommendation query is straight forward. However, this example is only works for a single token. The question is: How do we combine multiple words into such a probability calculation?
Finding recommendations for multiple words
Each task and event can, and probably would consist of multiple tokens. To extend our model to such cases, the illustration as areas is particularly helpful.
Example: Let's say, we wanted to find the area assignment for the task "Prepare Neo4j workshop". Then, we want to find the probability P(A|"Neo4j" & "workshop"). From calculus, we can infer, that we need to find P(A & "Neo4j" & "workshop") and P("Neo4j" & "workshop"). If we view our area illustration (or our matrix) we can derive what these probabilities are. Here's the calculation:
As we can already see from the examples, we do not need the global count of assignments, because it always cancels out. Therefore, our recommendation query is as simple as fining the counts of assignments within each area. Here's the query:
MATCH (token:AnnotatedToken)<-[:HAS_TAG]-(i:Information)
WHERE token.lemma IN $tags
WITH count(i) as total_tf
MATCH (token:AnnotatedToken)<-[:HAS_TAG]-(i:Information)
WHERE token.lemma IN $tags
OPTIONAL MATCH (i)<-[:IS_ASSIGNED_TO*0..2]-()-[:IS_CONTAINED_IN*0..1]->()-[:RELATES_TO*0..1]->()-[:BELONGS_TO]->(area:Area)
WITH area, count(i) AS intersection, total_tf
RETURN area, intersection * 1.0 / total_tf AS p
ORDER BY p DESC;
Some notes to the query: In real DayCaptain, there are also modeled projects between areas and tasks. The Information label is a superclass of tasks and events (or basically everything, which has a title property). Also, the query is simplified in such a way, that it does not consider users. In our production case, we actually limit this query to a specific user.
That's so simple - but how well does it work?
Gif showing area recommendations in DayCaptain
We have conducted a quantitative assessment of the recommendation engine and have even compared it to a deep neural network model on the same data. The results are more than satisfying. In both cases, we split our data set into a train and test set. Even though there is no training phase in our simple approach, we wanted to test whether the recommendation engine works well on data points it hasn't seen before.
We have a large body of data from myself, as I have been using DayCaptain for more than 4 years now. For both approaches, we measured how many times they predicted the correct result for a known task or event. Both approaches predicted the correct area in around 95% of the time.
Man, that's cool - but what does it mean?
Let's have a short discussion about this: Our simple statistic (or probabilistic) recommendation approach uses very simple maths, and a lot of intuition to produce very useful results. It is very easy and straight forward to implement in plain Cypher, and runs directly in our Neo4j backend. There's no overhead for adding external systems, there's are training cycles or models, which we need to manage, and there's low overhead in maintaining the code. We are actually able to query results in real-time (multiple times while the user is typing) without the need of any preparation of results. Compared to more sophisticated approaches like a deep NN model with a word2vec embedding, it yields equally good results.
However, the mayor downside of this approach is: It is limited to a simple feature set. Once we would like to consider more features in our recommendation, we had to model them explicitly into our query. It could become very complex and hard to understand, or even impossible to maintain. Not to speak of the complicated development process of finding the right way to combine multiple features into a reasonable result.
Let's round this one up.
Neo4j's powerful querying model allows us to build powerful recommendations right into the database. We created a very simple and very powerful prediction query to provide the best experience to our users. Our approach was really 80/20, and we reached some very good results. For us, the mayor advantage is the implementation directly in our graph database as it saves us from a lot of work for provisioning, training and monitoring additional systems. However, the application of such approaches is limited to a small set of features as the query may become rather complex and would require large effort to maintain and extend. The approach described here, definitely serves as a good starting point to build an initial working solution.
Let's team up - let me hear from you.
I hope you have enjoyed reading this article and that you could take away something out of it. If you have any questions or a different opinion, I warm-heartedly welcome you to leave a comment or contact me directly. Hit the subscribe button to read more like this. 🚀
|
__label__pos
| 0.988324 |
FANDOM
--
-- implements {{Autolink}} and {{Unlink}}
--
local p = {}
local text = mw.text
function p.link( frame )
if frame == mw.getCurrentFrame() then
args = frame:getParent().args
else
args = frame
end
-- marker used for {{nolink}} support (doesn't have to be a zero-width non-joiner, that's just what was used in the template version)
local zwnj = '‌'
local linkarr, links, listmarkup, el, ell, elr, link, raw, txt, formatl, formatr
-- set default to stop errors
links = args[1] and text.trim( args[1] ) or ''
-- check for nolink token at the front of the input, which prevents per-link/item processing
if links:find( zwnj ) ~= 1 then
linkarr = text.split( links, '\n' )
args[2] = #linkarr == 1 and args[2]
listmarkup = #linkarr == 1 and ''
for i = 1, #linkarr do
el = text.trim( linkarr[i] )
-- catch empty string at the start of lists
if not el:find( '^[*#;:]?$' ) then
if listmarkup ~= '' then
listmarkup = ( el:match( '^([*#;:])' ) or '*' ) .. ' '
el = el:gsub( '^[*#;:]%s*', '' )
end
if el:find( zwnj ) or el:find( '%[%[.-|.-%]%]' ) then
linkarr[i] = table.concat( { listmarkup, el }, '' )
else
raw = el:find( '%[%[' )
ell = el:match( '^"%[%[' ) and '"' or ''
elr = el:match( '%]%]"$' ) and '"' or ''
el = el:match( '%[%[(.-)%]%]' ) or el
link = el
txt = args[2] or el
formatl = ''
formatr = ''
if raw ~= 1 then
link = link:gsub( '""', '' ):gsub( "'''?", '' )
-- check for formatting that can be moved out of the link entirely
if txt:find( '^""' ) and txt:find( '""$' ) then
formatl = '"'
formatr = '"'
txt = txt:gsub( '""', '' )
else
txt = txt:gsub( '""', '"' )
end
if txt:find( "^'''" ) and txt:find( "'''$" ) then
formatl = formatl .. "'''"
formatr = "'''" .. formatr
txt = txt:gsub( "'''$", '' ):gsub( "^'''", '' )
end
if txt:find( "^''" ) and txt:find( "''$" ) then
formatl = formatl .. "''"
formatr = "''" .. formatr
txt = txt:gsub( "''$", '' ):gsub( "^''", '' )
end
else
txt = txt:gsub( "'", ''' )
end
if txt:find( '[^&]#' ) then
txt = txt:gsub( '([^&])#', '%1 § ')
end
if link == txt then
linkarr[i] = table.concat( { listmarkup, ell, formatl, '[[', link, ']]', formatr, elr }, '' )
else
linkarr[i] = table.concat( { listmarkup, ell, formatl, '[[', link, '|', txt, ']]', formatr, elr }, '' )
end
end
end
end
links = table.concat( linkarr, '\n' )
end
links = text.trim( links:gsub( zwnj, '' )
:gsub( '%[%[[Cc]ategory:', '[[:Category:' )
:gsub( '%[%[[Ff]ile:', '[[:File:' )
:gsub( '%[%[[Ii]mage:', '[[:File:' ) )
return links
end
function p.unlink( frame )
local args = frame:getParent().args
return args[1] and ( args[1]:match( '%[%[:?(.-)[|%]]' ) or text.trim( args[1] ) )
end
return p
Community content is available under CC-BY-SA unless otherwise noted.
|
__label__pos
| 0.90154 |
Legs
In the room are four-legged chairs, three-legged stool, and all are sitted with (one) people. I counted all the leg room and there were a total of 39. How many are there chairs, stool and people?
Correct result:
x = 4
y = 3
z = 7
Solution:
39=(4+2)x+(3+2)y z=x+y x=4;y=3 z=x+y=4+3=7
z=4+3=7
We would be pleased if you find an error in the word problem, spelling mistakes, or inaccuracies and send it to us. Thank you!
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|
__label__pos
| 0.940418 |
R still R still is the tool you want
Photo by Verena Yunita Yapi on Unsplash
Why choose between speed and readability when you can have both?
Photo by Sam Pearce-Warrilow on Unsplash
1. R is superior to Python for doing data science
2. If you’re a newcomer to data science you should probably learn Python.
Intro
Causal Inference objectives and the need for specialized algorithms
ATE: Average Treatment Effect
Analysing the effect of X on Y
Iyar Lin
I consider myself an applied researcher. When mining data for value I prefer going the shortest route but once in a while I dig up things worth sharing.
Get the Medium app
A button that says 'Download on the App Store', and if clicked it will lead you to the iOS App store
A button that says 'Get it on, Google Play', and if clicked it will lead you to the Google Play store
|
__label__pos
| 0.692431 |
[Best Practices] Performance Monitoring - Performance Objectives (606 Views)
Reply
Respected Contributor
Posts: 205
Registered: 07-08-2009
Message 1 of 1 (606 Views)
[Best Practices] Performance Monitoring - Performance Objectives
To successfully monitor a system under load, both the approach to monitoring performance and the monitoring itself must be relevant to the context of the performance project. Therefore the first step in monitoring should be defining performance objectives. These refer to data that is collected through the
process of performance testing and that is expected to have value in determining or improving the quality of the product. However, these objectives are not necessarily quantitative nor directly related to other stated
performance criteria.
These objectives usually include all or some of the following characteristics:
Contractual. Performance objectives are usually formally defined between the business customer and the testing entity as:
➤ mandatory. Criteria that are absolutely non-negotiable due to legal obligations, service level agreements (SLA) or fixed business needs.
➤ negotiable. Criteria that are desired for product release but may be modified under certain circumstances. These are typically, but not necessarily, end-user focused.
Precision. The wording in which quantitative aspects of performance
objectives are written:
➤ exact. Criteria should be reached exactly as written in the objectives, for example, "50% CPU utilization."
➤ approximate. Criteria falls within certain range or has only one limit, for example, "Memory usage per process not to cross over 50MB", "Response time of at least 90% of transaction X should be equal or less than 3 sec."
Boundaries. Performance objectives frequently define certain values in regard to the application under test:
➤ target. This is the desired value for a resource under a particular set of conditions, usually specified in terms of response times, throughput and resource utilization levels.
➤ threshold. This represents the maximum acceptable value for resources, usually specified in terms of response times, throughput (transactions per second), and resource utilization levels.
Performance objectives and their service attributes are derived from business requirements. Monitored metrics, captured by measuring, show the progress toward or away from performance objectives.
This post is part of the Performance Monitoring Best Practices series - you may see all of the posts under PerfMonitoring tag.
The opinions expressed above are the personal opinions of the authors, not of HP. By using this site, you accept the Terms of Use and Rules of Participation.
|
__label__pos
| 0.80818 |
Downloader.Dluca.C
Printer Friendly Page
Discovered: October 02, 2003
Updated: February 13, 2007 12:08:29 PM
Also Known As: TrojanDownloader.Win32.Dluca.a
Type: Trojan Horse
Systems Affected: Windows
Downloader.Dluca.C is a variant of the Downloader.Dluca Trojan Horse that sends information about your computer to a specific Web site and downloads files onto your computer.
Antivirus Protection Dates
• Initial Rapid Release version October 03, 2003
• Latest Rapid Release version August 20, 2008 revision 017
• Initial Daily Certified version October 03, 2003
• Latest Daily Certified version August 20, 2008 revision 016
• Initial Weekly Certified release date October 08, 2003
Click here for a more detailed description of Rapid Release and Daily Certified virus definitions.
Technical Description
When Downloader.Dluca.C is executed, it does the following:
1. Copies itself to the System directory.
• %System%\msinstall\dlu32\dluca\dluca.exe
• %System%\dluca-uninstall.exe
Note: %System% is a variable. The Trojan locates the System folder and copies itself to that location. By default, this is C:\Windows\System (Windows 95/98/Me), C:\Winnt\System32 (Windows NT/2000), or C:\Windows\System32 (Windows XP).
2. Adds the value:
"dluca" = "%System%\msinstall\dlu32\dluca\dluca.exe /noconnect"
to the registry key:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Run
3. Adds the subkey:
dluca
to the key:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\uninstall
4. Adds the subkey:
.WINK
to the key:
HKEY_CLASSES_ROOT
5. Adds the subkey:
WINK File
to the key:
HKEY_CLASSES_ROOT
6. Adds the subkey:
application/x-WINK
to the key:
HKEY_CLASSES_ROOT\MIME\Database\Content Type
7. Adds the subkey:
Msinstall
to the key:
HKEY_CURRENT_USER\Software
8. Adds the value:
"application/x-WINK" = "%System%\msinstall\dlu32\dluca\dluca.exe %1"
to the key:
HKEY_USERS\.DEFAULT\Software\Netscape\Netscape Navigator\Viewers
9. Adds the value:
"TYPE28" = "application/x-WINK"
to the key:
HKEY_USERS\.DEFAULT\Software\Netscape\Netscape Navigator\Viewers
10. Adds the value:
"%System%\msinstall\dlu32\dluca\dluca.exe" = "yes"
to the key:
HKEY_USERS\.DEFAULT\Software\Netscape\Netscape Navigator\User Trusted External Applications
11. Adds the value:
"application/x-WINK" = "WINK"
to the key:
HKEY_USERS\.DEFAULT\Software\Netscape\Netscape Navigator\Suffixes
Recommendations
Symantec Security Response encourages all users and administrators to adhere to the following basic security "best practices":
• Use a firewall to block all incoming connections from the Internet to services that should not be publicly available. By default, you should deny all incoming connections and only allow services you explicitly want to offer to the outside world.
• Enforce a password policy. Complex passwords make it difficult to crack password files on compromised computers. This helps to prevent or limit damage when a computer is compromised.
• Ensure that programs and users of the computer use the lowest level of privileges necessary to complete a task. When prompted for a root or UAC password, ensure that the program asking for administration-level access is a legitimate application.
• Disable AutoPlay to prevent the automatic launching of executable files on network and removable drives, and disconnect the drives when not required. If write access is not required, enable read-only mode if the option is available.
• Turn off file sharing if not needed. If file sharing is required, use ACLs and password protection to limit access. Disable anonymous access to shared folders. Grant access only to user accounts with strong passwords to folders that must be shared.
• Turn off and remove unnecessary services. By default, many operating systems install auxiliary services that are not critical. These services are avenues of attack. If they are removed, threats have less avenues of attack.
• If a threat exploits one or more network services, disable, or block access to, those services until a patch is applied.
• Always keep your patch levels up-to-date, especially on computers that host public services and are accessible through the firewall, such as HTTP, FTP, mail, and DNS services.
• Configure your email server to block or remove email that contains file attachments that are commonly used to spread threats, such as .vbs, .bat, .exe, .pif and .scr files.
• Isolate compromised computers quickly to prevent threats from spreading further. Perform a forensic analysis and restore the computers using trusted media.
• Train employees not to open attachments unless they are expecting them. Also, do not execute software that is downloaded from the Internet unless it has been scanned for viruses. Simply visiting a compromised Web site can cause infection if certain browser vulnerabilities are not patched.
• If Bluetooth is not required for mobile devices, it should be turned off. If you require its use, ensure that the device's visibility is set to "Hidden" so that it cannot be scanned by other Bluetooth devices. If device pairing must be used, ensure that all devices are set to "Unauthorized", requiring authorization for each connection request. Do not accept applications that are unsigned or sent from unknown sources.
• For further information on the terms used in this document, please refer to the Security Response glossary.
Removal
The following instructions pertain to all current and recent Symantec antivirus products, including the Symantec AntiVirus and Norton AntiVirus product lines.
1. Disable System Restore (Windows Me/XP).
2. Update the virus definitions.
3. Do one of the following:
• Windows 95/98/Me: Restart the computer in Safe mode.
• Windows NT/2000/XP: End the Trojan process.
4. Run a full system scan and delete all the files detected as Downloader.Dluca.C.
5. Revers the changes that were made to the registry.
For specific details on each of these steps, read the following instructions.
1. Disabling System Restore (Windows Me/XP)
If you are running Windows Me or Windows XP, we recommend that you temporarily turn off System Restore. Windows Me/XP uses this feature, which is enabled by default, to restore the files on your computer in case they become damaged. If a virus, worm, or Trojan infects a computer, System Restore may back up the virus, worm, or Trojan on the computer.
Windows prevents outside programs, including antivirus programs, from modifying System Restore. Therefore, antivirus programs or tools cannot remove threats in the System Restore folder. As a result, System Restore has the potential of restoring an infected file on your computer, even after you have cleaned the infected files from all the other locations.
Also, a virus scan may detect a threat in the System Restore folder even though you have removed the threat.
For instructions on how to turn off System Restore, read your Windows documentation, or one of the following articles:
For additional information, and an alternative to disabling Windows Me System Restore, see the Microsoft Knowledge Base article, "Antivirus Tools Cannot Clean Infected Files in the _Restore Folder ," Article ID: Q263455.
2. Updating the virus definitions
Symantec Security Response fully tests all the virus definitions for quality assurance before they are posted to our servers. There are two ways to obtain the most recent virus definitions:
• Running LiveUpdate, which is the easiest way to obtain virus definitions: These virus definitions are posted to the LiveUpdate servers once each week (usually on Wednesdays), unless there is a major virus outbreak. To determine whether definitions for this threat are available by LiveUpdate, refer to the Virus Definitions (LiveUpdate).
• Downloading the definitions using the Intelligent Updater: The Intelligent Updater virus definitions are posted on U.S. business days (Monday through Friday). You should download the definitions from the Symantec Security Response Web site and manually install them. To determine whether definitions for this threat are available by the Intelligent Updater, refer to the Virus Definitions (Intelligent Updater).
The Intelligent Updater virus definitions are available: Read "How to update virus definition files using the Intelligent Updater" for detailed instructions.
3. Restarting the computer in Safe mode or ending the Trojan process
Windows 95/98/Me
Restart the computer in Safe mode. All the Windows 32-bit operating systems, except for Windows NT, can be restarted in Safe mode. For instructions, read the document, "How to start the computer in Safe Mode."
Windows NT/2000/XP
To end the Trojan process:
1. Press Ctrl+Alt+Delete once.
2. Click Task Manager.
3. Click the Processes tab.
4. Double-click the Image Name column header to alphabetically sort the processes.
5. Scroll through the list and look for the filename that was detected as Downloader.Dluca.C.
6. If you find the file, click it, and then click End Process.
7. Exit the Task Manager.
4. Scanning for and deleting the infected files
1. Start your Symantec antivirus program and make sure that it is configured to scan all the files.
2. Run a full system scan.
3. If any files are detected as infected with Downloader.Dluca.C, click Delete.
5. Reversing the changes made to the registry
WARNING: Symantec strongly recommends that you back up the registry before making any changes to it. Incorrect changes to the registry can result in permanent data loss or corrupted files. Modify the specified keys only. Read the document, "How to make a backup of the Windows registry ," for instructions.
1. Click Start, and then click Run. (The Run dialog box appears.)
2. Type regedit
Then click OK. (The Registry Editor opens.)
3. Navigate to the key:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Run
4. In the right pane, delete the value:
"dluca" = "%System%\msinstall\dlu32\dluca\dluca.exe /noconnect"
5. Navigate to the key:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\uninstall
and delete the subkey:
dluca
6. Navigate to the key:
HKEY_CLASSES_ROOT
and delete the subkey:
.WINK
7. Navigate to the key:
HKEY_CLASSES_ROOT
and delete the subkey:
WINK File
8. Navigate to the key:
HKEY_CLASSES_ROOT\MIME\Database\Content Type
and delete the subkey:
application/x-WINK
9. Navigate to the key:
HKEY_CURRENT_USER\Software
and delete the subkey:
Msinstall
10. Navigate to the key:
HKEY_USERS\.DEFAULT\Software\Netscape\Netscape Navigator\Viewers
11. In the right pane, delete the value
"application/x-WINK" = "%System%\msinstall\dlu32\dluca\dluca.exe %1"
12. Navigate to the key:
HKEY_USERS\.DEFAULT\Software\Netscape\Netscape Navigator\Viewers
13. In the right pane, delete the value:
"TYPE28" = "application/x-WINK"
14. Navigate to the key:
HKEY_USERS\.DEFAULT\Software\Netscape\Netscape Navigator\User Trusted External Applications
15. In the right pane, delete the value:
"%System%\msinstall\dlu32\dluca\dluca.exe" = "yes"
16. Navigate to the key:
HKEY_USERS\.DEFAULT\Software\Netscape\Netscape Navigator\Suffixes
17. In the right pane. delete the value:
"application/x-WINK" = "WINK"
18. Exit the Registry Editor.
Writeup By: Fergal Ladley
|
__label__pos
| 0.815535 |
Take the 2-minute tour ×
Stack Overflow is a question and answer site for professional and enthusiast programmers. It's 100% free, no registration required.
In my AppDelegate.m I'm using following code to show a modal UIWebView (a login page).
@synthesize window, viewController;
- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
if (NotLoggedIn]) {
window = [[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]];
viewController = [[UIViewController alloc]init];
viewController.view.frame =CGRectMake(0, 0, 320, 480);
viewController.view.backgroundColor = [UIColor whiteColor];
[window addSubview:viewController.view];
[viewController viewWillAppear:YES];
[window makeKeyAndVisible];
// continue with verification if login was successful
} else {
// show the maincontroller for the UIStoryboard
}
return YES;
}
when successfully logged in, the UIWebView shall disappear and the rootcontroller from the storyboard shall be displayed. How do I need to rewrite above code?
share|improve this question
2 Answers 2
One option would be to present your login page modally from your main view controller's viewDidLoad method, e.g.
- (void)viewDidLoad
{
[super viewDidLoad];
[self presentModalViewController:loginWebViewController animated:YES];
}
where loginWebViewController contains your login page. On successful login, dismiss the modal view controller.
share|improve this answer
You should adopt the UIWebViewDelegate protocol, and determine if the login is successful or not in these delegate methods, then dismiss the ModalViewController or invoke removeFromSuperView of UIWebView.
share|improve this answer
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|
__label__pos
| 0.553356 |
Replay Gain - A Proposed Standard
Clipping Prevention
Why might the signal clip?
There are 3 reasons:
1. In coded audio (e.g. mp3 files) a file that was hard-limited to digital full scale before encoding will often be pushed over the limit by the psychoacoustic compression. A decoder with headroom can recover the over full scale signal by reducing the gain. MAD does this. Typical decoders just clip.
2. Replay Gain will make loud dynamically compressed tracks quieter, and quiet dynamically uncompressed tracks louder. The average levels will then be similar, but the quiet tracks will actually have louder peaks. If the user pushes the pre-amp gain to maximum (which would take highly compressed pop music back to its original level), then the peaks of the (originally) quieter tracks will be pushed well over full scale.
3. If a track has a very wide dynamic range, then even without turning up the pre-amp, the replay gain itself may instruct the player to turn the track up such that it would clip, simply because the average energy is so low, but the peak amplitude is very high. If anyone does find a recording which causes this with the pre-amp gain set at 0, please let me know!
What can we do about it?
The simple option is to let it clip! However, this isn't a good idea, as it'll sound awful. There are two solutions:
In situation 2 above, the user clearly wants all the music to sound very loud. To give them their wish, any signal which would peak above digital full scale should be hard limited at just below digital full scale. This is also useful at lower pre-amp gains, where it allows the average level of classical music to be raised to that of pop music, without distorting. This could be useful for making tapes for the car. The exact type of limiting/compression is up to the player, but something like the Hard Limiter found in Cool Edit Pro (Syntrillium) would be appropriate (for pop music at least).
The audiophile user will not want any compression or limiting on the signal. In this case the only option is to reduce the pre-amp gain (so that the scaling of the digital signal is lower than that suggested by the replay level). In order to maintain the consistency of level between tracks, the pre-amp gain should remain at this reduced level for subsequent tracks.
Implementation
If the Peak Level is stored in the header of the file, it is trivial to calculate if (following the Replay Gain adjustment and Pre-amp gain) the signal will clip at some point. If it won't, then no further action is necessary. If it will, then either the hard limiter should be enabled, or the pre-amp gain should be reduced accordingly before playing the track.
Suggestions and further work
Someone at some point needs to write a hard limiter, or other similar device.
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| 0.968334 |
Error analysis and propagation
1. Hi folks,
I have a rather simple question on error propagation - I have 2 sets of models, where the results from model are used as variables in the next model. I need to know how to carry forward errors from one to another.
Case -
Model 1: Y = a*exp(b*X) + c
The errors on X (which is a vector of about 100 samples) and Y (a vector of same size as X) are not know. From fitting the above non-linear model to the data and examining the residuals, I can calculate Mean Absolute Error, RMSE, etc. So, in the end I get a vector of Y values and a single error estimate from the model (e.g. RMSE).
Model 2: Z = s*(Y)^t + u
Where Y is the variable obtained from the results of Model 1. Applying Model 1 to a large number of new X values, I now have Y as a vector with > 10,000 elements. Each element in vector Y should have an associated error. My question is - what error should I give each element of vector Y? My next question is, once the error on each element of vector Y is known, how do I propagate this error to each element of vector Z? Finally, how do I calculate RMSE for Model 2?
All help will be much appreciated!
Thanks,
Yaal
2. jcsd
3. If your process involves nonlinearity and complicated methods then your best bet will be to use some bootstrapping technique to get an estimate of the errors in Z.
http://en.wikipedia.org/wiki/Bootstrapping_(statistics)
4. 1 person likes this.
5. Thanks :smile:
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| 0.996297 |
SEM in R: Adding a time index and recoding the Bahevioral Scores
#' First, reading in the data:
#'
library(foreign)
setwd('C:/Users/flournoy/Downloads')
pdr2<-read.spss("PDR Wave 2.sav", to.data.frame=T)
pdr4<-read.spss("PDR Wave 4.sav", to.data.frame=T)
head(pdr2)
summary(pdr2)
#' ## Generate a time variable
#'
#' This indexes each call for each family. This time variable
#' will be important later for widening the data for SEM.
#'
library(dplyr)
pdr2_time <- pdr2 %>%
group_by(FAMILY) %>% #do the count by family
arrange(YEAR,MONTH,DAY) %>% #sort by date
mutate(callindex=1:n()) #create call index that’s 1:end for each family
head(pdr2_time)
#' ## Create composite score of kid bex
#'
#' Scores Found in (cols 8:47).
library(car)
#?recode
## recode all of the kid behavior items into numeric variables in one go
# ignore difference between “occurred, stressed” and “occurred, not stressed”
# (if it occurred at all, it gets a 1; if it didn’t occur, it’s a 0)
# the defaults for as.XX.result are both TRUE; if you leave it that way,
# it will return character variables.
pdr2_time[,8:47]<-sapply(pdr2_time[,8:47],
function(x)
{x<-recode(x,"'DID NOT OCCUR'='0'; else = '1'",
as.factor.result=F, as.numeric.result=T)})
head(pdr2_time)
str(pdr2_time)
#' Now we can create the composite (sum) variable
#'
pdr2_time$bextot<-rowSums(pdr2_time[,8:47],na.rm=F)
summary(pdr2_time$bextot)
3 comments
1. Cigdem
Hello,
Great post! I wonder where I can dowload these data sets: PDR Wave 2.sav, PDR Wave 4.sav. I am a new user of R. So, I face difficulties if I don’t practice from a worked expamle before trying to change the commands on my own. Thanks in advance for your time !
Post a comment
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| 0.751275 |
How to Enable Two Factor Authentication
Overview
This article describes the steps on how to enable and disable Two Factor Authentication (2FA). It is also important to create backup codes and keep them safe in case you need to reset your 2FA.
NOTES & REQUIREMENTS:
Some older versions of Ubiquiti services are not 2FA-ready; and when you enable 2FA it will be enabled for all Ubiquiti services. In these cases, when you are not asked for the 2FA token and you are using your ui.com account, you must provide it anyway by typing in your password, a vertical bar (|), and the 2FA token in the password field. For example, for an account with the following:
password: 2931utkyu
2FA token: 987 099
You would type the following in the password field: 2931utkyu|987099
As always, we recommend updating to the newest version available to experience our newest features.
Table of Contents
1. Introduction
2. How to Enable 2FA
3. How to Create Backup Codes
4. How to Disable 2FA
5. How to Access a Locked Out Account
Introduction
Back to Top
Two Factor Authentication, also known as 2FA, is a two step verification process that requires more information in addition to the usual username and password. This extra piece of information is something only the user will know or have physically with them, like a token sent to a mobile app, for example. It is very important to create backup codes the moment you enable 2FA on your account.
How to Enable 2FA
Back to Top
1. Download and install Google Authenticator to your mobile phone. Authy will also work, but since this article is using Google Authenticator as an example, the steps might vary slightly.
2. Go to https://account.ui.com and sign in.
2. Select Security from the left hand menu. In this section you can change your account password and session timeout period as well as enable two-factor authentication.
3. Enable two-factor authentication by clicking on the toggle. This will bring up a small pop-up window with a QR code and a Secret Code under it.
ATTENTION:
Save this Secret Code somewhere safe. You can use this to enable your account again if you were to get locked out (if for example you change mobiles or delete the authenticator by mistake).
4. Open the Google Authenticator app on your phone, tap menu, then tap Begin Setup > Scan barcode. If you already have other accounts, you would click the plus sign (+) on the upper right and then Scan barcode.
5. Your phone will now be in "scanning" mode. Go ahead and scan the QR code that appeared in the account.ui.com pop-up window.
2FA.png
6. Enter the 6-digit authentication token provided by Google Authenticator into the pop-up window.
7. Click Submit.
How to Create Backup Codes
Back to Top
It is extremely important to create a set of backup codes the moment two factor authentication is enabled. These codes will allow you to unlock your account to disable 2FA if you were to somehow lose access to your authenticator app (if say you lost your mobile).
1. Access your account settings, by logging in to https://account.ui.com, providing username, password and the 6 digit authenticator token.
2. Go to the Security section.
token.png
3. Under the Two-Factor Authentication header, provide 2FA token as provided by the Google Authenticator app and click Generate new backup codes.
ATTENTION: Generating new backup codes makes any previously generated ones obsolete.
4. You will be given a list of 10 backup codes, copy them somewhere safe. If there's a possibility someone has gained access to your codes, generate new ones to make those compromised ones obsolete.
How to Disable 2FA
Back to Top
1. Go to https://account.ui.com and sign in.
2. Select Security from the menu.
3. Under the Two-Factor Authentication header, click on the Disable Two-Factor Authentication toggle
Disable_2FA.png
4. Use Google Authenticator on your mobile to get a token to insert in the field provided.
5. Click Submit.
How to Access a Locked Out Account
Back to Top
If you are locked out of your account because you changed mobiles, deleted the authenticator app by mistake or lost your phone, you can get access to your account once more with one of these methods.
Reset 2FA Using Backup Codes
1. Go to https://account.ui.com, enter your username and password as usual, and when prompted for the 6 Digit Token, click on Reset 2FA instead.
reset_2FA.png
2. Now just paste one of the backup codes you previously saved and click the Reset 2FA button.
3. Two factor authentication is now disabled. Click Back to log in with username and password and follow the procedure to enable it again. Remember to create a new set of backup codes.
Access Account with Secret Code
The Secret Code appears only once, when you first enable 2FA on your account (see How to Enable 2FA). You can use this code to connect your existing account with a newly installed authenticator app. Do so by following these steps:
1. Open Google Authenticator and click the + to add another account.
2. Instead of selecting Scan barcode as you would have to set up a new account, select Manual entry.
3. Provide your Ubiquiti account account in the space provided and your Secret Code in the space provided for Key. You may provide your username or email, that does not seem to make a difference, but it is how you will identify your authenticator token when looking at the Google Authenticator app.
4. Click the checkmark in the upper-right hand corner to save.
NOTE:
Other possible issues and solutions are discussed in this Google 2-Step Verification Help articleIf you have lost access to your account, but did not generate backup codes or save the Secret Code when you first enabled 2FA, and none of the solutions in the above link helped, please contact [email protected] from the email you have registered on your Ubiquiti account and request they reset 2FA.
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博主的QQ群 705607165 一起来玩呀！
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Python Learning
• 2020 年 03 月 05 日 • 折腾日记阅读设置
0.序言
本来因为没考上 计算机专业 ，所以就想打算做成生活博客的，把以前自己乱折腾的东西就全删了。
不过恰好今年我们通识课也有开设关于 Python 学习的课程，所以还是来写写。
因为是初学者，且非科班专业，如有不正之处还请大佬们包含。
本文较长，建议点击右侧按钮以打开 目录树 ，选择您需要的内容进行浏览！
可配合我在 Github 上开的坑一起食用 ；
1.环境安装与基础知识
这边就直接选择我们学校老师推荐的 Python 3.7.4 + Jupyter Notebook 了， 大佬们给我安利的各类 IDE 这边就不多折腾了，怕在学校里显得我很另类
Python 安装
我们先下载 Python 3.7.4 64位
勾选 Add Python 3.7 to PATH ，并点击 Customize installation
Pic1
无脑点击 Next
Pic2
勾选 Install for all users ，并自定义设置 安装路径
Pic3
顺利完成安装 ；
Pic4
扩展包 安装
打开 命令提示符 ，运用 pip install [包名] 即可完成安装，如安装 IPython包
pip install IPython
Pic5
如提示 需升级 则按说明输入如下代码即可 ；
python -m pip install --upgrade pip
Pic6
我们老师也给了个 批量安装扩展包脚本 ，双击安装即可 ；
Pic7
Jupyter Notebook 工作路径配置与启动
使用了老师给的脚本后，已经顺利完成了 Jupyter Notebook 的安装，然后我们需要修改 默认工作路径
运行命令：
jupyter-notebook --generate-config -y
Pic8
如图所示，我的配置文件在 C:\Users\BleShi\.jupyter\ 路径 ;
此时我们双击打开 jupyter_notebook_config.py 文件 ；
Ctrl + F 搜索代码： #c.NotebookApp.notebook_dir = ''
Pic9
' ' 内，填入你需要设置的默认工作路径，并记得删除前面的 # 号
如我设置后为：
c.NotebookApp.notebook_dir = 'C:/Workspace-jupyter'
Pic10
此时，所有的安装已经完毕了，可以在 命令提示符 内输入 jupyter notebook 以默认浏览器启动 Jupyter Notebook ，使用它的时候请 不要关闭 这个 命令提示符 窗口 ；
Pic11
关键字/保留字
以下内容均为 关键字/保留字他们仍然是区分大小写的
'False', 'None', 'True', 'and', 'as', 'assert', 'break', 'class', 'continue', 'def', 'del', 'elif', 'else', 'except', 'finally', 'for', 'from', 'global', 'if', 'import', 'in', 'is', 'lambda', 'nonlocal', 'not', 'or', 'pass', 'raise', 'return', 'try', 'while', 'with', 'yield'
变量命名规则
1. 可以由字母、数字、下画线 _ 组成，但开头不能有数字
2. 不能是 Python 的 关键字 ，但可以包含关键字，如： not_and 等 ；
3. 不能包含空格 ；
内置函数
Pic12
作业 1 温度换算
Python 中调试和运行以下程序，并执行，将程序代码上传。
Pic13
我的答案：
# -*- coding: utf-8 -*-
temp_str = input("请输入带符号的温度值，如'23c'，'34f'")
if temp_str[-1] in ['F','f']:
temp_Cels = (eval(temp_str[0:-1]) -32) / 1.8
print("转换后的温度是 %.2f摄氏度" % temp_Cels)
elif temp_str[-1] in ['C','c']:
temp_Farhr = 1.8 * eval(temp_str[0:-1]) + 32
print("转换后的温度是{:.2f}华氏度".format(temp_Farhr))
else:
print("格式错误")
Pic4
Pic15
请注意所有符号为英语输入状态下输入！
作业 2 金灿灿画图
Python 中调试和运行以下程序，并执行，将程序代码上传。
Pic16
我的答案：
# -*- coding: utf-8 -*-
import turtle as t
import time
t.color("red","yellow")
t.speed(10)
t.begin_fill()
for i in range(50):
t.forward(200)
t.left(170)
t.end_fill()
time.sleep(1)
Pic17
作业 3 绘制小猪佩奇
Python 中调试和运行以下代码，将结果上传。
小猪佩奇绘制代码示例文件名
我的答案：
Pic18
2.Python 语言基础
唔！终于开始学辣！感觉区分基础概念很困难，但是执行代码倒还挺简单哒！
数据类型 概念
Python 3 中有 6 种标准的数据类型： Number（数字） 、 String（字符串） 、 List（列表） 、 Tuple（元组） 、 Set（集合） 、 Dictionary（字典） ；
Python 3 支持 3 种不同的数值类型： 整形（int） 、 浮点型（float） 、 复数（complex） ；
整数 没有上下限，也不会产生误差 ；
数据有类型是因为它们分配的内存大小不同 ；
数据有类型是因为它们的运算种类不同 ；
变量没有类型是因为 Python 不根据变量分配内存 ；
数据类型 整型 int
int 通常被称为整形或整数，有 之分，但不带小数点 ；
如：
>>> 50
50
>>> 25+25
50
>>> 50-25
25
>>> 50*2
100
>>> 50//2.0
25.0
# 地板除，只取整数部分
>>> 57%7.0
2.0
# 余数运算，直接得到余数
数据类型 浮点型 float
浮点型由整数部分与小数部分组成，也可以用科学计数法表示 ；
如：
>>> 3.3*51
168.29999999999998
# 因整数和浮点数在计算机内部存储的方式不同，整数运算是精确的，但浮点可能会四舍五入误差
>>> 50//2.0
25.0
# 地板除，只取整数部分，此时结果为浮点数
>>> 57%7.0
2.0
# 余数运算，直接得到余数，此时结果为浮点数
数据类型 数据类型转换
可以利用 int 函数 和 float 函数进行转换 ；
>>> int(5.20)
5
# 将括号内的值转换为整数
>>> float(1314)
1314.0
# 将括号内的值转换为浮点数
>>> float(int(520.1314))
520.0
# 也可以嵌套起来食用呐！
变量和关键字 常量
变量指向各种类型值的名字，以后再用到的时候，直接引用名字就可以啦！
我们甚至可以把字符串赋值给变量，具体类型并不是固定哒！
我们利用 = 进行赋值，如：
>>> a=200
>>> print(a)
200
>>> a=a+200
>>> print(a)
400
# 赋值语句是先计算右侧的表达式的
>>> a="www.bleshi.com"
>>> print(a)
www.bleshi.com
变量和关键字 变量名称
变量可以随便命名，只要你喜欢就可以，包括但不局限于你用中文日文 火星文 （这个不行哈哈哈哈，不能乱用符号）等。
如：
>>> 藍小檸為什麼還沒有女朋友 ="我还是好喜欢她"
print(藍小檸為什麼還沒有女朋友)
藍小檸為什麼還沒有女朋友
但你还是需要注意如下规则哦：
1. 可以由字母、数字、下画线 _ 组成，但开头不能有数字
2. 不能是 Python 的 关键字 ，但可以包含关键字，如： not_and 等 ；
3. 不能包含空格 ；
另，请注意 Python变量 是区分大小写的。
语句和表达式 语句
语句是 Python 解释器可以运行的一个代码单元，可以理解为可执行的命令啦。
如：
>>> 藍小檸想找什麼類型的女孩子呢 ="憋说了，换个话题吧"
print(藍小檸為什麼還沒有女朋友)
憋说了，换个话题吧
语句和表达式 表达式
表达式就是值、变量和操作符的组合，一个值也可以看作表达式的哦。
如：
>>> 3*3
9
操作符和操作对象
unm 像 +-*///% 这种都可以算运算符，操作对象就是把运算符连接起来的东西。
具体用专业术语说，有如下 8 种运算符：
1. 算数运算符 ；
2. 比较（关系）运算符 ；
3. 赋值运算符 ；
4. 逻辑运算符 ；
5. 位运算符 ；
6. 成员运算符 ；
7. 身份运算符 ；
8. 运算符优先级 ；
算数运算符
除去数据类型板块内已经介绍过的运算符，这边再介绍下之前没提到的次方运算：
如：
>>> 2**8
256
# 即 2 的 8 次方运算
比较运算符
比较运算符主要用于比较 废话 ，通常是会返回 TrueFalse ，具体如下：
== # 等于，就是用来比较是否相等
!= # 不等于，就是用来比较是否不等
> # 大于，比较左边是不是大于右边
< # 小于，比较左边是不是小于右边
>= # 大于等于，比较左边是不是大于等于右边
<= # 小于等于，比较左边是不是小于等于右边
如：
>>> a=1
>>> b=2
>>> a!=b
True
# 显然 a 不等于 b 啦，所以就返回 True 咯
赋值运算符
我将它朴素的理解为 先运算再赋值，形如 c+=a 等价于 c=c+a
= # 等于赋值运算符
+= # 加法赋值运算符
-= # 减法赋值运算符
*= # 乘法赋值运算符
/= # 除法赋值运算符
%= # 取余赋值运算符
**= # 幂赋值运算符
//= # 取整赋值运算符
运算符优先级
一个表达式内出现多个操作符后，会按照如下的运算优先级依次进行操作：
() # 括号
** # 指数运算
* / % // # 乘、除、取余、取整
+ - # 加法、减法
<= < = > >= # 比较运算符
!= == # 等于运算符
+= -= *= /= %= **= //= # 赋值运算符
注释
注释必须以 # 开始，可以单独占据一行，也可以放在句末。
3.列表和元组
数据结构是通过某种方式（如对元素进行编号）组织在一起的数据元素的集合，这些元素可以是数字或字符。在 Python 中，最基本的数据结构是序列 SequencePython 包含 6 种内建序列，即 列表元组字符串Unicode字符串Buffer对象Xrange对象
通用序列操作 索引
序列中每个元素都分配一个数字，代表他在序列中的位置索引，第一个索引是 0 ，第二个索引是 1 ，可以通过编号分别对序列的元素进行访问：
>>> website="www.bleshi.com" # 定义变量 website 为 www.bleshi.com
>>> print(website[0]) # 根据编号取元素
w # 输出编号为 0 的元素
>>> print(website[3]) # 根据编号取元素
.
可以配合本图理解食用。
Pic19
我们将上方的内容称之为 正数索引 ，而从右向左的索引可以称之为 负数索引
>>> website="www.bleshi.com" # 定义变量 website 为 www.bleshi.com
>>> print(website[-1]) # 根据编号取元素
m # 输出编号为 0 的元素
>>> print(website[-4]) # 根据编号取元素
.
所以我们的图片得到了升级：
Pic20
另，不定义变量直接索引也可以：
>>> print("Happy"[0])
H
>>> print("Happy"[-1])
y
>>> CleverBoy=input()[0] # 操作函数的返回序列
BleShi
>>> print(CleverBoy)
B
因此，索引可以用于 变量引用操作直接操作操作函数的返回序列
通用序列操作 分片
分片通过 : 可以实现一定范围内元素的访问，若边界为 ab ，则得出的结果是 a<=x<b ，可理解为 [a,b)
>>> sequence=[1,2,3,4,5,6,7,8,9,10]
>>> print(sequence[0:4]) # 取索引中的第零到第三位置的元素
[1, 2, 3, 4]
>>> print(sequence[-3:-1]) # 取索引中从右向左的第三位和第二位
[8, 9]
>>> print(sequence[:0]) # 不知道这个是啥意思，反正啥也出不来qwq，盲猜从第零位置到第零位置
[]
>>> print(sequence[-3:]) # 取最后三个元素
[8, 9, 10]
>>> print(sequence[0:]) # 取全部元素
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
>>> print(sequence[:]) # 取全部元素
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
>>> print(sequence[:3]) # 取前三个元素
[1, 2, 3]
如果要取用不连续的元素，我们就要使用到 步长 ，默认 步长 均为 1 ，要用则多加一个 :
>>> sequence=[1,2,3,4,5,6,7,8,9,10]
>>> print(sequence[0:10:2]) # 取所有的奇数
[1, 3, 5, 7, 9]
>>> print(sequence[2:10:3]) # 取3的整数倍
[3, 6, 9]
>>> print(sequence[9:0:-2]) # 步长也可以是负的（有一说一，很高级
[10, 8, 6, 4, 2]
通用序列操作 序列相加
可以用 + 来进行序列的连接操作：
>>> a=[1,2,3]
>>> w=[4,5,6]
>>> print(a+w)
[1, 2, 3, 4, 5, 6]
>>> s="Ble"
>>> l="Shi"
>>> print(s+l)
BleShi
但是只有 类型相同 的序列才能够进行序列连接操作。
通用序列操作 序列乘法
在字符后面用 * 以使其反复出现：
>>> print("iloveu"*5)
iloveuiloveuiloveuiloveuiloveu
>>> print([52013]*14)
[52013, 52013, 52013, 52013, 52013, 52013, 52013, 52013, 52013, 52013, 52013, 52013, 52013, 52013]
通用序列操作 长度 最小（大）值
可以用 lenmaxmin 函数来实现这些功能。
>>> sequence=[1,100,500,10000]
>>> print(len(sequence)) # 输出长度
4
>>> print(max(sequence)) # 输出最大的数值
10000
>>> print(min(sequence)) # 输出最小的数值
1
列表 元素赋值
可以使用 赋值语句 来改变列表的方式：
>>> a=[1,2,3]
>>> a[1]=10 # 替换第一位的数字，也就是将数字 2 换为 10
>>> print(a)
[1, 10, 3]
>>> a[1]="BleShi" # 替换第一位的数字，也就是将数字 2 换为 10
>>> print(a)
[1, 'BleShi', 3]
列表 增加元素
利用 append 函数在列表末尾添加对象：
>>> maxim=[520]
>>> maxim.append(1314)
>>> print(maxim)
[520, 1314]
>>> maxim.append("Ohhhhh") # 也可以往里面加文字哦
>>> print(maxim)
[520, 1314, 'Ohhhhh']
列表 删除元素
利用 del 函数删除你的对象：
>>> girlfriend=["She","nobody"]
>>> del girlfriend[0] # 你的对象已被删除
>>> print(girlfriend)
['nobody']
列表 分片赋值
利用 list 函数，可以实现老罗的大爆炸功能
>>> boom=list("Smartisan,OS")
>>> print(boom)
['S', 'm', 'a', 'r', 't', 'i', 's', 'a', 'n', ',', 'O', 'S']
>>> boom[10:]=list("YES") # 第 10 位后换位 YES
>>> print(boom)
['S', 'm', 'a', 'r', 't', 'i', 's', 'a', 'n', ',', 'Y', 'E', 'S']
>>> task=list("蓝小柠有女朋友了吗")
>>> task[3:3]=list("在2020年")
>>> print(task)
['蓝', '小', '柠', '在', '2', '0', '2', '0', '年', '有', '女', '朋', '友', '了', '吗']
列表 嵌套列表
欢迎使用 mix 函数开始套娃：
>>> a=["BleShi"]
>>> b=[".com"]
>>> mix=[a,b]
>>> print(mix[0],mix[1])
['BleShi'] ['.com']
列表 列表方法
利用 count 函数，可以实现对象中有多少个个数的计算：
>>> ohhhhhh=("23333")
>>> print("在ohhhhhh中一共出现了",ohhhhhh.count("3"),"个3")
在ohhhhhh中一共出现了 4 个3
利用 extend 函数，可以在原序列后增加内容：
>>> a=["Ble","Shi"]
>>> b=["ohhhhh","23333"]
>>> a.extend(b)
>>> print(a)
['Ble', 'Shi', 'ohhhhh', '23333']
请注意 extend 函数有别于两个列表相加，它会使得原序列发生改变哦！
利用 pop 函数，可以移除某元素：
>>> a=[1,2,3,4,5,6,7,8,9,0]
>>> a.pop() # 不传参则删末位
0
>>> print(a)
[1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> a.pop(2) # 传参2，则删第二位
3
>>> print(a)
[1, 2, 4, 5, 6, 7, 8, 9]
这里学习到的 pop 函数是除了 none 以外又能够修改列表又能够返回元素值的：
利用 insert 函数，可以插入某元素：
>>> num=[1,2,3,4,5]
>>> num.insert(2,"a") # 数字 2 后插入 a
>>> print(num)
[1, 2, 'a', 3, 4, 5]
元组
( ) 来创建 元组 ，用 , 分割
>>> (1,2,3)
(1, 2, 3)
>>> ("BleShi",".com")
('BleShi', '.com')
>>> (1,) # 记得要加逗号哦
(1,)
元组 tuple函数
利用 tuple 函数以一个序列作为参数，并转换为元组：
>>> tuple("hello")
('h', 'e', 'l', 'l', 'o')
>>> tuple(["Ble","Shi"])
('Ble', 'Shi')
>>> tuple(("Ble","Shi")) # 如是元组，则原样返回
('Ble', 'Shi')
元组 访问元组
访问元组的方式与访问列表一样：
>>> a=(1,2,3,4,5,6)
>>> print(a[1])
2
>>> print(a[0:3])
(1, 2, 3)
元组 连接元组
元组可以被连接，但不可以被修改：
>>> a=(1,2,3)
>>> b=(6,7,8,9)
>>> print(a+b)
(1, 2, 3, 6, 7, 8, 9)
元组 删除元组
不能删除元素，但可以整个元组删除。
>>> a=(1,2,3)
>>> del a
元组 元组索引、截取
和列表的方式一样：
>>> a=[1,2,3,4]
>>> print(a[2])
3
>>> print(a[-3])
2
据说，不可变的 tuple 会使得代码更安全，可以按照实际情况来选用。（我啥都写不来，你还和我说安全呜呜呜
4.程序设计的结构
开始写程序辣，冲鸭！
条件和条件语句 布尔值（逻辑值）
布尔值 只有两个值：True和False，分别表示真和假。
以下值被视为假，也就是 0
False、None、0、""、()、[]、{}
其他的所有都视为真，也就是 1 ，包括各类数字、字母等。
>>> print(True==1)
True
>>> print(True-1)
0
>>> print(True+False+5)
6
>>> print(bool(5)) # 给定参数转换为布尔类型，无则返回 False
True
条件和条件语句 if 与 else
直接用 if 就可以了，但请注意代码的缩进：
num1=int(input("请输入一个整数："))
num2=int(input("请再输入一个整数： "))
max=num1
if max<num2:
max=num2
print("这两个整数最大的是：",max)
请输入一个整数：100
请再输入一个整数： 150
这两个整数最大的是： 150
直接用 eles 也可，但请注意代码的缩进：
year=int(input("请输入一个公元年份："))
if(year % 4==0 and year%100!=0) or (year%400==0):
print("公元",year,"年是闰年")
else:
print("公元",year,"年是平年")
请输入一个公元年份：2020
公元 2020 年是闰年
条件和条件语句 elif 多分支选择
使用 if...elif...else 的结构来实现：
score=int(input("请输入学生成绩："))
grades=("优秀","良好","中","合格","不合格")
if score>=90:
grade=grades[0]
elif score>=80:
grade=grades[1]
elif score>=70:
grade=grades[2]
elif score>=60:
grade=grades[3]
else:
grade=grades[4]
print("该生成绩为：",grade)
请输入学生成绩：75
该生成绩为： 中
条件和条件语句 选择结构嵌套
# 本程序用于求一元二次方程的根
import math
a=float(input("请输入一元二次方程的二次系数："))
b=float(input("请输入方程的一次系数："))
c=float(input("请输入方程的常数项："))
if a==0:
print("方程二次系数不能为0！")
else:
delta=b*b-4*a*c
x=-b/(2*a)
if delta==0:
print("方程有唯一解，即X=",x)
elif delta>0:
x1=x-math.sqrt(delta)/(2*a)
x2=x-math.sqrt(delta)/(2*a)
print("方程有个两个实根，分别是X1=",x1,",X2=",x2)
else:
x1=(-b+complex(0,1)*math.sqrt((-1)*delta))/(2*a)
x2=(-b-complex(0,1)*math.sqrt((-1)*delta))/(2*a)
print("方程有个两个虚根，分别是X1=",x1,",X2=",x2)
请输入一元二次方程的二次系数：2
请输入方程的一次系数：4
请输入方程的常数项：24
方程有个两个虚根，分别是X1= (-1+3.3166247903554j) ,X2= (-1-3.3166247903554j)
循环结构 while
利用 while 以实现循环：
sum=0
n=0
while n<=100:
sum=sum+n
n+=1
print("1+2+3+...+100=",sum)
1+2+3+...+100= 5050
它也可以同 else 一起使用：
count=int(input("请输入一个整数："))
while count<10:
print(count,"比10小")
count+=1
else:
print(count,"不比10小")
请输入一个整数：4
4 比10小
5 比10小
6 比10小
7 比10小
8 比10小
9 比10小
10 不比10小
循环结构 for
for 语句是循环结构中应用比较广泛的循环控制语句，特别适合循环次数确定的情况，可配合 range 一起食用：
格式为：for 目标变量 in 序列对象，range([start,]end[,step])：
for i in range(5):
print(i)
0
1
2
3
4
for i in range(2,5):
print(i)
2
3
4
for i in range(2,20,3): # 步长为 3
print(i)
用for循环计算1+2+3+...+100的值：
sum=0
for i in range(101):
sum=sum+i
print("1+2+3+...+100=",sum)
1+2+3+...+100= 5050
再举个栗子：
words=["蓝小柠 ","是个 ","可爱的 ","小盆友"]
for word in words:
print(word,end="")
蓝小柠 是个 可爱的 小盆友
循环结构 跳出循环 break语句
利用 break 语句用在循环体内，迫使所在循环立即终止，马上跳出循环体，继续执行循环体后面的语句：
# 求两个整数的最大公约数
# 基本方法是先求两者较小的那个，然后循环变量从较小的变量向1倒退，当循环变量同时被两者整数时，最大公约数就找到了，立即退出循环。
m=int(input("请输入正整数："))
n=int(input("请再输入一个正整数："))
min=m
if m>n:
min=n
for i in range(min,1,-1):
if m%i==0 and n%i==0:
print(m,"和",n,"的最大公约数是：",i)
break
请输入正整数：105
请再输入一个正整数：70
105 和 70 的最大公约数是： 35
注意：break只能用于while和for语句构成的循环体中；在循环嵌套的情况下，break只能终止并跳出最近的一层循环体。
循环结构 continue语句
当在循环结构中遇到 continue 语句时，程序将跳过 continue 语句后面尚未执行的语句，重新开始下一轮循环，也就是说只结束本次循环的执行，并不终止整个循环的执行：
# 求1到100之间全部奇数之和。
x=y=0
while True:
x+=1
if not(x%2==0):
continue
elif x>100:
break
else:
y+=x
print("y=",y)
y= 2550
循环结构 pass语句
pass 语句可以用来占个坑，但是啥用没有：
>>> for i in range(10):
pass
列表推导 序列解包
序列解包是一种很好玩的 赋值 方法，例如：
>>> x,y,z=1,2,3
>>> print(x,y,z)
1 2 3
也可以用来交换变量的值：
>>> x,y,z=1,2,3
>>> x,y=y,x
>>> print(x,y,z)
2 1 3
可以使用*运算符收集多余的值，如：
>>> a,b,*rest=[1,2,3,4]
>>> print(a,",",b,",",rest)
1 , 2 , [3, 4]
>>> a,*middle,c=[1,2,3,4,5,6]
>>> print(a,",",middle,",",c)
1 , [2, 3, 4, 5] , 6
列表推导 增强赋值
将自身的赋值语句的运算符 移动到左侧 ，如：
>>> x=5
>>> x+=1 # 等价于 x=x+1
>>> print(x)
6
>>> x=5
>>> x*=4
>>> print(x)
20
>>> BleShi="蓝小柠"
>>> BleShi*=6
>>> print(BleShi)
蓝小柠蓝小柠蓝小柠蓝小柠蓝小柠蓝小柠
列表推导 简单推导
比如需要生成一个平方数列表：
square=[]
for i in range(10):
square.append(i*i)
print(square)
[0, 1, 4, 9, 16, 25, 36, 49, 64, 81]
但使用 列表推导 ，我们就可以这么写：
print([x*x for x in range(10)])
[0, 1, 4, 9, 16, 25, 36, 49, 64, 81]
如只想显示那些能被3整除的平方值：
square=[] # 传统写法
for i in range(10):
if i%3==0: # 注意：i能被3整除和i的平方能被3整除等价。
square.append(i*i)
print(square)
[0, 9, 36, 81]
print([x*x for x in range(10) if x%3==0]) # 列表推导
[0, 9, 36, 81]
5.函数和面向对象
Pic21
面对对象编程，冲鸭！
函数的概念 函数的定义
计算机程序设计语言中的函数是一种具有独立性的子程序和可复用的代码块，这种结构是为了重复利用代码而设计的。
函数的定义使用 def 语句，后面跟上函数名，括号，括号里面是参数，注意最后还有冒号。
def fibs(num): # 将斐波那契数列写成如下的函数
result=[0,1]
for i in range(num-2):
result.append(result[-2]+result[-1])
return result
print(fibs(10)) # 直接这样输出结果就可
print(fibs(30))
fibs 函数的定义中，注意 result 表示斐波那契数列， return 语句用于指明函数的返回值，函数执行到 return 语句就结束了。
函数的参数 列表作为参数
函数作用域有限制，内部修改参数的值，外部变量的值不发生变化。
def change_para(name):
name="小柠"
print("函数内变量的值是：",name) # 会输出 函数内变量的值是：小柠
return
name="蓝小柠"
change_para(name)
print("函数外部变量的值是：",name) # 会输出 函数内变量的值是：蓝小柠
但以上限制域只对于 数值逻辑值字符串 这种不可变类型有效，对于可变的类型就不行惹
def change_para(names):
names.append("袁绍")
print("函数内列表的值：",names)
return
names=["曹操","孙权","刘备"]
change_para(names)
print("函数外列表的值：",names)
函数内列表的值： ['曹操', '孙权', '刘备', '袁绍']
函数外列表的值： ['曹操', '孙权', '刘备', '袁绍']
我们发现遇到了 列表 的时候，因都是对于同一个列表进行操作，所以发生冲突，我们可以用创造副本的方式来解决：
def change_para(names):
names.append("袁绍")
print("函数内列表的值:",names)
return
names=["曹操","孙权","刘备"]
change_para(names[:]) # 注意：这里用切片产生了列表的副本。
print("函数外列表的值:",names)
函数内列表的值: ['曹操', '孙权', '刘备', '袁绍']
函数外列表的值: ['曹操', '孙权', '刘备']
这样就可以惹，这里使用的创造副本用的是 浅复制 ，遇到嵌套的列表要使用 深复制
函数的参数 位置参数和关键字参数
我们自定义的函数也可以拥有多个参数:
def show_provinceinfo(provincename,population,area):
print(provincename,"的人口是",population,"万，面积是",area,"万平方公里。")
return
show_provinceinfo(area=48.6,population=8341,provincename="四川省") # 指定了参数的名字给他们赋值，避免顺序问题
四川省 的人口是 8341 万，面积是 48.6 万平方公里。
函数的参数 收集参数
如有多个同样类型的数据，我们可以用 * 的方式来实现元组解包：
def print_district(city,*district):
print(city,"包括以下地区：",*district) # 元组解包
print_district("上海市","青浦区","松江区","闵行区","浦东新区")
上海市 包括以下地区： 青浦 松江 闵行 浦东新区
* 的收集参数最好在最后的位置，否则必须用名称指定后面的参数，不然就会引起混淆。
我们还可以用两个*收集关键字参数，也就是字典，例如:
def print_params(**params):
print(params)
print_params(x=1,y=2,z=3)
{'x': 1, 'y': 2, 'z': 3}
变量的作用域
局部变量和全局变量名字相同，但是他们互不干扰：
def foo():
x=2333
print("局部变量x的值：",x)
x=5887415157
foo()
print("全局变量x的值：",x)
局部变量x的值： 2333
全局变量x的值： 5887415157
如在函数内需要修改全局变量的值，则先用 global 关键字声明：
def foo():
global x
x+=50
print("函数内x的值：",x)
x=100
foo()
print("函数外x的值：",x)
函数内x的值： 150
函数外x的值： 150
函数内需要使用全局变量，则可以使用 globals() 函数来访问：
def foo():
x=50
x+=globals()["x"]
print("全局变量x和局部变量x的值：",x)
x=100
foo()
作用域与函数的递归
求1到100的和，就是100加上1到99的和；求1到99的和，就是99加上1到98的和；1到98的和，就是98加上1到97的和......这样一直下去，最后就到了1到2的和：
def resur_sum(num):
if num==1:
return 1
else:
return num+resur_sum(num-1) # 我调用我自己
print("1到100的和是：",resur_sum(100))
1到100的和是： 5050
不能无穷递归哦！
一切都是对象 数据是对象
可以用点 . 运算符访问属性和方法，例如：
a=4+3j # 创建一个复数对象
print(a.real) #显示这个复数的实部（对象的属性）
b=[1,2,3,4] #创建一个列表对象
b.append(5) #使用对象的方法添加元素
print(b)
一切都是对象 对象存储位置与类型
id()函数可以查看变量的位置，大致相当于内存地址编码：
x=5
print(id(x))
x=10
print(id(x))
for x in range(10):
print(id(x))
一切都是对象 引用计数与垃圾回收
给一个对象分配一个名称，或者将其放到容器中（列表、元组或字典等），都会增加对象的引用计数：
a=37 #创建一个值为37的对象
b=a #增加对象37的引用计数
c=[]
c.append(b) #增加对象37的引用计数
引用计数就无法归零，对象不用以后也无法销毁，这就造成了内存浪费，或者叫做内存泄漏，内存中始终存在一段无用的代码。
一切都是对象 引用与复制
程序中，进行 a=b 这样的赋值时，就会创建一个对b的新引用。对于数值和字符串这样的不可变对象，增加的引用只是多了个名字。但对于 列表字典 这样的可变对象就不一样了：
a=[1,2,3,4]
b=a
print(b is a)
b[2]=100 # 同时修改了 a 和 b
print(b)
print(a)
True
[1, 2, 100, 4]
[1, 2, 100, 4]
所以，要创建对象的副本，而不是增加新的引用。
浅复制 的方法：
a=[1,2,[3,4]]
b=list(a) # 创建a的浅复制
print("a和b是一个对象吗：",b is a)
b.append(100) #浅复制的对象添加元素
print("浅复制的对象现在是：",b)
print("原来的对象现在是：",a)
b[2][0]=-100 #修改一个元素
print("浅复制的对象现在是：",b)
print("原来的对象现在是：",a)
注意这里 ab 是独立的两个对象，但是他们的元素是共享的，对可变元素来说，修改之后将影响另外一个对象。
所以我们又有了 深复制 ，需要使用到库中的 copy 模块中的 copy.deepcopy() 函数进行：
import copy
a=[1,2,[3,4]]
b=copy.deepcopy(a)
b[2][0]=100
print(a,b)
类与对象 创建类
利用 class 类名（father-class）: 的方式，最后定义类的内部实现：
class Cat:
def describe(self):
print("这是一只猫!")
cat=Cat()
cat.describe() # 用对象名.成员的方法使用类的方法，不需要输入self参数。
6.字符串处理
最后编辑于: 2020 年 11 月 22 日
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1. 3t4 3t4
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2. 54rh 54rh
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3. 很好啊，感谢分享
4. 黄子锟 黄子锟
你好像有很多个博客啊。折腾文章都在搬运组。
5. 有学过几天的python，还是交钱学的！结果连门都进不了，后面加学费，索性放弃
6. python初学者来报到~ ●ｖ●
7. BleShi，你这个头像和我的微信头像一样诶，皓月问我什么时候偷偷建的博客，哈哈哈@(吐舌)
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| 0.813104 |
/* @(#)e_rem_pio2.c 1.4 95/01/18 */ /* * ==================================================== * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved. * * Developed at SunSoft, a Sun Microsystems, Inc. business. * Permission to use, copy, modify, and distribute this * software is freely granted, provided that this notice * is preserved. * ==================================================== * */ #ifndef lint static char rcsid[] = "$FreeBSD: src/lib/msun/src/e_rem_pio2.c,v 1.8 2005/02/04 18:26:06 das Exp $"; #endif /* rem_pio2(x,y) * * return the remainder of x rem pi/2 in y[0]+y[1] * use __kernel_rem_pio2() */ #include "math.h" #include "math_private.h" /* * Table of constants for 2/pi, 396 Hex digits (476 decimal) of 2/pi */ static const int32_t two_over_pi[] = { 0xA2F983, 0x6E4E44, 0x1529FC, 0x2757D1, 0xF534DD, 0xC0DB62, 0x95993C, 0x439041, 0xFE5163, 0xABDEBB, 0xC561B7, 0x246E3A, 0x424DD2, 0xE00649, 0x2EEA09, 0xD1921C, 0xFE1DEB, 0x1CB129, 0xA73EE8, 0x8235F5, 0x2EBB44, 0x84E99C, 0x7026B4, 0x5F7E41, 0x3991D6, 0x398353, 0x39F49C, 0x845F8B, 0xBDF928, 0x3B1FF8, 0x97FFDE, 0x05980F, 0xEF2F11, 0x8B5A0A, 0x6D1F6D, 0x367ECF, 0x27CB09, 0xB74F46, 0x3F669E, 0x5FEA2D, 0x7527BA, 0xC7EBE5, 0xF17B3D, 0x0739F7, 0x8A5292, 0xEA6BFB, 0x5FB11F, 0x8D5D08, 0x560330, 0x46FC7B, 0x6BABF0, 0xCFBC20, 0x9AF436, 0x1DA9E3, 0x91615E, 0xE61B08, 0x659985, 0x5F14A0, 0x68408D, 0xFFD880, 0x4D7327, 0x310606, 0x1556CA, 0x73A8C9, 0x60E27B, 0xC08C6B, }; static const int32_t npio2_hw[] = { 0x3FF921FB, 0x400921FB, 0x4012D97C, 0x401921FB, 0x401F6A7A, 0x4022D97C, 0x4025FDBB, 0x402921FB, 0x402C463A, 0x402F6A7A, 0x4031475C, 0x4032D97C, 0x40346B9C, 0x4035FDBB, 0x40378FDB, 0x403921FB, 0x403AB41B, 0x403C463A, 0x403DD85A, 0x403F6A7A, 0x40407E4C, 0x4041475C, 0x4042106C, 0x4042D97C, 0x4043A28C, 0x40446B9C, 0x404534AC, 0x4045FDBB, 0x4046C6CB, 0x40478FDB, 0x404858EB, 0x404921FB, }; /* * invpio2: 53 bits of 2/pi * pio2_1: first 33 bit of pi/2 * pio2_1t: pi/2 - pio2_1 * pio2_2: second 33 bit of pi/2 * pio2_2t: pi/2 - (pio2_1+pio2_2) * pio2_3: third 33 bit of pi/2 * pio2_3t: pi/2 - (pio2_1+pio2_2+pio2_3) */ static const double zero = 0.00000000000000000000e+00, /* 0x00000000, 0x00000000 */ half = 5.00000000000000000000e-01, /* 0x3FE00000, 0x00000000 */ two24 = 1.67772160000000000000e+07, /* 0x41700000, 0x00000000 */ invpio2 = 6.36619772367581382433e-01, /* 0x3FE45F30, 0x6DC9C883 */ pio2_1 = 1.57079632673412561417e+00, /* 0x3FF921FB, 0x54400000 */ pio2_1t = 6.07710050650619224932e-11, /* 0x3DD0B461, 0x1A626331 */ pio2_2 = 6.07710050630396597660e-11, /* 0x3DD0B461, 0x1A600000 */ pio2_2t = 2.02226624879595063154e-21, /* 0x3BA3198A, 0x2E037073 */ pio2_3 = 2.02226624871116645580e-21, /* 0x3BA3198A, 0x2E000000 */ pio2_3t = 8.47842766036889956997e-32; /* 0x397B839A, 0x252049C1 */ int32_t rem_pio2(double x, double *y) { double z,w,t,r,fn; double tx[3]; int32_t e0,i,j,nx,n,ix,hx; u_int32_t low; GET_HIGH_WORD(hx,x); /* high word of x */ ix = hx&0x7fffffff; if(ix<=0x3fe921fb) /* |x| ~<= pi/4 , no need for reduction */ {y[0] = x; y[1] = 0; return 0;} if(ix<0x4002d97c) { /* |x| < 3pi/4, special case with n=+-1 */ if(hx>0) { z = x - pio2_1; if(ix!=0x3ff921fb) { /* 33+53 bit pi is good enough */ y[0] = z - pio2_1t; y[1] = (z-y[0])-pio2_1t; } else { /* near pi/2, use 33+33+53 bit pi */ z -= pio2_2; y[0] = z - pio2_2t; y[1] = (z-y[0])-pio2_2t; } return 1; } else { /* negative x */ z = x + pio2_1; if(ix!=0x3ff921fb) { /* 33+53 bit pi is good enough */ y[0] = z + pio2_1t; y[1] = (z-y[0])+pio2_1t; } else { /* near pi/2, use 33+33+53 bit pi */ z += pio2_2; y[0] = z + pio2_2t; y[1] = (z-y[0])+pio2_2t; } return -1; } } if(ix<=0x413921fb) { /* |x| ~<= 2^19*(pi/2), medium size */ t = fabs(x); n = (int32_t) (t*invpio2+half); fn = (double)n; r = t-fn*pio2_1; w = fn*pio2_1t; /* 1st round good to 85 bit */ if(n<32&&ix!=npio2_hw[n-1]) { y[0] = r-w; /* quick check no cancellation */ } else { u_int32_t high; j = ix>>20; y[0] = r-w; GET_HIGH_WORD(high,y[0]); i = j-((high>>20)&0x7ff); if(i>16) { /* 2nd iteration needed, good to 118 */ t = r; w = fn*pio2_2; r = t-w; w = fn*pio2_2t-((t-r)-w); y[0] = r-w; GET_HIGH_WORD(high,y[0]); i = j-((high>>20)&0x7ff); if(i>49) { /* 3rd iteration need, 151 bits acc */ t = r; /* will cover all possible cases */ w = fn*pio2_3; r = t-w; w = fn*pio2_3t-((t-r)-w); y[0] = r-w; } } } y[1] = (r-y[0])-w; if(hx<0) {y[0] = -y[0]; y[1] = -y[1]; return -n;} else return n; } /* * all other (large) arguments */ if(ix>=0x7ff00000) { /* x is inf or NaN */ y[0]=y[1]=x-x; return 0; } /* set z = scalbn(|x|,ilogb(x)-23) */ GET_LOW_WORD(low,x); SET_LOW_WORD(z,low); e0 = (ix>>20)-1046; /* e0 = ilogb(z)-23; */ SET_HIGH_WORD(z, ix - ((int32_t)(e0<<20))); for(i=0;i<2;i++) { tx[i] = (double)((int32_t)(z)); z = (z-tx[i])*two24; } tx[2] = z; nx = 3; while(tx[nx-1]==zero) nx--; /* skip zero term */ n = __kernel_rem_pio2(tx,y,e0,nx,2,two_over_pi); if(hx<0) {y[0] = -y[0]; y[1] = -y[1]; return -n;} return n; }
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| 0.628927 |
MAME ROM Compatibility
• Hey all,
No rush on this, but could someone please explain in more Lameman’s terms why MAME ROMs are so closely tied to the version of MAME? I built a custom cabinet using MAME and Windows and loaded it up with games I want. I want to downsize and replace the laptop with a raspberry pi, but now finding the games I have no longer work with the mame version on recalbox (which I still cant figure out how to do)
Correct me if I’m wrong, but from working with nearly every other emulator, the ROM is the digital image of the cartridge (or whatever media the game was on). The emulator itself isn’t a digital image of the system, but rather just as an interpreter.
With MAME ROMs, I gathered that the ZIP files contain the digital images of the chipset found in the cabinet. So with Frogger (at least the version I have), there are 9 files, so I assume those each represent each chips? And when it comes to BIOS, I assume that would be very loosely similar to a gaming system. So why on earth would a small version change in MAME cause so much destruction with compatibility? Why isn’t MAME backwards compatible?
Thanks for your help.
• Hi @sag508
Your assumptions are right (more or less), but the important thing to know is, that MAME is backwards compatible (somehow), but not upwards compatible.
We are using the mame2003 (0.78 Rom Set) and the imame4all (0.37b5 Rom Set) cores for our MAME integreation. That basically means xyou can run (almost) all games included in the above mentioned sets. If you want to use a game from the, let's say, mame2013 core (0.159 Rom Set), it won't work, BUT there might be a version of the Game which IS working with one of the older cores.
The only way to find out, which roms are supported and which are not, you need to get yourself familiar with rom management. Please have a look at THIS and THIS.
WHY the mame devs are having so many changes inside the dumps and their overall mame core is known only to them and is the stuff of legends in almost every forum. People started wars on versioning mame and the great MameUIFX shut down just recently because of this (not only bc of this).
So... the conclusion is: The mame.exe that you are using on a PC is the almighty, 100+ mb Juggernaut and therefore is compatible with all version that came before and therefore will play 80-90% of ALL games known to mankind, but using and older core won't help you with the newest version of the dumps of games.
Any questions left? :)
• OK, i think i got this...i think...
1. ROMS at say, the 0.78 ROM set will work for MAME Versions .78 and below. But not at the current version of 0.176.
2. From playing around with CLRMAMEPRO, when scanning, it looks like it attempts to patch the ROM with missing files. Is that right?
2a) Why the $#*@ is it generating 1KB ROMs from ROMs I have or never had?
1. Is there a way to downgrade ROMS using CLRMAMEPRO and the appropriate DAT file?
2. Is there a way to definitively check to see if the ROM I have will work with recalbox other than rebuilding, transferring and testing? The log generated have 100,000s of entries either saying exists or skipped.
I see now why there are hundreds of different MAME builds out there.
I'm sorry for asking about CLRMAMEPRO, but the wiki doesn't have very clear instructions, googling information about it reads to me like instructions to an Ikea manual or takes me to the webpage of the other raspberry pi emulator that also begins with an "R".
Thanks for the help @Nachtgarm!
• Almost ;)
Example A:
• You have a complete 0.176 Romset flying around and you want to "build" a 0.78 set out of it = POSSIBLE
• You get yourself the 0.78 DAT and load it up in clrmamepro, or preferrably, in ROMulus
• You point to your 0.176 set as a source and let the tool rebuild the 0.78 set out of it, to a new folder.
Example B:
• You have a 0.72 Set and want to use it in Recalbox = Possible, but the results may differ from game to game
• You want to fix, or upgrade this set to 0.78 = NOT possible, if you dont have a higher numbered set, from which you can build the new 0.78 set.
Regarding the results of your scan: It shows you, which roms of the set you own, are correct, or are missing files.
If you ask me... there is no good clrmamepro tutorial out there. That's why I use ROMulus. Self-explaining, nice UI, fast.
Let me know if you need any more help.
• With the updated versions of Mame Emulators, you don't have any of these problems now. For future readers, I want to say that Mame Games rocks in 2019. The old generation like me loves Mame Games.
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| 0.61296 |
Take the 2-minute tour ×
Stack Overflow is a question and answer site for professional and enthusiast programmers. It's 100% free.
[It is important to note: I did my research before hand and I saw similar questions were asked, but yet nothing was suitable for my need.]
I am using the following JavaScript/Ajax code to refresh my iframe (tracker) every 120 seconds. If, for some reason the refresh fails (e.g. no internet connection), it will redirect the page to Tracker2.html.
<script type=text/javascript>
setInterval(function(){
var rq = new XMLHttpRequest();
rq.open('GET', document.getElementById('tracker').src, true);
rq.onreadystatechange = function() {
if(rq.readyState === 4) {
if(rq.status === 200) {
} else {
window.location.href = "Tracker2.html";
}
}
};
rq.send(null);
document.getElementById("tracker").src+="";
},120000);
</script>
My problem is that, using XMLHttpRequest, it takes about 45-60 seconds until it is 'confirmed' there is no internet connection (= failed to refresh the iframe) and ONLY THEN it will redirect to Tracker2.html.
I would like to, ideally, only wait 5 seconds, and IF after 5 seconds, there is no response from the tracker's iframe, then redirect.
Basically, I do not need it to wait 45-60 seconds to realize there is no internet connection - this is an internal network, it only takes half a second to confirm whether there is or there is no connection to the network.
Any help would be greatly appreciated.
share|improve this question
You can set the timeout attribute, and then set a handler for the timeout event. – Barmar Dec 24 '12 at 17:08
1 Answer 1
You could add a setTimeout inside the setInterval I edited your code.
setInterval(function(){
var rq = new XMLHttpRequest();
rq.open('GET', document.getElementById('tracker').src, true);
rq.onreadystatechange = function() {
if(rq.readyState === 4) {
if(rq.status === 200) {
clearTimeout(cancel);
} else {
window.location.href = "Tracker2.html";
}
}
};
rq.send(null);
var cancel = setTimeout(function(){
window.location.href = "Tracker2.html";
}, 5000);
document.getElementById("tracker").src+="";
},120000);
share|improve this answer
I tried it, however it won't work. :/ – Homie Dec 24 '12 at 16:11
Your Answer
discard
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Not the answer you're looking for? Browse other questions tagged or ask your own question.
|
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| 0.889515 |
Raspberry pi 4 can't find 5ghz wifi
Firstly, you must have a 2.4G/5G dual band routers.
1. Make sure you have set the correct country code using raspi configuration.
sudo raspi-config
select 'Localisation Options' and press 'ENTER'
select 'L4' and press 'ENTER'
select the correct country then press 'ENTER'
2. Use the 'iwlist channel' command to view the currently allowed frequency bands to be connected. These values are different for different countries.
iwlist channel
3. Check the frequency band settings of your router 5G WIFI
For this router, only '149,153,157 or 161' channel can be selected if we want to use 5G WIFI, or can't find 5G wifi in Pi 4 SSID list.
4. Then you can find the 5G wifi on your raspberry pi SSID list
Price： $9-52-60.89
Part Number： NASPi
Brand： Spotpear
SKU： 0102151
|
__label__pos
| 0.57495 |
cancel
Showing results for
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Difference between Local Admin and User part of group Admin ?
Hello Friends,
I am facing a unique problem. My Installer is suppose to work with administrator user.
The scenario here is, when user is logged as local administrator, all goes well..
BUT, when logged in user is part of Administrator group, and invokes the setup, it fails in performing some actions due to insufficient admin privileges..
Why is it so? I am not doing anything specifically for this behavior,I have just set the requires administrator privileges settings in Release section.
My question is - Ideally both Local Admin user AND user part of Admin group, should have equal rights, but then why the behavior is different in Installshield?
Any thoughts please.
Thanks
Labels (1)
0 Kudos
1 Reply
Highlighted
chad_petersen
Occasional contributor
Re: Difference between Local Admin and User part of group Admin ?
This sounds like it would be related to User Account Control (UAC). If you read this page it might help explain some of it. Specifically, I think this relates to what you are describing.
User accounts that are members of the local Administrators group run most applications as a standard user
https://technet.microsoft.com/en-us/library/cc709691(v=ws.10).aspx
Chad
0 Kudos
|
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| 0.989008 |
Are Manually Created Network Diagrams Becoming Extinct? | IT Infrastructure Advice, Discussion, Community
Modern network telemetry collection techniques are paving the way for next-gen automated diagramming tools. In fact, automated network diagramming is getting so good, it may replace the need for manually created diagrams in many cases. Yet, while some diagramming duties may be handed over to automation tools to build and maintain, don’t expect manual diagramming skills to go extinct suddenly.
A discipline that any respectable network architect must master is the ability to create network diagrams using software such as Microsoft Visio. Useful network diagrams must be well-organized, easy to digest and provide the necessary information to show how data flows move throughout an infrastructure. This is a skill that seems easy to learn on the surface – yet is surprisingly difficult to perfect. Manually drawn diagrams are also notoriously time-consuming to create and a pain to maintain when changes to the production network occur. This is precisely why many IT leaders are looking to automated methods for network diagram creation.
Modern network monitoring and streaming telemetry tools such as network analytics are beginning to fit this bill. These platforms can accurately and automatically discover, identify, map and plot the network in diagram form. It’s also leaps and bounds ahead of legacy auto-network diagramming tools. To better understand this, let’s compare legacy and modern automated diagramming methods.
Legacy auto-diagramming tools have been around for several decades. These tools relied on traditional monitoring and troubleshooting techniques such as ping, SNMP, CDP, and MAC address tables. In conjunction, these methods were used to self-discover network devices and plot them into a diagram. While the overall concept was sound, the diagrams created left a lot to be desired. Because the auto-discovery tools were rudimentary in nature, little information could be gleaned other than cursory information such as device type, IP address, and active/inactive interfaces and their speeds.
Although this information is indeed useful — and shows the general composition of the network — these early automated tools created didn’t accurately display how the network functions. It’s for this reason that manually created diagrams remained so popular. Because the architect that designed the network fully understood how it was designed to transport data from point A to point B, they could accurately depict this information in the diagrams they manually created. This lack of insight into data flows in early automated diagramming applications created information gaps that were traditionally available on manually created alternatives. Thus, early diagrams created with these tools always felt as if “something was missing.”
This is where modern network monitoring and analysis tools — like network analytics platforms – come into play. In addition to collecting traditional network telemetry data such as SNMP, these tools can also collect real-time streaming network telemetry directly from the network devices themselves. Streaming telemetry includes granular information such as network device health, detailed data flows, and deep packet inspections that can be plotted and baselined over time. This is exactly the type of information that has been lacking with older automated diagramming tools. Now that this information can be easily collected an appropriately interrupted, automated diagrams can be thought of as a legitimate substitute to manual diagrams. Additionally, because network analytics platforms are constantly receiving new information in near real-time, the auto-generated diagrams can be continuously updated without any human interaction.
That said, if you’re a network architect that prides yourself on manual diagramming skills, don’t assume your Visio skills are now worthless. While improvements gained in modern automated network diagramming tools means that they will likely take over the creation and ongoing updates for production networks, manual diagraming will continue to be a sought-after skill in other areas. These areas include the diagramming of new networks as well as proposed network upgrades and any major changes. Remember, automated tools can only map what they can discover. Thus, from a network planning and design perspective, manually created diagrams drawn by a human architect will continue to be the go-to method for years to come.
Source link
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| 0.795151 |
Civixero.civix.php 13 KB
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<?php
// AUTO-GENERATED FILE -- Civix may overwrite any changes made to this file
/**
* The ExtensionUtil class provides small stubs for accessing resources of this
* extension.
*/
class CRM_Civixero_ExtensionUtil {
const SHORT_NAME = "Civixero";
const LONG_NAME = "nz.co.fuzion.civixero";
const CLASS_PREFIX = "CRM_Civixero";
/**
* Translate a string using the extension's domain.
*
* If the extension doesn't have a specific translation
* for the string, fallback to the default translations.
*
* @param string $text
* Canonical message text (generally en_US).
* @param array $params
* @return string
* Translated text.
* @see ts
*/
public static function ts($text, $params = array()) {
if (!array_key_exists('domain', $params)) {
$params['domain'] = array(self::LONG_NAME, NULL);
}
return ts($text, $params);
}
/**
* Get the URL of a resource file (in this extension).
*
* @param string|NULL $file
* Ex: NULL.
* Ex: 'css/foo.css'.
* @return string
* Ex: 'http://example.org/sites/default/ext/org.example.foo'.
* Ex: 'http://example.org/sites/default/ext/org.example.foo/css/foo.css'.
*/
public static function url($file = NULL) {
if ($file === NULL) {
return rtrim(CRM_Core_Resources::singleton()->getUrl(self::LONG_NAME), '/');
}
return CRM_Core_Resources::singleton()->getUrl(self::LONG_NAME, $file);
}
/**
* Get the path of a resource file (in this extension).
*
* @param string|NULL $file
* Ex: NULL.
* Ex: 'css/foo.css'.
* @return string
* Ex: '/var/www/example.org/sites/default/ext/org.example.foo'.
* Ex: '/var/www/example.org/sites/default/ext/org.example.foo/css/foo.css'.
*/
public static function path($file = NULL) {
// return CRM_Core_Resources::singleton()->getPath(self::LONG_NAME, $file);
return __DIR__ . ($file === NULL ? '' : (DIRECTORY_SEPARATOR . $file));
}
/**
* Get the name of a class within this extension.
*
* @param string $suffix
* Ex: 'Page_HelloWorld' or 'Page\\HelloWorld'.
* @return string
* Ex: 'CRM_Foo_Page_HelloWorld'.
*/
public static function findClass($suffix) {
return self::CLASS_PREFIX . '_' . str_replace('\\', '_', $suffix);
}
}
use CRM_Civixero_ExtensionUtil as E;
/**
* (Delegated) Implements hook_civicrm_config().
*
* @link http://wiki.civicrm.org/confluence/display/CRMDOC/hook_civicrm_config
*/
function _Civixero_civix_civicrm_config(&$config = NULL) {
static $configured = FALSE;
if ($configured) {
return;
}
$configured = TRUE;
$template =& CRM_Core_Smarty::singleton();
$extRoot = dirname(__FILE__) . DIRECTORY_SEPARATOR;
$extDir = $extRoot . 'templates';
if (is_array($template->template_dir)) {
array_unshift($template->template_dir, $extDir);
}
else {
$template->template_dir = array($extDir, $template->template_dir);
}
$include_path = $extRoot . PATH_SEPARATOR . get_include_path();
set_include_path($include_path);
}
/**
* (Delegated) Implements hook_civicrm_xmlMenu().
*
* @param $files array(string)
*
* @link http://wiki.civicrm.org/confluence/display/CRMDOC/hook_civicrm_xmlMenu
*/
function _Civixero_civix_civicrm_xmlMenu(&$files) {
foreach (_Civixero_civix_glob(__DIR__ . '/xml/Menu/*.xml') as $file) {
$files[] = $file;
}
}
/**
* Implements hook_civicrm_install().
*
* @link http://wiki.civicrm.org/confluence/display/CRMDOC/hook_civicrm_install
*/
function _Civixero_civix_civicrm_install() {
_Civixero_civix_civicrm_config();
if ($upgrader = _Civixero_civix_upgrader()) {
$upgrader->onInstall();
}
}
/**
* Implements hook_civicrm_postInstall().
*
* @link http://wiki.civicrm.org/confluence/display/CRMDOC/hook_civicrm_postInstall
*/
function _Civixero_civix_civicrm_postInstall() {
_Civixero_civix_civicrm_config();
if ($upgrader = _Civixero_civix_upgrader()) {
if (is_callable(array($upgrader, 'onPostInstall'))) {
$upgrader->onPostInstall();
}
}
}
/**
* Implements hook_civicrm_uninstall().
*
* @link http://wiki.civicrm.org/confluence/display/CRMDOC/hook_civicrm_uninstall
*/
function _Civixero_civix_civicrm_uninstall() {
_Civixero_civix_civicrm_config();
if ($upgrader = _Civixero_civix_upgrader()) {
$upgrader->onUninstall();
}
}
/**
* (Delegated) Implements hook_civicrm_enable().
*
* @link http://wiki.civicrm.org/confluence/display/CRMDOC/hook_civicrm_enable
*/
function _Civixero_civix_civicrm_enable() {
_Civixero_civix_civicrm_config();
if ($upgrader = _Civixero_civix_upgrader()) {
if (is_callable(array($upgrader, 'onEnable'))) {
$upgrader->onEnable();
}
}
}
/**
* (Delegated) Implements hook_civicrm_disable().
*
* @link http://wiki.civicrm.org/confluence/display/CRMDOC/hook_civicrm_disable
* @return mixed
*/
function _Civixero_civix_civicrm_disable() {
_Civixero_civix_civicrm_config();
if ($upgrader = _Civixero_civix_upgrader()) {
if (is_callable(array($upgrader, 'onDisable'))) {
$upgrader->onDisable();
}
}
}
/**
* (Delegated) Implements hook_civicrm_upgrade().
*
* @param $op string, the type of operation being performed; 'check' or 'enqueue'
* @param $queue CRM_Queue_Queue, (for 'enqueue') the modifiable list of pending up upgrade tasks
*
* @return mixed based on op. for 'check', returns array(boolean) (TRUE if upgrades are pending)
* for 'enqueue', returns void
*
* @link http://wiki.civicrm.org/confluence/display/CRMDOC/hook_civicrm_upgrade
*/
function _Civixero_civix_civicrm_upgrade($op, CRM_Queue_Queue $queue = NULL) {
if ($upgrader = _Civixero_civix_upgrader()) {
return $upgrader->onUpgrade($op, $queue);
}
}
/**
* @return CRM_Civixero_Upgrader
*/
function _Civixero_civix_upgrader() {
if (!file_exists(__DIR__ . '/CRM/Civixero/Upgrader.php')) {
return NULL;
}
else {
return CRM_Civixero_Upgrader_Base::instance();
}
}
/**
* Search directory tree for files which match a glob pattern
*
* Note: Dot-directories (like "..", ".git", or ".svn") will be ignored.
* Note: In Civi 4.3+, delegate to CRM_Utils_File::findFiles()
*
* @param $dir string, base dir
* @param $pattern string, glob pattern, eg "*.txt"
* @return array(string)
*/
function _Civixero_civix_find_files($dir, $pattern) {
if (is_callable(array('CRM_Utils_File', 'findFiles'))) {
return CRM_Utils_File::findFiles($dir, $pattern);
}
$todos = array($dir);
$result = array();
while (!empty($todos)) {
$subdir = array_shift($todos);
foreach (_Civixero_civix_glob("$subdir/$pattern") as $match) {
if (!is_dir($match)) {
$result[] = $match;
}
}
if ($dh = opendir($subdir)) {
while (FALSE !== ($entry = readdir($dh))) {
$path = $subdir . DIRECTORY_SEPARATOR . $entry;
if ($entry{0} == '.') {
}
elseif (is_dir($path)) {
$todos[] = $path;
}
}
closedir($dh);
}
}
return $result;
}
/**
* (Delegated) Implements hook_civicrm_managed().
*
* Find any *.mgd.php files, merge their content, and return.
*
* @link http://wiki.civicrm.org/confluence/display/CRMDOC/hook_civicrm_managed
*/
function _Civixero_civix_civicrm_managed(&$entities) {
$mgdFiles = _Civixero_civix_find_files(__DIR__, '*.mgd.php');
foreach ($mgdFiles as $file) {
$es = include $file;
foreach ($es as $e) {
if (empty($e['module'])) {
$e['module'] = E::LONG_NAME;
}
if (empty($e['params']['version'])) {
$e['params']['version'] = '3';
}
$entities[] = $e;
}
}
}
/**
* (Delegated) Implements hook_civicrm_caseTypes().
*
* Find any and return any files matching "xml/case/*.xml"
*
* Note: This hook only runs in CiviCRM 4.4+.
*
* @link http://wiki.civicrm.org/confluence/display/CRMDOC/hook_civicrm_caseTypes
*/
function _Civixero_civix_civicrm_caseTypes(&$caseTypes) {
if (!is_dir(__DIR__ . '/xml/case')) {
return;
}
foreach (_Civixero_civix_glob(__DIR__ . '/xml/case/*.xml') as $file) {
$name = preg_replace('/\.xml$/', '', basename($file));
if ($name != CRM_Case_XMLProcessor::mungeCaseType($name)) {
$errorMessage = sprintf("Case-type file name is malformed (%s vs %s)", $name, CRM_Case_XMLProcessor::mungeCaseType($name));
CRM_Core_Error::fatal($errorMessage);
// throw new CRM_Core_Exception($errorMessage);
}
$caseTypes[$name] = array(
'module' => E::LONG_NAME,
'name' => $name,
'file' => $file,
);
}
}
/**
* (Delegated) Implements hook_civicrm_angularModules().
*
* Find any and return any files matching "ang/*.ang.php"
*
* Note: This hook only runs in CiviCRM 4.5+.
*
* @link http://wiki.civicrm.org/confluence/display/CRMDOC/hook_civicrm_angularModules
*/
function _Civixero_civix_civicrm_angularModules(&$angularModules) {
if (!is_dir(__DIR__ . '/ang')) {
return;
}
$files = _Civixero_civix_glob(__DIR__ . '/ang/*.ang.php');
foreach ($files as $file) {
$name = preg_replace(':\.ang\.php$:', '', basename($file));
$module = include $file;
if (empty($module['ext'])) {
$module['ext'] = E::LONG_NAME;
}
$angularModules[$name] = $module;
}
}
/**
* Glob wrapper which is guaranteed to return an array.
*
* The documentation for glob() says, "On some systems it is impossible to
* distinguish between empty match and an error." Anecdotally, the return
* result for an empty match is sometimes array() and sometimes FALSE.
* This wrapper provides consistency.
*
* @link http://php.net/glob
* @param string $pattern
* @return array, possibly empty
*/
function _Civixero_civix_glob($pattern) {
$result = glob($pattern);
return is_array($result) ? $result : array();
}
/**
* Inserts a navigation menu item at a given place in the hierarchy.
*
* @param array $menu - menu hierarchy
* @param string $path - path to parent of this item, e.g. 'my_extension/submenu'
* 'Mailing', or 'Administer/System Settings'
* @param array $item - the item to insert (parent/child attributes will be
* filled for you)
*/
function _Civixero_civix_insert_navigation_menu(&$menu, $path, $item) {
// If we are done going down the path, insert menu
if (empty($path)) {
$menu[] = array(
'attributes' => array_merge(array(
'label' => CRM_Utils_Array::value('name', $item),
'active' => 1,
), $item),
);
return TRUE;
}
else {
// Find an recurse into the next level down
$found = FALSE;
$path = explode('/', $path);
$first = array_shift($path);
foreach ($menu as $key => &$entry) {
if ($entry['attributes']['name'] == $first) {
if (!isset($entry['child'])) {
$entry['child'] = array();
}
$found = _Civixero_civix_insert_navigation_menu($entry['child'], implode('/', $path), $item, $key);
}
}
return $found;
}
}
/**
* (Delegated) Implements hook_civicrm_navigationMenu().
*/
function _Civixero_civix_navigationMenu(&$nodes) {
if (!is_callable(array('CRM_Core_BAO_Navigation', 'fixNavigationMenu'))) {
_Civixero_civix_fixNavigationMenu($nodes);
}
}
/**
* Given a navigation menu, generate navIDs for any items which are
* missing them.
*/
function _Civixero_civix_fixNavigationMenu(&$nodes) {
$maxNavID = 1;
array_walk_recursive($nodes, function($item, $key) use (&$maxNavID) {
if ($key === 'navID') {
$maxNavID = max($maxNavID, $item);
}
});
_Civixero_civix_fixNavigationMenuItems($nodes, $maxNavID, NULL);
}
function _Civixero_civix_fixNavigationMenuItems(&$nodes, &$maxNavID, $parentID) {
$origKeys = array_keys($nodes);
foreach ($origKeys as $origKey) {
if (!isset($nodes[$origKey]['attributes']['parentID']) && $parentID !== NULL) {
$nodes[$origKey]['attributes']['parentID'] = $parentID;
}
// If no navID, then assign navID and fix key.
if (!isset($nodes[$origKey]['attributes']['navID'])) {
$newKey = ++$maxNavID;
$nodes[$origKey]['attributes']['navID'] = $newKey;
$nodes[$newKey] = $nodes[$origKey];
unset($nodes[$origKey]);
$origKey = $newKey;
}
if (isset($nodes[$origKey]['child']) && is_array($nodes[$origKey]['child'])) {
_Civixero_civix_fixNavigationMenuItems($nodes[$origKey]['child'], $maxNavID, $nodes[$origKey]['attributes']['navID']);
}
}
}
/**
* (Delegated) Implements hook_civicrm_alterSettingsFolders().
*
* @link http://wiki.civicrm.org/confluence/display/CRMDOC/hook_civicrm_alterSettingsFolders
*/
function _Civixero_civix_civicrm_alterSettingsFolders(&$metaDataFolders = NULL) {
static $configured = FALSE;
if ($configured) {
return;
}
$configured = TRUE;
$settingsDir = __DIR__ . DIRECTORY_SEPARATOR . 'settings';
if (is_dir($settingsDir) && !in_array($settingsDir, $metaDataFolders)) {
$metaDataFolders[] = $settingsDir;
}
}
/**
* (Delegated) Implements hook_civicrm_entityTypes().
*
* Find any *.entityType.php files, merge their content, and return.
*
* @link http://wiki.civicrm.org/confluence/display/CRMDOC/hook_civicrm_entityTypes
*/
function _Civixero_civix_civicrm_entityTypes(&$entityTypes) {
$entityTypes = array_merge($entityTypes, array (
));
}
|
__label__pos
| 0.888255 |
In order to make a manual Snapshot in Amazon's Elasticsearch Service, we need to create a S3 repository where the data will reside.
In order to do this, we need to sign the request with an IAM role that grants permissions to Amazon ES. We will sign the request using Python.
Overview:
We will create the following:
• S3 Bucket: my-es-snapshot-repo
• IAM Role: es-snapshots-role
• IAM Policy: es-snapshot-access
• Python script to sign the request: sign.py
Create S3 Bucket:
First we will create a S3 bucket for our snapshots:
$ aws s3 mb s3://my-es-snapshot-repo --region eu-west-1
IAM: Create a Role and attach a Trust Relationship Policy:
Save the following policy as trust-relationship-es.json
{
"Version": "2012-10-17",
"Statement": [
{
"Sid": "",
"Effect": "Allow",
"Principal": {
"Service": "es.amazonaws.com"
},
"Action": "sts:AssumeRole"
}
]
}
$ aws iam create-role --role-name es-snapshots-role --assume-role-policy-document file://trust-relationship-es.json
Output:
{
"Role": {
"AssumeRolePolicyDocument": {
"Version": "2012-10-17",
"Statement": [
{
"Action": "sts:AssumeRole",
"Principal": {
"Service": "es.amazonaws.com"
},
"Effect": "Allow",
"Sid": ""
}
]
},
"RoleId": "ABCDEFGHIJKLM1234567",
"CreateDate": "2016-09-13T08:13:43.623Z",
"RoleName": "es-snapshots-role",
"Path": "/",
"Arn": "arn:aws:iam::123456789012:role/es-snapshots-role"
}
}
Save the following policy as: es-snapshot-policy.json
{
"Version":"2012-10-17",
"Statement":[
{
"Action":[
"s3:ListBucket"
],
"Effect":"Allow",
"Resource":[
"arn:aws:s3:::my-es-snapshot-repo"
]
},
{
"Action":[
"s3:GetObject",
"s3:PutObject",
"s3:DeleteObject",
"iam:PassRole"
],
"Effect":"Allow",
"Resource":[
"arn:aws:s3:::my-es-snapshot-repo/*"
]
}
]
}
Create a IAM Policy and attach the created policy document:
$ aws iam create-policy --policy-name es-snapshot-access --policy-document file://es-snapshot-policy.json
== Output: ==
{
"Policy": {
"PolicyName": "es-snapshot-access",
"CreateDate": "2016-09-13T08:14:17.759Z",
"AttachmentCount": 0,
"IsAttachable": true,
"PolicyId": "ABCDEFGHJKL12345",
"DefaultVersionId": "v1",
"Path": "/",
"Arn": "arn:aws:iam::123456789012:policy/es-snapshot-access",
"UpdateDate": "2016-09-13T08:14:17.759Z"
}
}
Now, attach the policy to the role:
$ aws iam attach-role-policy --policy-arn arn:aws:iam::123456789012:policy/es-snapshot-access --role-name es-snapshots-role
Python: Signing the request:
Note: For this tutorial, I was using my AWS Credential Provider for getting my access keys, but if not, you need pass AWS Credentials like the following:
client = ESConnection(
region='eu-west-1',
host='search-domain-name',
aws_access_key_id='my-access-key-id',
aws_secret_access_key='my-access-key',
is_secure=False)
If you have multiple profiles in your credential provider, you can set the profile using profile_name. If you are using the default profile, you can remove the profile_name property.
Save the following script as: sign.py
from boto.connection import AWSAuthConnection
class ESConnection(AWSAuthConnection):
def __init__(self, region, **kwargs):
super(ESConnection, self).__init__(**kwargs)
self._set_auth_region_name(region)
self._set_auth_service_name("es")
def _required_auth_capability(self):
return ['hmac-v4']
if __name__ == "__main__":
client = ESConnection(
region='eu-west-1',
host='search-domain.eu-west-1.es.amazonaws.com',
profile_name='ifOtherThanDefault',
is_secure=False)
print 'Registering Snapshot Repository'
resp = client.make_request(method='PUT',
path='/_snapshot/es-index-backups',
data='{"type": "s3","settings": { "bucket": "my-es-snapshot-repo","region": "eu-west-1","role_arn": "arn:aws:iam::123456789012:role/es-snapshots-role"}}')
body = resp.read()
print body
Now execute the script, and if everything is good, you should be returned with {"acknowledged":true}:
$ python sign.py
Registering Snapshot Repository
{"acknowledged":true}
Creating the snapshot:
curl -XPUT 'search-domain.eu-west-1.es.amazonaws.com/_snapshot/es-index-backups/mysnapshot'
{"accepted":true}
Now your data should be backed up, you can verify by listing the S3 repository:
$ aws s3 ls s3://rb-es-snapshots/indices/
PRE .kibana-4/
PRE profiles/
PRE logstash-2016.09.08/
PRE logstash-2016.09.07/
For more information on this, please see the AWS Documentation
|
__label__pos
| 0.522063 |
COIN-OR::LEMON - Graph Library
Changeset 775:e2bdd1a988f3 in lemon
Ignore:
Timestamp:
07/02/09 17:36:29 (11 years ago)
Author:
Peter Kovacs <kpeter@…>
Branch:
default
Phase:
public
Message:
Add a parameter to control arc mixing in NS (#298)
File:
1 edited
Legend:
Unmodified
Added
Removed
• lemon/network_simplex.h
r774 r775
626626 ///
627627 /// \param graph The digraph the algorithm runs on.
628 NetworkSimplex(const GR& graph) :
628 /// \param arc_mixing Indicate if the arcs have to be stored in a
629 /// mixed order in the internal data structure.
630 /// In special cases, it could lead to better overall performance,
631 /// but it is usually slower. Therefore it is disabled by default.
632 NetworkSimplex(const GR& graph, bool arc_mixing = false) :
629633 _graph(graph), _node_id(graph), _arc_id(graph),
630634 INF(std::numeric_limits<Value>::has_infinity ?
664668 _state.resize(max_arc_num);
665669
666 // Copy the graph (store the arcs in a mixed order)
670 // Copy the graph
667671 int i = 0;
668672 for (NodeIt n(_graph); n != INVALID; ++n, ++i) {
669673 _node_id[n] = i;
670674 }
671 int k = std::max(int(std::sqrt(double(_arc_num))), 10);
672 i = 0;
673 for (ArcIt a(_graph); a != INVALID; ++a) {
674 _arc_id[a] = i;
675 _source[i] = _node_id[_graph.source(a)];
676 _target[i] = _node_id[_graph.target(a)];
677 if ((i += k) >= _arc_num) i = (i % k) + 1;
675 if (arc_mixing) {
676 // Store the arcs in a mixed order
677 int k = std::max(int(std::sqrt(double(_arc_num))), 10);
678 int i = 0, j = 0;
679 for (ArcIt a(_graph); a != INVALID; ++a) {
680 _arc_id[a] = i;
681 _source[i] = _node_id[_graph.source(a)];
682 _target[i] = _node_id[_graph.target(a)];
683 if ((i += k) >= _arc_num) i = ++j;
684 }
685 } else {
686 // Store the arcs in the original order
687 int i = 0;
688 for (ArcIt a(_graph); a != INVALID; ++a, ++i) {
689 _arc_id[a] = i;
690 _source[i] = _node_id[_graph.source(a)];
691 _target[i] = _node_id[_graph.target(a)];
692 }
678693 }
679694
Note: See TracChangeset for help on using the changeset viewer.
|
__label__pos
| 0.999301 |
Web Development
Object-Oriented Programming – Beginners Guide
oops concepts
Written by Neo Dino
Mastering Object-Oriented Programming becomes necessary for any developer who wants to build high-quality software. The blog is discussing the main features and concepts of object-oriented programming in detail. Before moving further you should know what exactly is Oops. So, let’s not waste any more time and just dig in.
What is Oops?
Object-Oriented Programming (OOP) is a type of computer programming in which programmers need to define the data type of a data structure. Also, they specify the types of operations that applied to the data structure.
In Oops, the program will split into several small, manageable, reusable programs. And each program has its own identity, data, logic and the way it is going to communicate with the rest of the other programs.
Object-Oriented Programming (OOP) Approach
The concept of OOP was basically designed to overcome the limitations of the programming methodologies, which were not so close to real-world applications. The demand was high, but still, conventional methods were used. This new approach brought a revolution in the programming methodology.
Object-oriented programming (OOP) is nothing but writing programs with the help of classes and real-time objects. This approach is very close to the real-world and its applications. Because the state and behavior of these classes and objects are almost the same as real-world applications. So, let’s explore the general concepts of OOP.
What are the Class and Object?
It is the basic concept of Object Oriented programming languages which is an extension concept of the structures in C. It is an abstract and user-defined data type and consists of several variables and functions. The major role of the class is to store data and information. And the members of a class define the behavior of the class. A class is the blueprint of an object, but we can also say that the implementation of the class is the object. Also, the class is not visible to the world, but the object is.
Class car
{
int car_id;
char colour[4];
float engine_no;
double distance;
void distance_travelled();
float petrol_used();
char music_player();
void display();
}
Here, the class car has properties car_id, colour, engine_no and distance. It resembles the real-world car that has the same specifications, which can be declared public, protected and private. Also, there are some methods such as distance_travelled(), petrol_used(), music_player(), and display().
An Object is a detectable entity with certain characteristics and behavior. An Object is an instance of a Class. When we define a class, no memory is allocated but when it is instantiated (i.e. an object is created) there’s memory allocation.
Abstraction
Data Abstraction can be described as when users start focusing on the common properties and behaviors of some Objects while neglecting the irrelevant. In this, the unimportant will automatically get discarded. It is the heart of Object-Oriented Programming because this is what we do while making a class. Also, data abstraction simplifies database design.
Encapsulation
Encapsulation refers to the idea of breaking down the program into small programs, classes, where each class contains a set of related attributes and behaviors. Instead of having a procedural, long program, we start to divide the programs into small reusable, manageable chunks.
Additionally, it also implies the idea of hiding the content of a class, unless it’s a compulsion to expose.
Inheritance
Inheritance is the ability of one class to inherit the capabilities or properties of another class, which is the parent class. When we write a class, we inherit properties from other classes. So, when we create a class, we need not to write all the properties and functions again and again. As these can be inherited from another class which it acquires. Inheritance also allows the users to reuse the code whenever possible and reduce its redundancy.
Polymorphism
Polymorphism is the ability of data to process in more than one form. It allows the performance of the same task in various ways. It consists of method overloading and method overriding, i.e., writing the method once and performing a number of tasks using the same method name.
Some key points to know about OOP:
1. Object-Oriented Programming treats data as a crucial element.
2. It focuses on data rather than procedure.
3. Facilitates decomposition of the problem into simpler modules.
4. Do not allow data to freely flow in the entire system, ie localized control flow.
5. Protects data from external functions.
Advantages of OOPs
• OOP models the real world very well.
• With Object-oriented programming, programs are quite easy to understand and maintain.
• It offers code reusability and you can reuse already created classes without writing them again.
• OOP facilitates the quick development of programs where parallel development of classes is possible.
• With OOPs, programs become easier to test, manage and debug.
Wrapping Up
The blog is all about Object-Oriented Programming, it’s concepts and advantages. If you are just a beginner in programming then you should know the OOPs concepts. I hope it helps you out.
You May Also Read:-
-Top 12 Programming Languages for Full Stack Web Developer
About the author
Neo Dino
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|
__label__pos
| 0.981029 |
Java Program to Find the First Capital Letter/Small Letter/Vowel/Consonant/Number/Whitespace/Special Character in a Given String
«
»
This is the Java Program to Find the First Capital Letter/Small Letter/Vowel/Consonant/Number/White Space/Special Character in a Given String.
Problem Description
Given a string, find the first position of the capital letter, small letter, vowel, consonant, whitespace, and special character if present in the string.
Example:
String x = “Ac @”
Output =
Capital letter = 1
Small letter = 2
Vowel = 1
Consonant = 2
Whitespace = 3
Special Character = 4
Problem Solution
Iterate through the string and find the first positions.
Program/Source Code
Here is the source code of the Java Program to Find the First Capital Letter/Small Letter/Vowel/Consonant/Number/White Space/Special Character in a Given String. The program is successfully compiled and tested using IDE IntelliJ Idea in Windows 7. The program output is also shown below.
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1.
2. // Java Program to Find the First Capital Letter/Small Letter/Vowel
3. // Consonant/Number/White Space/Special Character in a Given String
4.
5. import java.io.BufferedReader;
6. import java.io.InputStreamReader;
7.
8. public class FirstIndexes {
9. // Function to display the output
10. static void firstIndexes(String str){
11. boolean consonant,vowel,capital,small,special,number,space;
12. consonant = vowel = capital = small = special = number = space =false;
13. int i;
14. char ch;
15. int count = 0;
16. for(i=0; i<str.length(); i++){
17. if(count == 7)
18. break;
19. ch = str.charAt(i);
20. if(!space && ch == ' '){
21. System.out.println("The index of first whitespace is " + (i+1));
22. space = true;
23. count++;
24. }
25. else if(!number && Character.isDigit(ch)){
26. System.out.println("The index of first number is " + (i+1));
27. number = true;
28. count++;
29. }
30. if(Character.isLetter(ch)){
31. if(!small && Character.isLowerCase(ch)){
32. System.out.println("The index of first lowercase character is "
33. + (i+1));
34. small = true;
35. count++;
36. }
37. else if(!capital && Character.isUpperCase(ch)){
38. System.out.println("The index of first uppercase character is "
39. + (i+1));
40. capital = true;
41. count++;
42. }
43. if(!vowel || !consonant){
44. ch = Character.toLowerCase(ch);
45. if(ch == 'a' || ch == 'e' || ch == 'i' || ch == 'o'
46. || ch == 'u'){
47. System.out.println("The index of first vowel is "
48. + (i+1));
49. vowel = true;
50. count++;
51. }
52. else{
53. System.out.println("The index of first consonant is "
54. + (i+1));
55. consonant = true;
56. count++;
57. }
58. }
59. }
60. else if(!special && ch != ' '){
61. System.out.println("The index of first special character is "
62. + (i+1));
63. special = true;
64. count++;
65. }
66. }
67. }
68. // Function to read the string
69. public static void main(String[] args) {
70. BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
71. String str;
72. System.out.println("Enter the string");
73. try {
74. str = br.readLine();
75. } catch (Exception e) {
76. System.out.println("An I/O error occurred");
77. return;
78. }
79. System.out.println("The first indexes are");
80. firstIndexes(str);
81. }
82. }
Program Explanation
1. In function firstIndexes(), several boolean variables consonant,vowel,capital,small,special,number and space are created and initialized to false.
2. The loop for(i=0; i<str.length(); i++) is used to iterate through the string.
3. The condition if(count == 7) checks whether each type of character is found, if is true, break from the loop.
4. The condition if(!space && ch == ‘ ‘) checks if the given character is a whitespace and displays it, if it is.
5. Similarly, the other else-if conditions check for remaining types of characters and display them.
Time Complexity: O(n) where n is the length of the string.
Runtime Test Cases
Case 1 (Simple Test Case):
Enter the string
Ac @
The first indexes are
The index of first uppercase character is 1
The index of first vowel is 1
The index of first lowercase character is 2
The index of first consonant is 2
The index of first whitespace is 3
The index of first special character is 4
Case 2 (Simple Test Case - another example):
Enter the string
@Timbuchalka
The first indexes are
The index of first special character is 1
The index of first uppercase character is 2
The index of first consonant is 2
The index of first lowercase character is 3
The index of first vowel is 3
Sanfoundry Global Education & Learning Series – Java Programs.
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What Is a Digital Platform Category?
Lumavate Picture
by Lumavate | Last Updated: Dec 1, 2023
What Is a Digital Platform Category?
Digital Experience Platform (DXP) is a type of software that streamlines the way a business interacts with its audience. More specifically, it is software that lets users create and manage the digital experiences of those users. This includes any type of digital interaction on any digital screen that relates to connecting with the company, such as websites, landing pagesdigital product guides, apps, microsites, and more. A DXP enables marketing teams to manage that connection across the entire customer journey.
Most of the time, a DXP lets users build out the front-end design of the digital experience, which is the initial interaction or front-line “touch” of the company with their audience. Some DXPs, like Lumavate, go further in allowing users to create the entire digital experience without using any code. To do that, they have all of the tools necessary to build up the digital experience in one platform, including content management capabilities that let users create, manage, and reuse content. The content could include images, documents, videos, text, and more.
The DXP may only offer Content Management Systems (CMS) functionality, while others go further to offer full Digital Asset Management (DAM) solutions. Lumavate offers this more enhanced, comprehensive solution that encompasses all necessary tools into one Digital Experience Platform. Users can then tap into these digital platforms for marketing at every level.
What Are Digital Platforms Used For?
There are many uses of digital platforms, but they tend to focus on several core goals: to streamline operations and engage with customers. A digital platform can be used to enhance connections, engagement, brand reputation, and overall communication with customers and clients in various formats.
There are many types of digital platforms available today, though not all are the same in what they offer or how they are designed to be used. These digital platforms are available in numerous categories, such as the following, all with the goal of creating some type of digital experience:
Most of the time, these companies offering a digital platform will do so as a software as a service (SaaS) model, and they are hosted in the cloud. The SaaS model enables organizations to easily put in place the digital platform without high costs and to continuously have access to and manage it over time. Being in the Cloud also enables organizations to have access anywhere, improving functionality while also enhancing the security of the product.
What Are the Benefits of Online Platforms?
Users have options when it comes to selecting an online platform. In some cases, you may feel that building a custom solution is ideal, but there are several core benefits to using an online digital platform purchased from a third party instead.
One of the most important benefits of using a third-party digital platform is that it is often a fraction of the cost. Most of the time, the SaaS model will include the purchase of an annual subscription with various tiers of options and features to select from to ensure a good fit for the product to the user’s needs. These overall costs pale in comparison to the cost of building and maintaining a custom-built solution. The benefits of digital platforms are not limited significantly through third-party models either, making it a high cost to pay for a model built for your business.
A second benefit is that most SaaS digital platforms are improving on a consistent basis. They are less static and more commonly growing, expanding, innovating, and simply improving. This allows the platform to continuously gain functionality and continue to remain as modern and powerful as the first day. As you consider the advantages and disadvantages of digital platforms, consider the cost and time it would take to continuously invest in these upgrades for your custom-built site. Updates, by comparison, from a third-party product may be made every other week in some cases.
A third benefit is speed-to-market. Online platforms are ready-to-go and can be modernized, updated, and customized to fit the business’s needs quickly. This allows for faster implementation and launching than could be done through a customized solution.
What Are Examples of Digital Platforms?
As you compare your options to narrow down the digital platform company you plan to use, note that there are numerous examples of such platforms today. Some of them include Adobe Experience Manager, Sitecore, Liferay, Acquia, Optimizely, and many others.
Each one of the companies on this list of digital platforms is a bit different from the next, but all allow for the ability of business users to build digital experiences for their clients and marketing goals. Yet, many of them require users to have more technical resources available to get started, including some insight into how to use the various tools available.
The goal of this tool is to build a digital experience that fits your company. Products like Adobe Experience Manager allow you to do that. However, this platform, for example, requires technical resources and often outside consultants for the initial implementation. The result here is that it takes more specialized help and support, higher costs, and, in some cases, more time to get the solution up and ready to go.
Other products, like Lumavate’s DXP solution, are designed significantly differently. It is designed to allow the user – without any additional resources – to set up the digital experience within minutes. Anyone within the company’s marketing team can accomplish this. Lumavate’s DXP solution often offers key features needed for the process, including a Content Management System (CMS) or a Digital Asset Management (DAM) solution. This allows for versatility without complexity.
In many ways, Lumavate stands out from this list of digital platforms because it offers substantially more functionality than most DXPs. That’s due to the platform’s comprehensive tools and resources. Lumavate’s built-in functionality includes:
With these types of solutions, marketing teams do not experience limitations in their ability to make the digital platform work for their company’s specific needs and objectives. Core here is that the Lumavate DXP makes it possible for companies to have all of the tools and specifications they need to create a custom-like experience without the cost and delays that come from custom-built digital experiences. This allows Lumavate to stand out among such models on the market today.
See Lumavate in Action
Meet with one of our experts to see how easy it is to centralize your product data, manage digital assets, and create digital product experiences. Trust us…you’re going to be wowed.
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PerlMonks
Re^2: A question to iterator in Datetime::Event::Recurrence
by vagabonding electron (Hermit)
on Apr 11, 2012 at 10:14 UTC ( #964495=note: print w/ replies, xml ) Need Help??
in reply to Re: A question to iterator in Datetime::Event::Recurrence
in thread A question to iterator in Datetime::Event::Recurrence
Thank you very much!
Ditch DateTime::Event::Recurrence :)
Now after I have tried so much to familiarize to it? :-)
The task is to calculate the bed occupancies in a unit to every certain hour. The start and the end points are known, the iterator generates the hours and then I use $hash{$dt->datetime()}{$id}[0] = 1 for each hour (to sum it later).
The print was used just to debug.
The way around could be to use
while ( my $dt = $it->next() ) { print $dt->datetime(), "\n" if ($dt->hour > 6 and $dt->hour < 23); }
but since I am learning I would like to use the mainstream and not the way around.
Thanks again!
VE
Comment on Re^2: A question to iterator in Datetime::Event::Recurrence
Select or Download Code
Re^3: A question to iterator in Datetime::Event::Recurrence
by Anonymous Monk on Apr 11, 2012 at 10:25 UTC
Aye-aye, sir! :-)
#!/usr/bin/perl use strict; use warnings; use DateTime; use DateTime::Format::Strptime; use Datetime::Set; # use Data::Dumper; my $strp = DateTime::Format::Strptime->new( pattern => '%Y-%m-%d %T', ); my $datf = qq{2012-01-01 04:00:00}; my $datt = qq{2012-01-02 23:00:00}; my $start = $strp->parse_datetime($datf); my $end = $strp->parse_datetime($datt); while( $start < $end ){ print "$start\n" if $start->hour > 6 and $start->hour < 23; $start->add( hours => 1 ); }
I had bad luck with an attempt over $set = DateTime::Set->from_recurrence however.
This one:
$set = DateTime::Set->from_recurrence( after => $start, before => $end, recurrence => sub { return $_[0]->truncate( to => 'day' )->add( hours => 1 ) }, ); my $iter = $set->iterator; while ( my $dt = $iter->next ) { print $dt->ymd; };
produced an infinite loop with the error message "iterator can't find a previous value". I intended to add later
my $set2 = $set->grep( sub { return ( $_->hour > 7 or $_->hour < 23); } ); my $iter = $set2->iterator; while ( my $dt = $iter->next ) { print $dt->ymd; };
Where is my mistake?
Many thanks again - also for answering my previous question!
VE
The "recurrence" function should return the "next" recurrence. This is one way to implement it:
#!/usr/bin/perl use strict; use warnings; use DateTime; use DateTime::Format::Strptime; use Datetime::Set; # use Data::Dumper; my $strp = DateTime::Format::Strptime->new( pattern => '%Y-%m-%d %T', ); my $datf = qq{2012-01-01 04:00:00}; my $datt = qq{2012-01-02 23:00:00}; my $start = $strp->parse_datetime($datf); my $end = $strp->parse_datetime($datt); my $iter; my $set = DateTime::Set->from_recurrence( after => $start, before => $end, recurrence => sub { return $_[0]->add( hours => 1 )->truncate( to => 'hour' ) }, ); print "first set:\n"; $iter = $set->iterator; while ( my $dt = $iter->next ) { print $dt->datetime, "\n"; }; my $set2 = $set->grep( sub { return ( $_->hour > 7 && $_->hour < 23); } ); print "second set:\n"; $iter = $set2->iterator; while ( my $dt = $iter->next ) { print $dt->datetime, "\n"; };
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| 0.71217 |
communication
Hi Experts,
Two questions:
1 Can I open AOL or Yahoo messanger with known ID to whom I would communicate to ? How ?
2 If first answer is not then is there any samples how to create my own communication app ?
Thank you.
LVL 1
fpoyavoAsked:
Who is Participating?
I wear a lot of hats...
"The solutions and answers provided on Experts Exchange have been extremely helpful to me over the last few years. I wear a lot of hats - Developer, Database Administrator, Help Desk, etc., so I know a lot of things but not a lot about one thing. Experts Exchange gives me answers from people who do know a lot about one thing, in a easy to use platform." -Todd S.
jaynusCommented:
Can you expand on what exactly your trying to accomplish?
To open some sort of application to chat with a known person or...?
0
fpoyavoAuthor Commented:
I would use existing AOL IM.
1 I know userid and would open from my app IM using this id.
0
jaynusCommented:
Easiest way for AOL IM would be to force a URL open of
aim:goim?screenname=UserScreeName&message=Hello+World
If that doesnt suffice, let me know.
0
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GREAT. IT WORKS.
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does hd advance have better compatibility than hd loader?
Discussion in 'PS2 - Hardware boot discussion' started by jiggle-o, Dec 30, 2004.
1. jiggle-o
jiggle-o Guest
I have the hd loader and cannot get it to play the newer games such as any tom clancy any of the better ones and the original makers say that hd loader disables the network adapter but I can still get online for socom 1 so how is that if it really does disable it ? also on another thread a lot of peop[le are looking to download it but I have no idea how to host it so that they may so if someone would care to explain that to me I would be glad to do it.
2. Ag3ntNe0
Ag3ntNe0 Guest
As far as I know they are both the same in compatibility.
If socom online works for you then you must be using the disc because none of my games saved on the hard drive can go online.
3. jiggle-o
jiggle-o Guest
no I'm telling you that socom 1 works online directly from hard drive I gave the disc to a frien since I don't need it so there's not even a chance that could be in. I know what you're saying because nothing else will.
4. jonjakjam
jonjakjam Regular member
Joined:
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Yes socom1 works online from the HD because it doesnt use DNAS to connect.
And Splinter cell is not compatible with HDL/HDA.
5. heybilly
heybilly Regular member
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splinter cell is compatible if u patch the iso... so are a few other games... guys do some google research or search the forums first
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| 0.590407 |
programplug Logo
SQL Create Table
Create Table In SQL:
CREATE TABLE statement in SQL is used to create the new table in the database.
To create a table you need to define its coloumns and each coloumn's with their data type.
Syntax:
CREATE TABLE table_name(
column1 datatype,
column2 datatype,
column3 datatype,
column4 datatype,
column5 datatype,
.....
columnN datatype,
PRIMARY KEY( for one or more columns )
);
You can check,If you have successfully created the table by looking at the message displayed by the SQL server. Another way to chech by DESC command as follows:-
FIELD TYPE NULL KEY DEFAULT
ID Int(11) NO PRI
NAME Varchar(25) NO
AGE Int(11) NO
SUBJECT Varchar(20) YES NULL
MARKS decimal(20, 2) YES NULL
Now, you have STUDENT table available in your database through that you can use to save the required information related to students record.
|
__label__pos
| 0.963827 |
ReadFile?
This is a discussion on ReadFile? within the Windows Programming forums, part of the Platform Specific Boards category; I was wondering how to use ReadFile. I want to read the data in a .txt file and output it ...
1. #1
Programming is fun, mkay?
Join Date
Oct 2001
Posts
490
Question ReadFile?
I was wondering how to use ReadFile. I want to read the data in a .txt file and output it in my EDIT box. I know how to use the LB_ADDSTRING part, but I need to know how to use ReadFile to get the file's data. Any help is appreciated!
Website(s): http://www16.brinkster.com/trifaze/
E-mail: [email protected]
---------------------------------
C++ Environment: MSVC++ 6.0; Dev-C++ 4.0/4.1
DirectX Version: 9.0b
DX SDK: DirectX 8.1 SDK
2. #2
Banned maes's Avatar
Join Date
Aug 2001
Posts
744
here is a function that will read the file for you:
the text is being put in the global variable text, and the function returns the filelength
Code:
char *text;
int ReadTextFile(char FileName[MAX_PATH])
{
char *pBuffer;
int iFileLength;
HANDLE hFile;
DWORD dwBytesRead;
if (INVALID_HANDLE_VALUE ==
(hFile = CreateFile (FileName, GENERIC_READ, FILE_SHARE_READ,
NULL, OPEN_EXISTING, 0, NULL)))
return -1;
iFileLength = GetFileSize (hFile, NULL) ;
pBuffer = (char *)malloc (iFileLength + 2) ;
ReadFile (hFile, pBuffer, iFileLength, &dwBytesRead, NULL) ;
pBuffer[iFileLength] = '\0' ;
pBuffer[iFileLength + 1] = '\0' ;
text = (char *)malloc (iFileLength + 2) ;
strcpy(text,pBuffer);
free(pBuffer);
CloseHandle (hFile) ;
return iFileLength;
}
bye
3. #3
train spotter
Join Date
Aug 2001
Location
near a computer
Posts
3,861
Instead of malloc use GlobalAlloc()
ie
Code:
HGLOBAL hMem=NULL;
int *pInt;
hMem=GlobalAlloc(GHND,sizeof(int)*1000);
if(hMem==NULL)
{
iError=GetLastError();
sprintf(sBuffer,"Mem Alloc Error#%d in 'MyFunction'.",iError);
MessageBox(hDlg,sBuffer,"My App Error",MB_OK|MB_ICONERROR);
}
else pInt=(int *)GlobalLock(hMem);
Always check mem allocs for errors. Remember to free the mem later. ( With GlobalFree() and GlobalHandle() if needed )
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I don't understand something in C++, gcc doesn't like how do I proceed.
I did :
if (!fModeMdi)
MyFirstClass* main = (MyFirstClass*) fMaino;
else
MySecondClass* main = (MySecondClass*) fMdio;
...
...
int i = main->GetNum();
and I get this error :
file.C:211:16: warning: unused variable 'main' [-Wunused-variable]
file.C:213:15: warning: unused variable 'main' [-Wunused-variable]
file.C:219:9: error: 'main' was not declared in this scope
I cannot declare main in my header, because his type depends on fModeMdi boolean.
How can I solve this please ?
share|improve this question
Is there an inheritance hierarchy for the two classes? – hmjd Jun 26 '12 at 12:34
So should provided the classes definitions, as well as some context in which "main" is used. We can't be guessing how your code is implemented. – mfontanini Jun 26 '12 at 12:40
add comment
6 Answers
If MyFirstClass and MySecondClass are related through inheritance, then you can do what @unkulunkulu suggested in his answer.
However, if MyFirstClass and MySecondClass are unrelated classes, then you could use template as:
if (!fModeMdi)
{
do_work(static_cast<MyFirstClass*>(fMaino));
}
else
{
do_work(static_cast<MySecondClass*>(fMaino));
}
where do_work is a function template, implemented as:
template<typename T>
void do_work(T *obj)
{
int i = obj->GetNum();
//do rest of the work here....
}
Note that this template solution would work even if they're related!!
share|improve this answer
add comment
How about defining the variable before the if statement, and assigning it inside of it?
MyFirstClass* main = 0; // use nullptr if you have access to a C++11 compiler
if (!fModeMdi)
main = (MyFirstClass*) fMaino;
else
main = (MySecondClass*) fMdio;
Since you defined it inside the if statement, after it, the variable already went out of scope and can no longer be referenced.
share|improve this answer
2
What if MySecondClass is NOT derived from MyFirstClass (directly or indirectly)? – Nawaz Jun 26 '12 at 12:35
As long as we can assume it's OK to cast a MySecondClass * to a MyFirstClass *. – JoeFish Jun 26 '12 at 12:35
Yes, you're right. I assumed that, since OP is using 2 variables named the same way, those types were compatible. If he provided those classes definitions, maybe another approach could be used. – mfontanini Jun 26 '12 at 12:37
@mfontanini: See my solution. – Nawaz Jun 26 '12 at 12:42
@mfontanini: Note that c-style cast may not work here. What you need here is dynamic_cast if objects involved are polymorphic classes. – Nawaz Jun 26 '12 at 12:58
show 1 more comment
Assign the value of i with in the loop.
int i;
if (!fModeMdi){
MyFirstClass* main = (MyFirstClass*) fMaino;
i = main->GetNum();
}else{
MySecondClass* main = (MySecondClass*) fMdio;
i = main->GetNum();
}
share|improve this answer
add comment
The following should work. In c++ the scope of a variable is inside the bracket { }, that is it is only recognized inside the bracket. Once you get out, the program has no idea about it.
MyFirstClass* main =0;
MySecondClass* main2 =0;
if (!fModeMdi)
main = (MyFirstClass*) fMaino;
else
main2 = (MySecondClass*) fMdio;
share|improve this answer
There is now at least one uninitialised pointer. – hmjd Jun 26 '12 at 12:35
sorry, fixed the the uninitialized – Nick Jun 26 '12 at 12:37
add comment
C++ is a statically-typed language, at this line
int i = main->GetNum();
the compiler has to know the type of main at compile time (statically, hence the name). You cannot make type of main depend on some value fModeMdi, which is known only at runtime. If each of your classes contain a method GetNum and others, which you use after the if statement, you can consider moving them to a base class like this:
class MyBaseClass {
public:
virtual int GetNum() = 0;
}
class MyFirstClass : public MyBaseClass {
// ...
};
class MySecondClass : public MyBaseClass {
// ...
};
MyBaseClass* main = 0;
if (!fModeMdi)
main = (MyFirstClass*) fMaino;
else
main = (MySecondClass*) fMdio;
...
...
And then this is legal
int i = main->GetNum();
Actually, proper design (moving the common methods to a base class) will probably eliminate the need for this if statement altogether. This is what called the polymorphism, its whole purpose is to eliminate the need for these if or switch statements.
share|improve this answer
Crikey this is almost word-for-word what I was going to post. You're quick! – JoeFish Jun 26 '12 at 12:37
add comment
If main may not be polymorphic, then what appears to be the right solution is a functor (or function object). boost function and boost bind provide these libraries.
Ignoring the memory leak in the below program, we bind the object new A() or new B() to their respective GetNum() calls and wrap them in a callable object, f. We call f() when needed.
#include <boost/function.hpp>
#include <boost/bind.hpp>
#include <iostream>
class A {
public:
int GetNum() { return 0; }
};
class B {
public:
int GetNum() { return 0; }
};
int main(int args, char** argv)
{
bool p = true;
boost::function<int()> f;
int i;
if ( p ) {
f = boost::bind(&A::GetNum, new A());
}
else {
f = boost::bind(&B::GetNum, new B());
}
i = f();
std::cout<<i<<std::endl;
return 0;
}
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Greasemonkey Hacks/Accessibility
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Greasemonkey Hacks
Contents
Hacks 67–76: Introduction
I have cared about accessibility for almost 13 years, ever since I worked at AT&T as a relay operator for the deaf and hearing impaired. My manager was deaf, and I learned enough American Sign Language to communicate with him in his native language. (He also read lips and spoke perfect English.) One of my co-workers, with whom I became close friends, had been blind since birth. He did the same job I did, using a device that converted the words on the screen to Braille characters. We often worked different shifts, but I learned enough Braille to write him letters, which he looked forward to reading when he came in to work the next day.
Accessibility isn't just wheelchair ramps and bigger bathroom stalls. It crosses all disciplines, it affects all workplaces, and it makes no exception for gender, race, ethnicity, or income. With more and more information being published online, with more and more vital online services being developed, web accessibility is more important than ever.
Tip
In the year 2004, an estimated 7.9% (plus or minus 0.2 percentage points) of civilian, noninstitutionalized men and women, aged 18 to 64 in the United States reported a work limitation. In other words, that's 14,152,000 out of 179,133,000 (or about 1 in 13) people. [1]
The hacks in this chapter are a compilation of accessibility-related scripts I've written and found online. Some of them are tools for web developers, to help them make their own pages more accessible. Some of them leverage the accessibility features already present on the Web. The last one [Hack #76] is a proof-of-concept I developed to showcase the power of Greasemonkey as an accessibility enablement technology.
Customizing the Web isn't just fun and games. For some people, it provides the only way to use the Web at all.
Highlight Images Without Alternate Text
Quickly see which of your images are missing the required alt attribute.
If you're a web developer, you should already know that web accessibility is important. One of the primary mechanisms for enabling blind and disabled users to view your pages is to provide alternate text for every image. This is so important that the alt attribute is actually a required attribute of every <img> element. Even spacer images need an explicit alt="" attribute to tell text-only browsers and screen readers to skip over the image when they display the page or read it aloud.
Validating your page with the W3C's HTML validator (http://validator.w3.org) will tell you if an <img> element is missing the required alt attribute, but it will also tell you every other single thing you did wrong. If you aren't coding exactly to the HTML specification, the really important errors (such as missing alt attributes) will get lost in a sea of arcane rules and trivial mistakes.
The Code
This user script will run on all pages by default, but you should probably modify the @include line to include just the pages you're currently developing. The bulk of the script logic is contained in the XPath query, "//img[not(@alt)]", which finds all <img> elements that do not include any alt attribute. It will not find images that contain a blank alt attribute, which is perfectly legitimate for spacer images used solely for page layout. It will also not find images whose alternate text is useless to blind users, such as alt="filename.gif" or alt="include alternate text here".
Save the following user script as highlightnoalt.user.js:
// ==UserScript==
// @name Highlight No Alt
// @namespace http://diveintomark.org/projects/greasemonkey/
// @description highlight images without alternate text
// @include *
// ==/UserScript==
var snapBadImages = document.evaluate("//img[not(@alt)]",
document, null, XPathResult.UNORDERED_NODE_SNAPSHOT_TYPE, null);
for (var i = snapBadImages.snapshotLength - 1; i >= 0; i--) {
var elmBadImage = snapBadImages.snapshotItem(i);
elmBadImage.style.MozOutline = "2px solid red";
elmBadImage.title = 'Missing ALT attribute! src="' +
elmBadImage.src + '"';
}
Running the Hack
After installing the user script (Tools → Install This User Script), go to http://www.amazon.com. You will see a number of images highlighted with a thick red border, as shown in Figure 8-1.
Figure 8-1. Inaccessible images highlighted on Amazon.com
Inaccessible images highlighted on Amazon.com
This immediately highlights several accessibility problems on Amazon's home page. In the upper-left corner, they are cross-selling one of their new partner sites for buying gourmet food online. In the upper-right corner, they have an image link to a list of most wished-for items. Each of these images is missing the required alt attribute. In the absence of an alt attribute, screen readers will read the filename from the src attribute instead, which, as you can see, is completely meaningless.
Hacking the Hack
There are many different avenues to explore in highlighting broken images. You could expand the XPath query to find images with a blank alt attribute. These are legitimate for spacer images, but they should never occur on images that convey information (such as the "Shop in Gourmet Food" image in Figure 8-1):
var snapBadImages = document.evaluate("//img[not(@alt) or @alt='']",
document, null, XPathResult.UNORDERED_NODE_SNAPSHOT_TYPE, null);
You could also use a similar technique to find images that are missing other attributes, such as width and height. width and height attributes are not strictly required, but it helps browsers lay out the page more quickly if they know in advance how large an image will be:
var snapBadImages = document.evaluate("//img[not(@width) or not(@height)]",
document, null, XPathResult.UNORDERED_NODE_SNAPSHOT_TYPE, null);
You should modify the rest of the script accordingly—for instance, to change the tool tip to indicate the problem:
elmBadImage.title = 'Missing width or height! src="' + elmBadImage.src + '"';
Add an Access Bar with Keyboard Shortcuts
Display shortcut keys defined by a page.
An increasing number of sites define keyboard shortcuts, called access keys, for commonly used features. This is an accessibility aid for people who have difficulty using a mouse. For example, a site could define a shortcut to jump to the site's accessibility statement and another one to set focus to the site's search box (or jump to a separate search page). Unfortunately, there is no easy way to know which shortcuts the site has defined! This hack makes the keyboard shortcuts visible.
Tip
Learn more about defining keyboard shortcuts at http://diveintoaccessibility.org/15.
The Code
This user script runs on all pages. The code is divided into three parts:
1. Find all elements that define a keyboard shortcut with the accesskey attribute.
2. Loop through each of these elements and find the most logical label for the shortcut.
3. Add CSS styles to the page so the list of keyboard shortcuts appears in a fixed bar along the bottom of the browser window.
Step 2 is the hard part, because different HTML elements can define an accesskey attribute. Form elements like input, textarea, and select can each define an accesskey. The form element might or might not have an associated label that contains a text description of the form field. If so, the label might contain a title attribute that gives even more detailed information about the input field. If not, the label might simply contain text. Or the form field might have no associated label at all, in which case the value attribute of the input element is the best we can do.
On the other hand, the label itself can define the accesskey, instead of the input element the label describes. Again, we'll look for a description in the title attribute of the label element, but fall back to the text of the label if no title attribute is present.
A link can also define an accesskey attribute. If so, the link text is the obvious choice. But if the link has no text (for example, if it contains only an image), then the link's title attribute is the next place to look. If the link contains no text and no title, we fall back to the link's name attribute, and, failing that, the link's id attribute.
Save the following user script as accessbar.user.js:
// ==UserScript==
// @name Access Bar
// @namespace http://diveintomark.org/projects/greasemonkey/
// @description show accesskeys defined on page
// @include *
// ==/UserScript==
function addGlobalStyle(css) {
var elmHead, elmStyle;
elmHead = document.getElementsByTagName('head')[0];
if (!elmHead) { return; }
elmStyle = document.createElement('style');
elmStyle.type = 'text/css';
elmStyle.innerHTML = css;
elmHead.appendChild(elmStyle);
}
var snapAccesskeys = document.evaluate(
"//*[@accesskey]",
document,
null,
XPathResult.UNORDERED_NODE_SNAPSHOT_TYPE,
null);
if (!snapAccesskeys.snapshotLength) { return; }
var arDescriptions = new Array();
for (var i = snapAccesskeys.snapshotLength - 1; i >= 0; i--) {
var elm = snapAccesskeys.snapshotItem(i);
var sDescription = '';
var elmLabel = document.evaluate("//label[@for='" + elm.id+ "']",
document,
null,
XPathResult.FIRST_ORDERED_NODE_TYPE,
null).singleNodeValue;
if (elmLabel) {
sDescription = label.title;
if (!sDescription) { sDescription = label.textContent; }
}
if (!sDescription) { sDescription = elm.textContent; }
if (!sDescription) { sDescription = elm.title; }
if (!sDescription) { sDescription = elm.name; }
if (!sDescription) { sDescription = elm.id; }
if (!sDescription) { sDescription = elm.href; }
if (!sDescription) { sDescription = elm.value; }
var htmlDescription = '<strong>[' +
elm.getAttribute('accesskey').toUpperCase() + ']</strong> ';
if (elm.href) {
keyboard shortcutsdisplaying on access barshtmlDescription += '<a href="' + elm.href + '">' +
sDescription + '</a>';
} else {
htmlDescription += sDescription;
}
arDescriptions.push(htmlDescription);
}
arDescriptions.sort();
var elmWrapper = document.createElement('div');
elmWrapper.id = 'accessibilityaccess bars with keyboard shortcutsaccessbar-div-0';
var html = '<div><ul><li class="first">' + arDescriptions[0] + '</li>';
for (var i = 1; i < arDescriptions.length; i++) {
html += '<li>' + arDescriptions[i] + '</li>';
}
html += '</ul></div>';
elmWrapper.innerHTML = html;
document.body.style.paddingBottom = "4em";
window.addEventListener(
"load",
function() { document.body.appendChild(elmWrapper); },
true);
addGlobalStyle(
'#accessbar-div-0 {'+
' position: fixed;' +
' left: 0;' +
' right: 0;' +
' bottom: 0;' +
' top: auto;' +
' border-top: 1px solid silver;' +
' background: black;' +
' color: white;' +
' margin: 1em 0 0 0;' +
' padding: 5px 0 0.4em 0;' +
' width: 100%;' +
' font-family: Verdana, sans-serif;' +
' font-size: small;' +
' line-height: 160%;' +
'}' +
'#accessbar-div-0 a,' +
'#accessbar-div-0 li,' +
'#accessbar-div-0 span,' +
'#accessbar-div-0 strong {' +
' background-color: transparent;' +
' color: white;' +
'}' +
'#accessbar-div-0 div {' +
' margin: 0 1em 0 1em;' +
'}' +
'#accessbar-div-0 div ul {' +
' margin-left: 0;' +
' margin-bottom: 5px;' +
' padding-left: 0;' +
' display: inline;' +
'}' +
'#accessibilityaccess bars with keyboard shortcutsaccessbar-div-0 div ul li {' +
' margin-left: 0;' +
' padding: 3px 15px;' +
' border-left: 1px solid silver;' +
' list-style: none;' +
' display: inline;' +
'}' +
'#accessbar-div-0 div ul li.first {' +
' border-left: none;' +
' padding-left: 0;' +
'}');
Running the Hack
After installing the user script (Tools → Install This User Script), go to http://diveintomark.org. At the bottom of the browser window, you will see a black bar displaying the keyboard shortcuts defined on the page, as shown in Figure 8-2.
Figure 8-2. Keyboard shortcuts defined on diveintomark.org
Keyboard shortcuts defined on diveintomark.org
How you actually use the defined keyboard shortcuts varies by platform. On Windows and Linux, you press Alt along with the defined key. On Mac OS X, you press Command and the key. On http://www.diveintomark.org, you can press Alt-0 to jump to the site's accessibility statement, as shown in Figure 8-3.
Figure 8-3. The accessibility statement for http://www.diveintomark.org
The accessibility statement for http://www.diveintomark.org
Pressing Alt-1 jumps back to the home page, and Alt-4 sets focus to the search box on the right side of the page.
Remove Conflicting Keyboard Shortcuts
Remove annoying access keys from web pages that define conflicting shortcuts.
"Add an Access Bar with Keyboard Shortcuts" [Hack #68] introduced the concept of site-specific keyboard shortcuts (called access keys, after the attribute used to define them). Like Greasemonkey itself, access keys can be used for good or for evil. A malicious web page could redefine all available access keys to point to a link that tries to download a harmful executable or pop up an advertising window. Or a web publisher could—with the best of intentions—end up defining access keys that conflict with standard keyboard shortcuts in your browser.
Wikipedia, an otherwise excellent online encyclopedia, is such a site. It defines several access keys, including some (such as Alt-E) that conflict with the keyboard shortcuts for opening menus in the Firefox menu bar. This hack removes all access keys from a page to avoid the possibility of such conflicts.
The Code
This user script runs on all pages. It uses an XPath expression to find all the elements with an accesskey attribute, and then removes the attribute. This is enough to get Firefox to remove the associated keyboard shortcut from the link or form field.
Save the following user script as unaccesskey.user.js:
// ==UserScript==
// @name Remove AccessKeys
// @namespace http://diveintomark.org/projects/greasemonkey/
// @description remove accesskey shortcuts from web pages
// @include *
// ==/UserScript==
var snapSubmit = document.evaluate("//*[@accesskey]",
document, null, XPathResult.UNORDERED_NODE_SNAPSHOT_TYPE, null);
for (var i = snapSubmit.snapshotLength - 1; i >= 0; i--) {
snapSubmit.snapshotItem(i).removeAttribute('accesskey');
}
Running the Hack
This hack runs on all platforms, but it is especially useful on Microsoft Windows, where keyboard shortcuts to open menus conflict with keyboard shortcuts defined on the web page itself.
Before installing the user script, go to http://en.wikipedia.org/wiki/Music_of_Mongolia. Press Alt-E to try to open the Edit menu, or Alt-T to open the Tools menu. Holy conflicts, Batman! The web page has redefined Alt-E to jump to the editing page, and Alt-T to jump to the discussion page.
Now, install the user script (Tools → Install This User Script), and refresh http://en.wikipedia.org/wiki/Music_of_Mongolia. You can now press Alt-E to open the Edit menu, or Alt-T to open the Tools menu. All the standard key combinations work as you would expect them to.
Wikipedia is the highest-profile site that creates this problem (and it was the inspiration for this hack), but the possibility for conflict exists on any site. I leave this script installed with the default @include *, but if you use site-specific keyboard shortcuts, you can change the @include configuration to target only the sites that cause this problem.
Make Image alt Text Visible
Display otherwise invisible information in image alt attributes as a tool tip.
In the HTML specifications, there are two attributes designed to allow text to be attached to an image: alt and title. The alt attribute is short for alternate, and it is designed to display when the image itself cannot. The title attribute is designed as an extra title to show when a user hovers his mouse over the image. Most browsers function this way. Microsoft's Internet Explorer, however, will treat an alt attribute as a title, and display it as a tool tip.(To be fair, Microsoft did this to emulate the broken behavior of Netscape 4.) As a result, many less-informed web site maintainers use alt as if it was made to display a tool tip. When using a compliant browser like Firefox, this information is inaccessible!
With the magic of Greasemonkey, though, we can resurrect this information. This hack makes all alt attributes for images appear as their tool tips, by assigning the text to the title attribute instead.
The Code
This user script runs on all pages. First, we execute an XPath query to find all the <img> and <area> elements; these are the elements usually assigned <alt> text where the author intended a <title>. Then, a simple for loop evaluates each element returned from the query. For each <img> or <area> that has an empty title attribute and a nonempty alt attribute, we copy the alt text into the title.
Save the following user script as alt-tooltips.user.js:
// ==UserScript==
// @name Alt Tooltips
// @namespace http://www.arantius.com/
// @description Display Alt text as tooltip if no title is available
// @include *
// ==/UserScript==
// based on code by Anthony Lieuallen
// and included here with his gracious permission
var res = document.evaluate("//area|//img",
document, null, XPathResult.UNORDERED_NODE_SNAPSHOT_TYPE, null);
var i, el;
for (i=0; el=res.snapshotItem(i); i++) {
if (''==el.title && ''!=el.alt) el.title='ALT: '+el.alt;
}
Running the Hack
Before installing this script, browse to any page that contains images with alt attributes, and they will be visible as tool tips when you hover your cursor over the image. For example, the Google home page uses an image with alt text, but no title. Pointing your mouse at the image does nothing, as shown in Figure 8-4.
Figure 8-4. Unmodified Google home page
Unmodified Google home page
Now install the script (Tools → Install This User Script) and refresh the Google home page. The alt text in the logo is revealed when you hover your mouse over the image, as shown in Figure 8-5.
Figure 8-5. Google home page with alt tool tips
Google home page with alt tool tips
Hacking the Hack
As shown in "Master XPath Expressions" [Hack #8], XPath is a language all its own. The logic used in the loop can be fit into a more complex XPath query:
var res = document.evaluate("(//img|//area)[not(@title) and not(''=@alt)]",
document, null, XPathResult.UNORDERED_NODE_SNAPSHOT_TYPE, null);
With this XPath query, we can simplify the code inside the loop:
var res = document.evaluate("(//img|//area)[not(@title) and not(''=@alt)]",
document, null, XPathResult.UNORDERED_NODE_SNAPSHOT_TYPE, null);
var i, el;
for (i=0; el=res.snapshotItem(i); i++) {
el.title='accessibilityimage alt textALT: '+el.alt;
}
I call this trading complexity. The overall code is not simpler; we've just moved the complexity from one part to another. The end result is the same, so it boils down to a matter of style.
Anthony Lieuallen
Add a Table of Contents to Long Pages
Create a menu out of a page's header tags.
I read a lot of specifications online. Not as part of my day job; I mean I do this for fun. There are good specifications, and there are bad specifications, but there is one thing you can say about virtually all of them: they are incredibly long. And most of them are published online as a single HTML page. Firefox's incremental find feature helps when I'm trying to find something specific (just press Ctrl-F and start typing), but I still often get lost in the endless scrolling.
One nice thing about W3C specifications in particular is that they use HTML correctly. Section and subsection titles are marked up with header tags: <h1>, <h2>, <h3>, <h4>, and so on. This hack takes those header tags and creates an in-page table of contents. Using the same technique as "Add an Access Bar with Keyboard Shortcuts" [Hack #68], the script adds a fixed bar at the bottom of the browser window that contains a drop-down menu of all the headers on the page. Selecting a header from the menu jumps directly to that section on the page.
The Code
This user script runs on all pages. It iterates through all the <h1>, <h2>, <h3>, and <h4> elements on the page, and creates a <select> menu in a fixed-position bar along the bottom of the browser window, just above the status bar. Items in the menu are indented based on the header level, so when you drop down the menu, it appears to be a hierarchical table of contents. Finally, we add a Hide TOC button on the right side of the table of contents bar. Clicking Hide TOC hides the bar temporarily until you refresh the page or follow a link to another page.
Save the following user script as autotoc.user.js:
// ==UserScript==
// @name AutoTOC
// @namespace http://runeskaug.com/greasemonkey
// @description Creates a table of contents for all headers on the page
// @include *
// ==/UserScript==
// based on code by Rune Skaug
// and included here with his gracious permission
//set the optional behaviour accessibilitytables of contents menusof the TOC box
// - true resets it to its initial state after you have selected a header
// - false does not reset it
var resetSelect = false;
//if true, shows a "Hide TOC" button on the right side of the bar
var showHide = true;
var hideText = "Hide TOC";
function f() {
// only on (X)HTML pages containing at least one heading -
// excludes XML files, text files, plugins and images
if ( document.getElementsByTagName("html").length &&
(document.getElementsByTagName('h1').length ||
document.getElementsByTagName('h2').length ||
document.getElementsByTagName('h3').length ||
document.getElementsByTagName('h4').length )) {
var aHs = getHTMLHeadings();
if (aHs.length>2) { // HTML document, more than two headings.
addCSS(
'#js-toc {position: fixed; left: 0; right: 0; top: auto; ' +
'bottom: 0; height: 20px; width: 100%; vertical-align: ' +
'middle; display: block; border-top: 1px solid #777; ' +
'background: #ddd; margin: 0; web pagesadding tables of contents menuspadding: 3px; ' +
'z-index: 9999; }\n#js-toc select { font: 8pt verdana, ' +
'sans-serif; margin: 0; margin-left:5px; ' +
'background-color: #fff; color: #000; float: ' +
'left; padding: 0; vertical-align: middle;}\n' +
'#js-toc option { font: 8pt verdana, sans-serif; ' +
'color: #000; }\n#js-toc .hideBtn { font: 8pt verdana, ' +
'sans-serif; float: right;' +
'margin-left: 5px; margin-right: 10px; padding: 2px 2px; ' +
'border: 1px dotted #333; background-color: #e7e7e7; }\n' +
'#js-toc .hideBtn a { color: #333; text-decoration: none; '+
'background-color: transparent;} ' +
'#js-toc .hideBtn a:hover { ' +
'color: #333; text-decoration: none; background-color: ' +
'transparent;}'
);
var toc = document.createElement(
showHide?'tocuserjselem':'div');
toc.id = 'js-toc';
tocSelect = document.createElement('select');
tocSelect.addEventListener("change", function() {
if (this.value) {
if (resetSelect) {
this.selectedIndex = 0;
}
window.location.href = '#' + this.value;
}
}, true);
tocSelect.id = 'navbar-toc-select';
tocEmptyOption = document.createElement('option');
tocEmptyOption.setAttribute('value','');
tocEmptyOption.appendChild(
document.createTextNode('Table accessibilitytables of contents menusof Contents'));
tocSelect.appendChild(tocEmptyOption);
toc.appendChild(tocSelect);
if (showHide) {
var hideDiv = document.createElement('div');
hideDiv.setAttribute('class','hideBtn');
var hideLink = document.createElement('a');
hideLink.setAttribute("href","#");
hideLink.addEventListener('click', function(event) {
document.getElementById('js-toc').style.display =
'none';
event.preventDefault();
}, true);
hideLink.appendChild(document.createTextNode(hideText));
hideDiv.appendChild(hideLink);
toc.appendChild(hideDiv);
}
document.body.style.web pagesadding tables of contents menuspaddingBottom = "27px";
document.body.appendChild(toc);
for (var i=0,aH;aH=aHs[i];i++) {
if (aH.offsetWidth) {
op = document.createElement("option");
op.appendChild(document.createTextNode(gs(aH.tagName)+
getInnerText(aH).substring(0,100)));
var refID = aH.id ? aH.id : aH.tagName+'-'+(i*1+1);
op.setAttribute("value", refID);
document.getElementById("navbar-toc-select").
appendChild(
op);
aH.id = refID;
}
}
}
}
GM_registerMenuCommand('AutoTOC: Toggle display',
autoTOC_toggleDisplay);
};
function autoTOC_toggleDisplay() {
if (document.getElementById('js-toc').style.display == 'none') {
document.getElementById('js-toc').style.display = 'block';
}
else {
document.getElementById('js-toc').style.display = 'none';
}
}
function getHTMLHeadings() {
function acceptNode(node) {
if (node.tagName.match(/^h[1-4]$/i)) {
return NodeFilter.FILTER_ACCEPT;
}
return NodeFilter.FILTER_SKIP;
}
outArray = new Array();
var els = document.getElementsByTagName("*");
var j = 0;
for (var i=0,el;el=els[i];i++) {
if (el.tagName.match(/^h[1-4]$/i)) {
outArray[j++] = el;
}
}
return outArray;
}
function addCSS(css) {
var head, styleLink;
head = document.getElementsByTagName('head')[0];
if (!head) { return; }
styleLink = document.createElement('link');
styleLink.setAttribute('rel','stylesheet');
styleLink.setAttribute('type','text/css');
styleLink.setAttribute('href','data:text/css,'+escape(css));
head.appendChild(styleLink);
}
function gs(s){
s = s.toLowerCase();
if (s=="h2") return "\u00a0 \u00a0 "
else if (s=="h3") return "\u00a0 \u00a0 \u00a0 \u00a0 "
else if (s=="h4") return "\u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 ";
return "";
}
function getInnerText(el) {
var s='';
for (var i=0,node; node=el.childNodes[i]; i++) {
if (node.nodeType == 1) s += getInnerText(node);
else if (node.nodeType == 3) s += node.nodeValue;
}
return s;
}
f();
Running the Hack
After installing the user script (Tools → Install This User Script), go to http://whatwg.org/specs/web-apps/current-work/. At the bottom of the browser window is a drop-down box labeled Table of Contents. Open the menu to see an outline of all the headers on the page, as shown in Figure 8-6.
Figure 8-6. Table of contents
Table of contents
You can select any of the headings in the menu to jump to that section on the page.
Use Real Headers on Google Web Search
Make Google's markup more semantic, and learn why it matters.
Google does an excellent job of indexing the Web, but it does a poor job of displaying the results. By poor, I mean not semantic. Why does semantic markup matter? Well, among other things, it enables hacks such as "Add a Table of Contents to Long Pages" [Hack #71] to extract meaningful information from the page.
It is also an accessibility issue. Screen readers for the blind have features that allow users to navigate a page by its header elements. Sighted users can simply glance at the page on screen and see how it's structured; screen readers can only "glance" at the page's markup. If a page uses poor markup, screen readers have a more difficult time determining how the page is structured, which makes it more difficult for blind users to navigate.
This hack changes Google search result pages to use reader header elements for each search result.
The Code
This user script runs on Google web search result pages. It uses hardcoded knowledge of Google's markup—each search result is wrapped in a <p class="g"> element—to wrap a real <h2> tag around the title of each result. It also adds an <h1>Search Results</h1> element at the top of the page. This <h1> is hidden from sighted users, but screen readers will still "see" it in the DOM and announce it to blind users.
Save the following user script as googleheaders.user.js:
// ==UserScript==
// @name Google Headings
// @namespace http://zeus.jesus.cam.ac.uk/~jg307/mozilla/userscripts/
// @description Add real heading elements to google search results
// @include http://google.tld/search*
// @include http://www.google.tld/search*
// ==/UserScript==
// based on code by James Graham
// and included here with his gracious permission
var mainHeading = document.createElement('h1');
var headingText = document.createTextNode('Search Results');
mainHeading.appendChild(headingText);
mainHeading.style.visibility="Hidden";
mainHeading.style.height="0";
mainHeading.style.width="0";
var body = document.getElementsByTagName('body')[0];
body.insertBefore(mainHeading, body.firstChild);
var resultsParagraphs = document.evaluate("//p[@class='g']",
document, null, XPathResult.ORDERED_NODE_SNAPSHOT_TYPE, null);
if (resultsParagraphs.snapshotLength) {
var heading = resultsParagraphs.snapshotItem(0);
var headingSize = document.defaultView.getComputedStyle(
heading, '').getPropertyValue("font-size");
var headingWeight = document.defaultView.getComputedStyle(
heading, '').getPropertyValue("font-weight");
}
for (var i = 0; i < resultsParagraphs.snapshotLength; i++) {
var paragraphNode = resultsParagraphs.snapshotItem(i);
var linkNode = paragraphNode.getElementsByTagName('a')[0];
var heading = document.createElement('h2');
heading.appendChild(linkNode.cloneNode(true));
heading.style.fontSize = headingSize;
heading.style.fontWeight = headingWeight;
heading.style.marginBottom = 0;
heading.style.marginTop = 0;
paragraphNode.replaceChild(heading, linkNode);
try {
paragraphNode.removeChild(
paragraphNode.getElementsByTagName('br')[0]);
}
catch(error) {
}
}
Running the Hack
After installing the user script (Tools → Install This User Script), go to http://www.google.com and search for accessibility. The script does not appear to have made any difference, as shown in Figure 8-7.
Figure 8-7. Real headers?
Real headers?
This is because the script goes to great length to style the new <h2> elements so they look similar to the page's default text style. However, if you install autotoc.user.js [Hack #71], the difference becomes obvious, as shown in Figure 8-8.
Because the page now uses properly structured markup, the autotoc.user.js script can construct a table of contents for the page. This is essentially what screen readers do for blind users, by looking for real header elements and allowing the user to jump to the next or previous header.
Figure 8-8. Real headers!
Real headers!
Add a Toolbar to Zoom Images Easily
Reduce or enlarge individual images with a single click.
In Firefox, you can make any page text larger by pressing Ctrl-equals sign (=), or make it smaller by pressing Ctrl-hyphen (-).However, this does nothing to the images on the page. If you want to enlarge or reduce an image, you're out of luck.
Here's a cool little hack that adds a toolbar to each image on a page to make it larger or smaller.
The Code
This user script runs on all pages. It finds all the images on the page with the document.images collection, and then adds a toolbar of buttons (really, just a <div> with some <a> elements styled to look like buttons). Before you protest, I realize that this script isn't keyboard-accessible. You can't win them all.
Save the following user script as zoom-image.user.js:
// ==UserScript==
// @name Zoom Image
// @namespace http://www.smartmenus.org/
// @description Displays an zoom toolbar over accessibilityimage zooming toolbarsimages
// ==/UserScript==
// based on code by Vasil Dinkov
// and included here with his gracious permission
// === User Configuration ===
const kZoomFactor = 1.7; // amount to zoom image on each click
const kMenuShowTimeOut = 1.2; // seconds before auto-hiding menu
const kMinimumImageWidth = 100; // minimal width of the menu-enabled images
const kMinimumImageHeight = 50; // minimal height of the menu-enabled images
// === Code ===
var gTimeoutID = gPixelLeft = gPixelTop = 0;
var gMenuBuilt = false;
var gElmToolbar = gCurrentImage = null;
function image_mouseover(o) {
if ((o.clientWidth<kMinimumImageWidth ||
o.clientHeight<kMinimumImageHeight) &&
!o.zoomed ||
gMenuBuilt &&
gElmToolbar.style.visibility == "visible") {
return;
}
gCurrentImage = o;
if (!gCurrentImage.original_width) {
gCurrentImage.original_width = o.clientWidth;
gCurrentImage.original_height = o.clientHeight;
}
gPixelLeft = o.offsetLeft;
gPixelTop = o.offsetTop;
var oParent = o.offsetParent;
while (oParent) {
gPixelLeft += oParent.offsetLeft;
gPixelTop += oParent.offsetTop;
oParent = oParent.offsetParent;
}
gTimeoutID = setTimeout(show_toolbar, kMenuShowTimeOut*1000);
}
function show_toolbar() {
if (!build_menu()) { return; }
gElmToolbar.style.top = gPixelTop+"px";
gElmToolbar.style.left = gPixelLeft+"px";
gElmToolbar.style.visibility = "visible";
}
function hide_toolbar(e) {
if (gTimeoutID) {
clearTimeout(gTimeoutID);
gTimeoutID = 0;
}
if (!build_menu()) { return; }
var relatedTarget = e?e.relatedTarget:0;
if (relatedTarget &&
(gElmToolbar==relatedTarget ||
gElmToolbar==relatedTarget.parentNode)) {
return;
}
gElmToolbar.style.visibility = "hidden";
accessibilityimage zooming toolbarsgCurrentImage = null;
}
function toolbar_mouseout(e) {
var relatedTarget = e.relatedTarget;
if (relatedTarget && relatedTarget != gCurrentImage) {
hide_toolbar(e);
}
}
function create_button(sCaption, sTitle, fOnClick) {
var elmButton = document.createElement("a");
elmButton.href = '#';
elmButton.className = "zoomtoolbarbutton";
elmButton.title = sTitle;
elmButton.appendChild(document.createTextNode(sCaption));
elmButton.addEventListener("mouseover", function() {
this.style.borderColor = "#4d4c76";
}, false);
elmButton.addEventListener("mousedown", function() {
this.style.borderColor = "#000";
this.style.background = "#eee4a5";
}, false);
elmButton.addEventListener("mouseup", function() {
this.style.borderColor = "#4d4c76";
this.style.background = "transparent";
}, false);
elmButton.addEventListener("mouseout", function() {
this.style.borderColor = "#ffffdd #C1B683 #C1B683 #ffffdd";
this.style.background = "transparent";
}, false);
elmButton.addEventListener("click", fOnClick, false);
return elmButton;
}
function build_menu() {
if (gMenuBuilt) { return true; }
gElmToolbar = document.createElement("div");
with (gElmToolbar.style) {
position = "absolute";
border = "1px solid";
borderColor = "#ffffdd #857A4A #857A4A #ffffdd";
backgroundColor = "#F5EBBC";
margin = 0;
padding = "2px";
zIndex = 10000000;
}
gElmToolbar.appendChild(create_button("+", "Zoom in", function(e) {
var width, height;
width = accessibilityimage zooming toolbarsgCurrentImage.clientWidth;
height = gCurrentImage.clientHeight;
gCurrentImage.style.width = width*kZoomFactor+"px";
gCurrentImage.style.height = height*kZoomFactor+"px";
gCurrentImage.zoomed = 1;
e.preventDefault();
}));
gElmToolbar.appendChild(create_button("-", "Zoom out", function(e) {
var width, height;
width = gCurrentImage.clientWidth;
height = gCurrentImage.clientHeight;
gCurrentImage.style.width = width / kZoomFactor + "px";
gCurrentImage.style.height = height / kZoomFactor + "px";
gCurrentImage.zoomed = 1;
e.preventDefault();
}));
gElmToolbar.appendChild(create_button("\u21B2", "Restore", function(e) {
gCurrentImage.style.width = gCurrentImage.original_width + "px";
gCurrentImage.style.height = gCurrentImage.original_height + "px";
gCurrentImage.zoomed = 0;
e.preventDefault();
}));
document.body.appendChild(gElmToolbar);
gElmToolbar.addEventListener("mouseout", toolbar_mouseout, false);
gMenuBuilt = true;
return true;
}
function addGlobalStyle(css) {
var head, styleLink;
head = document.getElementsByTagName('head')[0];
if (!head) { return; }
styleLink = document.createElement('link');
styleLink.setAttribute('rel', 'stylesheet');
styleLink.setAttribute('type', 'text/css');
styleLink.setAttribute('href', 'data:text/css,' + escape(css));
head.appendChild(styleLink);
}
for (var i = 0; i < document.images.length; i++) {
var elmImage = document.images[i];
elmImage.addEventListener("mouseover", function() {
image_mouseover(this);
}, false);
elmImage.addEventListener("mouseout", hide_toolbar, false);
}
addGlobalStyle('' +
'a.zoomtoolbarbutton {' +
' position: relative;' +
' top: 0px;' +
' font: 14px monospace;' +
' border: 1px solid;' +
' border-color: #ffffdd #c1b683 #c1b683 #ffffdd;' +
' padding: 0 2px 0 2px;' +
' margin: 0 2px 2px 2px;' +
' text-decoration: none;' +
' background-color: transparent;' +
' color: black;' +
'}');
Running the Hack
After installing this script (Tools → Install This User Script), go to http://www.oreilly.com. Hover your cursor over the tarsier logo in the top-left corner of the page to activate the zoom toolbar, as shown in Figure 8-9.
Figure 8-9. Image zoom toolbar
Image zoom toolbar
Click the plus (+) button to zoom in on the image, as shown in Figure 8-10.
Figure 8-10. Zoomed tarsier
Zoomed tarsier
You can also click the minus (–) button to reduce the image size, or click the ? button to restore the image to its original size.
Hacking the Hack
There are lots of interesting things to do with images besides zooming them. If you right-click on an image, Firefox gives you several choices: view the image in isolation, copy the image URL, save it to disk, and several others. We can't reproduce all of these functions in JavaScript, but we can do the first one: view the image in isolation.
Immediately before this line in the build_menu function:
document.body.appendChild(gElmToolbar);
add this code snippet:
gElmToolbar.appendChild(create_button("V", "View image", function(e) {
location.href = gCurrentImage.src;
e.preventDefault();
}));
Now, refresh http://www.oreilly.com. Hover over the tarsier again, and you will see an additional button labeled V in the image toolbar, as shown in Figure 8-11.
Figure 8-11. Enhanced image toolbar
Enhanced image toolbar
Click on the V toolbar button to view the image in isolation, as shown in Figure 8-12.
Figure 8-12. Tarsier in isolation
Tarsier in isolation
This is the same functionality provided by selecting View Image in the image's context menu. You can click the back button to return to the O'Reilly home page.
Make Apache Directory Listing Prettier
Enhance Apache's autogenerated directory listing pages with semantic, accessible tables.
Have you ever visited a page to find nothing but a plain list of files? If a folder has no default web page, the Apache web server autogenerates a directory listing with clickable filenames. Nothing fancy, but it works, so why complain? Because we can do better! This hack takes the raw data presented in Apache directory listings and replaces the entire page with a prettier, more accessible, more functional version.
The Code
This user script runs on all pages. Of course, not all pages are Apache directory listings, so the first thing the script does is check for some common signs that this page is a directory listing. Unfortunately, there is no foolproof way to tell; recent versions of Apache add a <meta> element in the <head> of the page to say that the page was autogenerated by Apache, but earlier versions of Apache did not do this. The script checks for three things:
• The title of the page starts with "Index of /".
• The body of the page contains a <pre> element. Apache uses this to display the plain directory listing.
• The body of the page contains links with query parameters. Apache uses these for the column headers. Clicking a column header link re-sorts the directory listing by name, modification date, or size.
If all three of these conditions are met, the script assumes the page is an Apache directory listing, and proceeds to parse the preformatted text to extract the name, modification date, and size of each file. It constructs a table (using an actual <table> element—what a concept) and styles alternating rows with a light-gray background.
Save the following user script as betterdir.user.js:
// ==UserScript==
// @name BetterDir
// @namespace http://diveintomark.org/projects/greasemonkey/
// @description make Apache 1.3-style directory listings prettier
// @include *
// ==/UserScript==
function addGlobalStyle(css) {
var elmHead, elmStyle;
elmHead = document.getElementsByTagName('head')[0];
if (!elmHead) { return; }
elmStyle = document.createElement('style');
elmStyle.type = 'text/css';
elmStyle.innerHTML = css;
elmHead.appendChild(elmStyle);
}
// if page title does not start with "Index of /", bail
if (!(/^Index of \//.test(document.title))) { return; }
// If we can't find the PRE element, this is either
// not a directory listing at all, or it's an
// Apache directory listingsApache 2.x listing with fancy table output enabled
var arPre = document.getElementsByTagName('pre');
if (!arPre.length) { return; }
var elmPre = arPre[0];
// find the column headers, or bail
var snapHeaders = document.evaluate(
"//a[contains(@href, '?')]",
document,
null,
XPathResult.UNORDERED_NODE_SNAPSHOT_TYPE,
null);
if (!snapHeaders.snapshotLength) { return; }
// Tables aren't evil, they're just supposed to be used for tabular data.
// This is tabular data, so let's make a TABLE element
var elmTable = document.createElement('table');
// give the table a summary, for accessibilityaccessibility
elmTable.setAttribute('summary', 'Directory listing');
var elmCaption = document.createElement('caption');
// the "title" of the table should go in a CAPTION element
// inside the TABLE element, for semantic purity
elmCaption.textContent = document.evaluate("//head/title",
document, null, XPathResult.FIRST_ORDERED_NODE_TYPE,
null).singleNodeValue.textContent;
elmTable.appendChild(elmCaption);
var elmTR0 = document.createElement('tr');
var iNumHeaders = 0;
for (var i = 0; i < snapHeaders.snapshotLength; i++) {
var elmHeader = snapHeaders.snapshotItem(i);
// column headers go into TH elements, for accessibility
var elmTH = document.createElement('th');
var elmLink = document.createElement('a');
elmLink.href = elmHeader.href;
elmLink.innerHTML = elmHeader.innerHTML;
// give each of the column header links a title,
// to explain what will happen when you click on them
elmLink.title = "Sort by " + elmHeader.innerHTML.toLowerCase();
elmTH.appendChild(elmLink);
elmTR0.appendChild(elmTH);
iNumHeaders++;
}
elmTable.appendChild(elmTR0);
var sPreText = elmPre.innerHTML;
if (/<hr/.test(sPreText)) {
sPreText = sPreText.split(/<hr.*?>/)[1];
}
var arRows = sPreText.split(/\n/);
var nRows = arRows.length;
var bOdd = true;
for (var i = 0; i < nRows; i++) {
var sRow = arRows[i];
sRow = sRow.replace(/^\s*|\s*$/g, '');
if (!sRow) { continue; }
if (/\<hr/.test(sRow)) { continue; }
var arTemp = sRow.split(/<\/a>/);
var sLink = arTemp[0] + '</a>';
if (/<img/.test(sLink)) {
sLink = sLink.split(/<img.*?>/)[1];
}
sRestOfLine = arTemp[1];
arRestOfCols = sRestOfLine.split(/\s+/);
var elmTR = document.createElement('tr');
var elmTD = document.createElement('td');
elmTD.innerHTML = sLink;
elmTR.appendChild(elmTD);
var iNumColumns = arRestOfCols.length;
var bRightAlign = false;
for (var j = 1 /* really */; j < iNumColumns; j++) {
var sColumn = arRestOfCols[j];
if (/\d\d:\d\d/.test(sColumn)) {
elmTD.innerHTML += ' ' + sColumn;
} else {
elmTD = document.createElement('td');
elmTD.innerHTML = arRestOfCols[j];
if (bRightAlign) {
elmTD.setAttribute('class', 'flushright');
}
elmTR.appendChild(elmTD);
}
bRightAlign = true;
}
while (iNumColumns <= iNumHeaders) {
elmTR.appendChild(document.createElement('td'));
iNumColumns++;
}
// zebra-stripe table rows, from
// http://www.alistapart.com/articles/zebratables/
// and http://www.alistapart.com/articles/tableruler/
elmTR.style.backgroundColor = bOdd ? '#eee' : '#fff';
elmTR.addEventListener('mouseover', function() {
this.className = 'ruled';
}, true);
elmTR.addEventListener('mouseout', function() {
this.className = '';
}, true);
elmTable.appendChild(elmTR);
bOdd = !bOdd;
}
// copy address footer -- probably a much easier way to do this,
// but it's not always there (depends on httpd.conf options)
var sFooter = document.getElementsByTagName('address')[0];
var elmFooter = null;
if (sFooter) {
elmFooter = document.createElement('address');
elmFooter.innerHTML = sFooter.innerHTML;
}
window.addEventListener('load',
function() {
document.body.innerHTML = '';
document.body.appendChild(elmTable);
if (elmFooter) {
document.body.appendChild(elmFooter);
}
},
true);
// now that everything is semantic and accessible,
// make it a little prettier too
addGlobalStyle(
'table {' +
' border-collapse: collapse;' +
' border-spacing: 0px 5px;' +
' margin-top: 1em;' +
' width: 100%;' +
'}' +
'caption {' +
' text-align: left;' +
' font-weight: bold;' +
' font-size: 180%;' +
' font-family: Optima, Verdana, sans-serif;' +
'}' +
'tr {' +
' padding-bottom: 5px;' +
'}' +
'td, th {' +
' font-size: medium;' +
' text-align: right;' +
'}' +
'th {' +
' font-family: Optima, Verdana, sans-serif;' +
' padding-right: 10px;' +
' padding-bottom: 0.5em;' +
'}' +
'th:first-child {' +
' padding-left: 20px;' +
'}' +
'td:first-child,' +
'td:last-child,' +
'th:first-child,' +
'th:last-child {' +
' text-align: left;' +
'}' +
'td {' +
' font-family: monospace;' +
' border-bottom: 1px solid silver;' +
' padding: 3px 10px 3px 20px;' +
' border-bottom: 1px dotted #003399;' +
'}' +
'td a {' +
' text-decoration: none;' +
'}' +
'tr.ruled {' +
' background-color: #88eecc ! important;' +
'}' +
'address {' +
' margin-top: 1em;' +
' font-style: italic;' +
' font-family: Optima, Verdana, sans-serif;' +
' font-size: small;' +
' background-color: transparent;' +
' color: silver;' +
'}');
Running the Hack
Before installing the user script, go to http://diveintomark.org/projects/greasemonkey/. There is no default page for this directory, so Apache automatically generates a plain-text directory listing, as shown in Figure 8-13.
Now, install the user script (Tools → Install This User Script) and refresh http://diveintomark.org/projects/greasemonkey/. The user script replaces the plain directory listing with an enhanced version, which contains a real table with alternating rows shaded, as shown in Figure 8-14.
Figure 8-13. Plain Apache directory listing
Plain Apache directory listing
When you hover over a file, the entire row is highlighted, as shown in Figure 8-15.
Also, when you hover over one of the column headers, you will see a tool tip explaining that you can click to sort the directory listing, as shown in Figure 8-16.
I've probably seen thousands of autogenerated directory listings, and it wasn't until I wrote this hack that I realized that you could click a column header to change the sort order. Usability matters!
Figure 8-14. Enhanced Apache directory listing
Enhanced Apache directory listing
Figure 8-15. Row highlighting
Row highlighting
Figure 8-16. Column sorting
Column sorting
Add a Text-Sizing Toolbar to Web Forms
Insert buttons before <textarea> elements to make the text larger or smaller.
I spend a lot of time—probably too much time—commenting on weblogs and web-based discussion forums. Despite several attempts to create some sort of universal commenting API, virtually all of these sites continue to use a simple web form with a <textarea> element for entering comments.
This hack alters web forms to add a toolbar above every <textarea> element. The toolbar lets you increase or decrease the text size of the <textarea>, without changing the style of the rest of the page. The buttons are fully keyboard-accessible; you can tab to them and press Enter instead of clicking them with your mouse.
Tip
I mention this up front, because accessibility matters, and also because it was harder than it sounds.
The Code
This user script runs on all pages. The code looks complicated, and it is complicated, but not for the reason you think. It looks complicated because of the large multiline gibberish-looking strings in the middle of it. Those are data: URIs, which look like hell but are easy to generate.(See "Embed Graphics in a User Script" [Hack #11] for more on data: URIs.)
The toolbar is displayed visually as a row of buttons, but each button is really just an image of something that looks pushable, wrapped in a link that executes one of our JavaScript functions. Since we'll be creating more than one button (this script has only two, but you could easily extend it with more functionality), I created a function to encapsulate all the button-making logic:
function createButton functioncreateButton(target, func, title, width, height, src)
The createButton function takes six arguments:
target
An element object; the <textarea> element that this button will control.
func
A function object; the JavaScript function to be called when the user clicks the button with the mouse or activates it with the keyboard.
title
A string; the text of the tool tip when the user moves her cursor over the button.
width
An integer; the width of the button. This should be the width of the graphic given in the src argument.
height
An integer; the height of the button. This should be the height of the graphic given in the src argument.
src
A string; the URL, path, or data: URI of the button graphic.
Creating the image is straightforward, but creating the link that contains the image is where the real complexity lies:
button = document.createElement('a');
button._target = target;
button.title = title;
button.href = '#';
button.onclick = func;
button.appendChild(img);
There are two things I want to point out here. First, I need to assign a bogus href attribute to the link; otherwise, Firefox would treat it as a named anchor and wouldn't add it to the tab index (i.e., you wouldn't be able to tab to it, making it inaccessible with the keyboard). Second, I'm setting the _target attribute to store a reference to the target <textarea>. This is perfectly legal in JavaScript; you can create new attributes on an object just by assigning them a value. I'll access the custom _target attribute later, in the onclick event handler.
If you read Mozilla's documentation on the Event object, you'll see that there are several target-related properties, including one simply called target. You might be tempted to use event.target to get a reference to the clicked link, but it behaves inconsistently. When the user tabs to the button and presses Enter, event.target is the link, but when the user clicks the button with the mouse, event.target is the image inside the link! In any case, event.currentTarget returns the link in all cases, so I use that.
Tip
See http://www.xulplanet.com/references/objref/event.html for documentation on the Event object.
Now the real fun begins. (And you thought you were having fun already!) I need to get the current dimensions and font size of the <textarea> so that I can make them bigger. Simply retrieving the appropriate attributes from textarea.style (textarea.style.width, textarea.style.height, and textarea.style.fontSize) will not work, because those only get set if the page actually defined them in a style attribute on the <textarea> itself. That's not what I want; I want the final style, after all stylesheets have been applied. For that, I need getComputedStyle:
s = getComputedStyle(textarea, "");
textarea.style.width = (parseFloat(s.width) * 1.5) + "px";
textarea.style.height = (parseFloat(s.height) * 1.5) + "px";
textarea.style.fontSize = (parseFloat(s.fontSize) + 7.0) + 'px';
Finally, do you remember that bogus href value I added to my button link to make sure it was keyboard-accessible? Well, it's now become an annoyance, because after Firefox finishes executing the onclick handler, it's going to try to follow that link. Since it points to a nonexistent anchor, Firefox is going to jump to the top of the page, regardless of where the button is. This is annoying, and to stop it, I need to call event.preventDefault() before finishing my onclick handler:
event.preventDefault();
All this was just for the sake of keyboard accessibility. What can I say? Some people build model airplanes. I build accessible web pages.
Save the following user script as zoomtextarea.user.js:
// ==UserScript==
// @name Zoom Textarea
// @namespace http://diveintomark.org/projects/greasemonkey/
// @description add controls to zoom textareas
// @include *
// ==/UserScript==
function addEvent(oTarget, sEventName, fCallback, bCapture) {
var bReturn = false;
if (oTarget.addEventListener) {
oTarget.addEventListener(sEventName, fCallback, bCapture);
bReturn = true;
} else if (oTarget.attachEvent) {
bReturn = oTarget.attachEvent('on' + sEventName, fCallback);
}
return bReturn;
}
function createButton(elmTarget, funcCallback, sTitle, iWidth, iHeight, urlSrc) {
var elmImage = document.createElement('img');
elmImage.width = iWidth;
elmImage.height = iHeight;
elmImage.style.borderTop = elmImage.style.borderLeft = "1px solid #ccc";
elmImage.style.borderRight = elmImage.style.borderBottom = "1px solid #888";
elmImage.style.marginRight = "2px";
elmImage.src = urlSrc;
var elmLink = document.createElement('a');
elmLink.title = sTitle;
elmLink.href = '#';
addEvent(elmLink, 'click', funcCallback, true);
elmLink.appendChild(elmImage);
return elmLink;
}
var arTextareas = document.getElementsByTagName('textarea');
for (var i = arTextareas.length - 1; i >= 0; i--) {
var elmTextarea = arTextareas[i];
function textarea_zoom_in(event) {
var style = getComputedStyle(elmTextarea, "");
elmTextarea.style.width = (parseFloat(style.width) * 1.5) + "px";
elmTextarea.style.height = (parseFloat(style.height) * 1.5) + "px";
elmTextarea.style.fontSize = (parseFloat(style.fontSize) + 7.0) +
'px';
event.preventDefault();
}
function textarea_zoom_out(event) {
var style = getComputedStyle(elmTextarea, "");
elmTextarea.style.width = (parseFloat(style.width) * 2.0 / 3.0) +
"px";
elmTextarea.style.height = (parseFloat(style.height) * 2.0 / 3.0) +
"px";
elmTextarea.style.fontSize = (parseFloat(style.fontSize) - 7.0) +
"px";
event.preventDefault();
}
elmTextarea.parentNode.insertBefore(
createButton(
elmTextarea,
textarea_zoom_in,
'Increase text size',
20,
20,
'data:image/gif;base64,'+
'R0lGODlhFAAUAOYAANPS1tva3uTj52NjY2JiY7KxtPf3%2BLOys6WkpmJiYvDw8fX19vb'+
'296Wlpre3uEZFR%2B%2Fv8aqpq9va3a6tr6Kho%2Bjo6bKytZqZml5eYMLBxNra21JSU3'+
'Jxc3RzdXl4emJhZOvq7KamppGQkr29vba2uGBgYdLR1dLS0lBPUVRTVYB%2Fgvj4%2BYK'+
'Bg6SjptrZ3cPDxb69wG1tbsXFxsrJy29vccDAwfT09VJRU6uqrFlZW6moqo2Mj4yLjLKy'+
's%2Fj4%2BK%2Busu7t783Nz3l4e19fX7u6vaalqNPS1MjHylZVV318ftfW2UhHSG9uccv'+
'KzfHw8qqqrNPS1eXk5tvb3K%2BvsHNydeLi40pKS2JhY2hnalpZWlVVVtDQ0URDRJmZm5'+
'mYm11dXp2cnm9vcFxcXaOjo0pJSsC%2FwuXk6AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA'+
'AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAC'+
'H5BAAAAAAALAAAAAAUABQAAAeagGaCg4SFhoeIiYqKTSQUFwgwi4JlB0pOCkEiRQKKRxM'+
'gKwMGDFEqBYpPRj4GAwwLCkQsijwQBAQJCUNSW1mKSUALNiVVJzIvSIo7GRUaGzUOPTpC'+
'igUeMyNTIWMHGC2KAl5hCBENYDlcWC7gOB1LDzRdWlZMAZOEJl83VPb3ggAfUnDo5w%2F'+
'AFRQxJPj7J4aMhYWCoPyASFFRIAA7'),
elmTextarea);
elmTextarea.parentNode.insertBefore(
createButton(
elmTextarea,
textarea_zoom_out,
'Decrease text size',
20,
20,
'data:image/gif;base64,'+
'R0lGODlhFAAUAOYAANPS1uTj59va3vDw8bKxtGJiYrOys6Wkpvj4%2BPb29%2FX19mJiY'+
'%2Ff3%2BKqqrLe3uLKytURDRFpZWqmoqllZW9va3aOjo6Kho4KBg729vWJhZK%2BuskZF'+
'R4B%2FgsLBxHNydY2Mj%2Ff396amptLS0l9fX9fW2dDQ0W1tbpmZm8DAwfT09fHw8n18f'+
'uLi49LR1V5eYOjo6VBPUa6tr769wEhHSNra20pJStPS1KuqrNPS1ZmYm%2B7t77Kys8rJ'+
'y%2Fj4%2BaSjpm9uca%2BvsMjHyqalqHRzdVJRU8PDxVRTVcvKzc3Nz0pKS9rZ3evq7MC'+
'%2FwsXFxp2cnnl4e1VVVu%2Fv8ba2uM7Oz29vcbu6vZqZmnJxc9vb3PHx8uXk5mhnamJh'+
'Y1xcXZGQklZVV29vcHl4eoyLjKqpq6Wlpl1dXuXk6AAAAAAAAAAAAAAAAAAAAAAAAAAAA'+
'AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA'+
'AAACH5BAAAAAAALAAAAAAUABQAAAeZgGaCg4SFhoeIiYqKR1IWVgcyi4JMBiQqA0heQgG'+
'KQTFLPQgMCVocBIoNNqMgCQoDVReKYlELCwUFI1glEYorOgopWSwiTUVfih8dLzRTKA47'+
'Ek%2BKBGE8GEAhFQYuPooBOWAHY2ROExBbSt83QzMbVCdQST8Ck4QtZUQe9faCABlGrvD'+
'rB4ALDBMU%2BvnrUuOBQkE4NDycqCgQADs%3D'),
elmTextarea);
elmTextarea.parentNode.insertBefore(
document.createElement('br'),
elmTextarea);
}
Running the Hack
After installing the user script (Tools → Install This User Script), go to http://philringnalda.com/blog/2005/06/ms_embraces_rss.php. At the bottom of the page is a form for submitting comments. Above the form, you will see two buttons inserted by the user script, as shown in Figure 8-17.
Type some text into the <textarea>, then click the first button (titled "Increase text size") to make the text and the <textarea> larger, as shown in Figure 8-18. Alternatively, while focus is in the <textarea>, you can press Shift-Tab twice to set focus to the Zoom In button, and then press Enter to activate the button.
Click the second button, titled "Decrease text size," to make the text smaller. Due to rounding, if you repeatedly zoom in and then repeatedly zoom out, the text and its surrounding box may end up a slightly different size. The zooming is not permanent, so you can refresh the page to return to the original size.
Figure 8-17. Zoom toolbar in web form
Zoom toolbar in web form
Figure 8-18. Zoomed web form
Zoomed web form
Make Google More Accessible for Low-Vision Users
Change Google's layout to make it easier for low-vision users to read.
As a class of disabilities, low-vision users are often ignored by accessibility experts. However, accessibility expert Joe Clark has recently published his research into the needs of web users with limited vision. He pioneered a technique known as the zoom layout: a special alternate style applied to a web page that specifically caters to low-vision users.
As I was learning about zoom layouts, it occurred to me that this would be a perfect application of Greasemonkey. (Actually, that thought occurs to me a lot these days.) This hack is my first attempt at transforming a site into a zoom layout.
The Code
This user script runs on several specific Google pages:
Tip
This hack is written to be cross-browser compatible. It works in Firefox with Greasemonkey, in Internet Explorer 6 for Windows with Turnabout, and in Opera 8 with its built-in support for User JavaScript. You can download Turnabout at http://reifysoft.com/turnabout.php, and Opera at http://www.opera.com.
Save the following user script as zoom-google.user.js:
// ==UserScript==
// @name Zoom Google
// @namespace http://diveintomark.org/projects/greasemonkey/
// @description make Google more accessible to low-vision users
// @include http://www.google.tld/
// @include http://www.google.tld/?*
// @include http://www.google.tld/webhp*accessibilityGoogle
// @include http://www.google.tld/imghp*
// @include http://www.google.tld/search*
// @include http://images.google.tld/
// @include http://images.google.tld/?*
// @include http://images.google.tld/images*
// ==/Googlelow-vision usersUserScript==
function addGlobalStyle(css) {
var elmHead, elmStyle;
elmHead = document.getElementsByTagName('head')[0];
elmStyle = document.createElement('style');
elmStyle.type = 'text/css';
elmHead.appendChild(elmStyle);
elmStyle.innerHTML = css;
}
function getElementsByClassName(sTag, sClassName) {
sClassName = sClassName.toLowerCase() + ' ';
var arElements = document.getElementsByTagName(sTag);
var iMax = arElements.length;
var arResults = new Array();
for (var i = 0; i < iMax; i++) {
var elm = arElements[i];
var sThisClassName = elm.className;
if (!sThisClassName) { continue; }
sThisClassName = sThisClassName.toLowerCase() + ' ';
if (sThisClassName.indexOf(sClassName) != -1) {
arResults.push(elm);
}
}
return arResults;
}
function removeFontTags() {
// remove font tags
var arFonts = document.getElementsByTagName('font');
for (var i = arFonts.length - 1; i >= 0; i--) {
var elmFont = arFonts[i];
var elmSpan = document.createElement('span');
elmSpan.innerHTML = elmFont.innerHTML;
elmFont.parentNode.replaceChild(elmSpan, elmFont);
}
}
function zoomStyle() {
addGlobalStyle('body { margin: 30px; } \n' +
'body, td { font-size: large ! important; } \n' +
'html>body, html>body td { font-size: x-large ! important; } \n' +
'body, div, td { background: navy ! important; ' +
'color: white ! important; } \n' +
'a:link { background: transparent ! important; ' +
'color: yellow ! important; } \n' +
'a:visited { background: transparent ! important; ' +
'color: lime ! important; } \n' +
'a.fl { background: transparent ! important; ' +
'color: white ! important; } \n' +
'input { font-size: large ! important; } \n' +
'html>body input { font-size: x-large ! important; } \n' +
'.g { width: auto ! important; } \n' +
'.n a, .n .i { font-size: large ! important; } \n' +
'html>body .n a, html.body .n .i { font-size: x-large ! important; } \n' +
'.j { width: auto ! important; }');
}
function accHomePage() {
// remove personalized header, if any
var arTable = document.getElementsByTagName('table');
for (var i = arTable.length - 1; i >= 0; i--) {
var elmTable = arTable = ar
var html = elmTable.innerHTML;
if (/\/accounts\/Logout/.test(html)) {
elmTable.parentNode.removeChild(elmTable);
}
}
// simplify logo
var arImages = document.getElementsByTagName('img');
for (var i = arImages.length - 1; i >= 0; i--) {
var elmLogo = arImages[i];
if (elmLogo.alt) {
var elmTextLogo = document.createElement('h1');
elmTextLogo.style.fontSize = '400%';
var sAlt = /Firefox/.test(elmLogo.alt) ? '' : elmLogo.alt;
elmTextLogo.appendChild(document.createTextNode(sAlt));
elmLogo.parentNode.replaceChild(elmTextLogo, elmLogo);
var elmLink = elmTextLogo.parentNode;
while (elmLink.nodeName != 'BODY' &&
elmLink.nodeName != 'HTML' &&
elmLink.nodeName != 'A') {
elmLink = elmLink.parentNode;
}
elmLink.style.textDecoration = 'none';
} else {
elmLogo.parentNode.removeChild(elmLogo);
}
}
// simplify search form
if (document.forms.length) {
var arTD = document.getElementsByTagName('td');
for (var i = arTD.length - 1; i >= 0; i--) {
var elmTD = arTD[i];
if (/Advanced/.test(elmTD.innerHTML)) {
elmTD.innerHTML = '';
}
}
}
}
function accSearchResults() {
// simplify logo
var elmLogo = document.getElementsByTagName('img')[0];
var elmTextLogo = document.createElement('h1');
elmTextLogo.appendChild(document.createTextNode('accessibilityGoogleGoogle'));
elmTextLogo.style.marginTop = '0.2em';
elmTextLogo.style.marginRight = '0.3em';
elmLogo.parentNode.replaceChild(elmTextLogo, elmLogo);
elmTextLogo.parentNode.style.textDecoration = 'none';
// simplify top form
var elmAdvancedWrapper = document.getElementsByTagName('table')[3];
var elmAdvanced = elmAdvancedWrapper.getElementsByTagName('td')[1];
elmAdvanced.parentNode.removeChild(elmAdvanced);
// remove "tip" if present
var elmTip = document.getElementsByTagName('table')[7];
if (/Tip/.test(elmTip.innerHTML)) {
elmTip.parentNode.removeChild(elmTip);
}
// remove ads, if any
var aw1 = document.getElementById('aw1');
while (aw1) {
var table = aw1.parentNode;
while (table.nodeName != 'TABLE') {
table = table.parentNode;
}
table.parentNode.removeChild(table);
aw1 = document.getElementById('aw1');
}
var tpa1 = document.getElementById('tpa1');
if (tpa1) {
while (tpa1.nodeName != 'DIV' && tpa1.nodeName != 'P') {
tpa1 = tpa1.parentNode;
}
tpa1.parentNode.removeChild(tpa1);
}
var tpa2 = document.getElementById('tpa2');
if (tpa2) {
while (tpa2.nodeName != 'DIV' && tpa2.nodeName != 'P') {
tpa2 = tpa2.parentNode;
}
tpa2.parentNode.removeChild(tpa2);
}
addGlobalStyle('iframe[name="google_ads_frame"] { ' +
'display: none ! important }');
// simplify results count
var elmDivider = document.getElementsByTagName('table')[5];
elmDivider.parentNode.removeChild(elmDivider);
var elmResultsContainer = document.getElementsByTagName('table')[5];
var arTD = elmResultsContainer.getElementsByTagName('td');
if (arTD.length > 1) {
var sResults = arTD[1].textContent;
var iParen = sResults.indexOf('(');
if (iParen != -1) {
sResults = sResults.substring(0, iParen);
}
var iDef = sResults.indexOf('[');
if (iDef != -1) {
sResults = sResults.substring(0, iDef);
}
var elmResults = document.createElement('h2');
elmResults.appendChild(document.createTextNode(sResults));
elmResultsContainer.parentNode.replaceChild(elmResults,
elmResultsContainer);
} else {
elmResultsContainer.parentNode.removeChild(elmResultsContainer);
}
// make search results use real headers
var arResults = getElementsByClassName('p', 'g');
for (var i = arResults.length - 1; i >= 0; i--) {
var elmResult = arResults[i];
var arLink = elmResult.getElementsByTagName('a');
if (!arLink.length) { continue; }
var elmLink = arLink[0];
var elmWrapper = document.createElement('div');
var elmHeader = document.createElement('h3');
elmHeader.style.margin = elmHeader.style.padding = 0;
elmHeader.innerHTML = '<a href="' + elmLink.href + '">' +
elmLink.innerHTML + '</a>';
var elmContent = elmResult.cloneNode(true);
elmContent.innerHTML = elmContent.innerHTML.replace(/<nobr>/g, '');
arLink = elmContent.getElementsByTagName('a');
if (!arLink.length) { continue; }
elmLink = arLink[0];
elmContent.removeChild(elmLink);
elmContent.style.marginTop = 0;
elmWrapper.appendChild(elmHeader);
elmWrapper.appendChild(elmContent);
elmResult.parentNode.replaceChild(elmWrapper, elmResult);
}
// simplify next page link
var arFont = document.getElementsByTagName('font');
for (var i = arFont.length - 1; i >= 0; i--) {
var elmFont = arFont[i];
var html = elmFont.innerHTML;
if (/Result\ \;Page\:/.test(html)) {
var elmTable = elmFont.parentNode;
while (elmTable.nodeName != 'TABLE') {
elmTable = elmTable = elm
}
var arTD = elmTable.getElementsByTagName('td');
if (arTD.length) {
var elmTD = arTD[arTD.length - 1];
var arNext = elmTD.getElementsByTagName('a');
if (arNext.length) {
var elmNext = arNext[0];
var elmTextNext = document.createElement('center');
elmTextNext.innerHTML = '<p style="font-size: ' +
'xx-large; margin-bottom: 4em;">b><a href="' +
elmNext.href + '">More Results ' +
'→</a></b></p>';
elmTable.parentNode.replaceChild(elmTextNext,
elmTable);
}
}
break;
}
}
// remove bottom ads
var arCenter = document.getElementsByTagName('center');
if (arCenter.length > 1) {
var elmCenter = arCenter[1];
elmCenter.parentNode.removeChild(elmCenter);
elmCenter = arCenter[0];
for (var i = 0; i < 4; i++) {
elmCenter.innerHTML = elmCenter.innerHTML.replace(/<br>/, '');
}
}
}
document.forms.namedItem('f') && accHomePage();
document.forms.namedItem('gs') && accSearchResults();
removeFontTags();
zoomStyle();
Running the Hack
After installing the user script (Tools → Install This User Script), go to http://www.google.com. The normally spartan search form has been magnified and simplified even further, as shown in Figure 8-19.
Accessibility studies have shown that low-vision users have an easier time reading light text on a dark background, so therefore the page is displayed as white-on-navy. Unvisited links are displayed in yellow; visited links are displayed in light green. The hack removes several elements from the page, including the Advanced Search link, plus any advertisements for Google services or other messages that occasionally appear below the search box.
Figure 8-19. Google home page, zoomed
Google home page, zoomed
When you execute a search, the search results are displayed differently, as shown in Figures 8-20 and 8-21, with the following notable differences:
• The entire page uses the same white-on-navy color scheme we used on the home page.
• The Google logo in the top-left corner is displayed as plain text instead of as an image.
• The top search form no longer includes the Advanced Search option.
• The sponsored links along the top and right are gone.
• The number of results is displayed much larger than before, and in the same white-on-navy color scheme.
• Links to search results pages are displayed in yellow (or green, if you've already visited that page). Other links within each search result, such as the "Cached" and "Similar pages" links, are displayed in white.
• The "Goooooooogle" navigation bar to see more results is replaced by a simple link titled "More results."
• The search box at the bottom of the page is gone.
Figure 8-20. Google search results, zoomed
Google search results, zoomed
Figure 8-21. Bottom of Google search results, zoomed
Bottom of Google search results, zoomed
If you click the Images link at the top of the page to search for the same keywords in Google Image Search, you will see that the image search results have been similarly hacked, as shown in Figure 8-22.
Figure 8-22. Google image results, zoomed
Google image results, zoomed
As with the web search results, the top navigation has been simplified, the number of results is more prominent, and the "Goooooooogle" navigation bar has been replaced by a single "More results" link that moves to the next page of images. The image thumbnails themselves cannot be magnified, since Google provides them only in a specific size.
Notes
1. Houtenville, Andrew J. "Disability Statistics in the United States." Ithaca, NY: Cornell University Rehabilitation Research and Training Center on Disability Demographics and Statistics (StatsRRTC), http://www.disabilitystatistics.org, April 4, 2005.
Personal tools
|
__label__pos
| 0.917992 |
show with app
function(input, output) {
# submit buttons do not have a value of their own,
# they control when the app accesses values of other widgets.
# input$num is the value of the number widget.
output$value <- renderPrint({ input$num })
}
fluidPage(
numericInput("num", label = "Make changes", value = 1),
# Copy the line below to place a submit into the UI.
submitButton("Apply Changes"),
hr(),
fluidRow(column(3, verbatimTextOutput("value")))
)
|
__label__pos
| 0.978001 |
Difference between revisions of "SVG Animation"
From Inkscape Wiki
Jump to navigation Jump to search
m
Line 7: Line 7:
Animation in SVG is defined in the Animation Module of the [http://www.w3.org/TR/SVG11/animate.html SVG specification]. Animation in SVG conforms to the [http://www.w3.org/TR/2001/REC-smil-animation-20010904/ SMIL specification] (Synchronized Multimedia Integration Language) and extends it (SVG is said to be a ''host'' language of SMIL).
Animation in SVG is defined in the Animation Module of the [http://www.w3.org/TR/SVG11/animate.html SVG specification]. Animation in SVG conforms to the [http://www.w3.org/TR/2001/REC-smil-animation-20010904/ SMIL specification] (Synchronized Multimedia Integration Language) and extends it (SVG is said to be a ''host'' language of SMIL).
Here I only focus on procedural animation and not programmatic animation (using a scripting language manipulating the DOM).
Here I only focus on procedural animation as defined by SMIL and not free-form scripted animation (using e.g. Javascript to manipulate the DOM).
In SMIL an animation is an object which defines the evolution of a given attribute of a given object (here an SVG element - path, rect, group...) over time. Multiple animations can control the evolution of particular attribute: for example, one will tell the object to turn for 1 second starting at t=0 second and another to move across the document for 2 seconds starting at a later time. So we have this relation:
In SMIL an animation is an object which defines the evolution of a given attribute of a given object (here an SVG element - path, rect, group...) over time. Multiple animations can control the evolution of particular attribute: for example, one will tell the object to turn for 1 second starting at t=0 second and another to move across the document for 2 seconds starting at a later time. So we have this relation:
Revision as of 13:07, 29 March 2008
This page is a work in progress. ToF
See also the UI ideas page (specific to SMIL animation).
SVG Animation
Animation in SVG is defined in the Animation Module of the SVG specification. Animation in SVG conforms to the SMIL specification (Synchronized Multimedia Integration Language) and extends it (SVG is said to be a host language of SMIL).
Here I only focus on procedural animation as defined by SMIL and not free-form scripted animation (using e.g. Javascript to manipulate the DOM).
In SMIL an animation is an object which defines the evolution of a given attribute of a given object (here an SVG element - path, rect, group...) over time. Multiple animations can control the evolution of particular attribute: for example, one will tell the object to turn for 1 second starting at t=0 second and another to move across the document for 2 seconds starting at a later time. So we have this relation:
1:n
Element <-------> Animation
For a given animation the element which is controlled is called the target element of the animation. The specific attribute/property which is controlled is called the target attribute/property.
In fact, it's completely legal to have two animations that control the same attribute of the same element, and they can overlap in time: at any particular time there exists a stack of animations applying to a particular attribute, and SMIL defines the order in which each animation is applied.
Animation elements
SVG defines 5 animation elements:
• animate
This element is used to animate any scalar attribute or property, such as the width attribute of an svg:rect element or the CCS property opacity
• animateMotion
This elements only controls the position of a SVG element, by telling it to follow a given path
• animateColor
This element controls the animation of color valued attributes and properties.
• animateTransform
This element controls the transformation which applies to the target element. It replaces or adds up to the transform attribute of the element.
• set
This element is used to set a particular value for the target attribute during a specified time interval. It is mainly used for animating attributes which do not span a continuous domain, such as boolean values like the visibility property.
Specifying the target element and property
There are 2 ways to tell which element a given animation will target. The first one is to embed the animation element as a child tag of the element to animate. Here is an example:
<circle cx="100px" cy="100px" r="20px">
<animate attributeName="r" attributeType="XML" begin="1s" dur="2s" from="20px" to="50px">
</circle>
The animate element targets the radius (r) property of the parent circle element. Starting 1 second after the document start, the radius will grow for 2 seconds from 20 pixels to 50 pixels. When nothing else is specified, the animation only applies in its interval of time, here [1-3] seconds; from 0 to 1 second and then from 3 second and after, the circle's radius is set back to its static attribute value, here 20 pixels.
The other way to specify the target element is to reference it in via an xlink:href attribute of the animation element. the previous animation could be written this way:
<circle id="balloon" cx="100px" cy="100px" r="20px" />
<animate xlink:href="#balloon" attributeName="r" attributeType="XML" begin="1s" dur="2s" from="20px" to="50px" />
To specify which property or attribute of the target element is controlled, we use the two attributes attributeName and attributeType. attributeName explains itself, it's the name of the controlled attribute. attributeType tells whether attributeName is searched among the SVG attributes of the element (attributeType="XML") or among the CSS properties of the element (attributeType="CSS"). (SPEC)
Timing an animation
The first thing to stress is that SMIL (and therefore SVG) animation is time-based, as opposed to frame-based. Frame-based animation is something traditional animators are used to, because they actually draw each image of the animation on a separate celluloid sheet, and the timing is related to the rate of images per second the camera will capture. When animation is done on a computer, the animator doesn't draw every image but only some key images (or keyframes) and then the animation software creates all the necessary images for him (the so-called in-betweens). That means the animator can focus back to a (real-)time based timing. Each keyframe is positioned on a timeline with real time values (instead of frame numbers). Of course at the end the animation is discrete and there is a fixed number of images computed inbetween two keyframes, but the neat property of specifying keyframes at time position is that the final animation frame rate is scalable and the animator doesn't have to care about it anymore. He just tells the software to produce 24 images each second if he targets a theatre film or say 10 images per second for low-bandwidth web animation. The animator doesn't need to reposition the keyframes.
In-betweening by interpolation
Creating the inbetween images on a computer is done by mean of interpolation. Interpolation is a mathematical tool that can reconstruct a the continuous evolution of a parameter given a set of discrete values of this parameter over time. The following image illustrates that. We know that some attribute A must take value A0, A1, A2 and A3 at times t0, t1, t2 and t3. The red curves is one of the possible continuous function that be used to compute inbetween values of A in the interval [t0, t3].
Interpolation.png
• The animation's time interval
The first aspect that characterizes an animation is the interval of time on which it applies: when does it begin, when does it end or how long does it run. Any of the above mentioned [[#Animation elements|animation elements] accept the attributes begin, end and dur.
There are many ways to define a particular point in time and this is what makes SMIL a very rich animation framework (see below). The simplest is to specify a clock-value which syntax is quite intuitive (SPEC). Depending on the combination of those 3 attributes, the interval of time of the animation will be [begin, end], [begin, begin+dur], [end-dur, end], etc.
• The attribute values
To specify the values taken by the attribute over the time interval, one need to tell the value that the parameter must have at the begin and at the end of the interval. here again those values are specified by combinations of three attributes: from, to and by. The attribute from correspond to the begin value, the attribute to correspond to the end value. The attribute by let you define either the initial and final values relative to the other one, that is to = from + by or from = to - by. The following image illusttrate this setup for our hypothetical attribute A
Time interval.png
The values of A are simply interpolated linearly in between begin and end. This is the more simple form of animation function that can defined: by the two extremal pairs of (time, attribute value): (begin, from) and (end, to).
More complex timing
It's possible to specify a whole set of (keytime, keyvalue) pairs to define the animation function. Each item of the pairs are actually given separately:
• the set of attribute values (keyvalues) are given by the values attribute.
• the set of time values (keytimes) are given by the keyTimes attribute.
Each set as the form of a semicolon-separated list of values. The thing to notice is that the keytimes are actually normalized to [0,1], which means that 0 maps to begin and 1 maps to end, and those values must start and end the list of keytimes. The image below illustrates all this:
Time interval2.png
The keyTimes list can actually be omitted, in which case the keytimes are evenly distributed in the interval [0,1] (or equivalently, evenly in the interval [begin, end])
The (keytime, keyvalue) pairs are joined by straight lines in the above graph, which means that linear interpolation is used for computing the inbetween values.
When applied to the position of an object, linear interpolation results in un-natural robot-like motions (which may be the effect wanted). SMIL allows smoother animations to be defined by using spline-based interpolation between two (keytime, keyvalue) pairs. For each interval (t0, A0) - (t1, A1) a spline can be defined by two control points defining the tangents of the spline at t0 and t1. Those control points are passed in the attribute keySplines.
Spline interpolation.png
Two other interpolation modes are possible:
• discrete: this is a degenerate form of interpolation, because the resulting animation function is not continuous. For each interval (t0,A0) - (t1, A1), the attribute value is set to A0 for the whole interval. It jumps to A1 right when t1 is reached and so on.
• paced: in this interpolation mode, the attribute is interpolated so that its evolution over time has constant speed. Because the animation function is defined implicitly, the attribute keyTimes is not taken into account. This mode is mainly useful for the animateMotion element.
Other aspects of animation
• repeating
• chaining animations.
• event based synchronisation
Examples
Examples showcasing some of the effects that can be achieved using procedural animation in SVG.
A ball bounces on the floor: there are 6 motions here: fall, squeeze, squeeze-offset and bounce unsqueeze and unsqueeze-offset that are the revert version of 3 first. A classic animation trick is to start squeezing the ball a little before it touches the floor, expressed here by the synchronization begin=fall.end - 0.1s.
<path
...
transform="translate(0,4.5456868)">
<animateTransform
type="translate"
additive="sum"
from="0, 0"
to="0, 136"
dur="1s"
begin="0s"
attributeType="XML"
attributeName="transform"
id="fall"
keySplines="0.5 0 0.8 0.5 "
calcMode="spline" />
<animateTransform
id="squeeze"
type="scale"
from="1,1"
to="1.1,0.9"
begin="fall.end-0.1s"
attributeName="transform"
attributeType="XML"
dur="0.2s"
additive="sum" />
<animateTransform
additive="sum"
dur="0.2s"
attributeType="XML"
attributeName="transform"
begin="fall.end-0.1s"
to="-13,0"
from="0,0"
type="translate"
id="squeeze-offset"
accumulate="sum" />
Bouncing ball1.png SVG file | ogg video
A ball bounces between two wall, following a path.
<path
...
sodipodi:type="arc"
id="ball"
...
>
<animateMotion
keyTimes="0; 0.20; 1"
keyPoints="0; 0.33; 1"
repeatCount="indefinite"
additive="sum"
rotate="0"
begin="0s"
dur="2s"
calcMode="linear"
id="animateMotion2680">
<mpath
xlink:href="#motion_path"
id="mpath2682" />
</animateMotion>
</path>
<path
d="M 82.080559,103.82375 ..."
id="motion_path" ... />
Bouncing ball2.png SVG file | ogg video
A ball bounces between 2 walls. The motion is defined as a combination a of vertical translation and a horizontal translation. This works because the second animation has an additive="sum" attribute, so the corresponding matrix is composited with the matrix defined by the underlying animations.
...
<!-- two animation sharing the same path -->
<animateMotion
id="animateMotion2302"
xlink:href="#arrow2"
dur="4s"
keyTimes="0; 0.5; 1"
keyPoints="0; 1; 0"
calcMode="linear"
repeatCount="indefinite">
<mpath
id="mpath2305"
xlink:href="#horiz-bounce" />
</animateMotion>
<animateMotion
calcMode="linear"
keyPoints="0; 1; 0"
keyTimes="0; 0.5; 1"
dur="4s"
xlink:href="#ball"
id="animateMotion2309"
repeatCount="indefinite">
<mpath
xlink:href="#horiz-bounce"
id="mpath2311" />
</animateMotion>
...
<animateMotion
keySplines="0.5 0 0.75 0.75, 0 0.5 0.5 0.75"
keyPoints="0; 1; 0"
calcMode="spline"
keyTimes="0; 0.5; 1"
id="animateMotion2298"
begin="0s"
dur="2s"
xlink:href="#ball"
additive="sum"
repeatCount="indefinite">
<mpath
id="mpath2300"
xlink:href="#vert-bounce" />
</animateMotion>
Bouncing ball3.png SVG file | ogg video
Useful tools
• Squiggle , as part of Batik 1.7 has a nearly complete support for SMIL Animation. The examples were all rendered using Squiggle.
• Inkscape ! I used the xml editor to manually add the animation elements for my test. It's a bit more cumbersome than editing the svg file in a text editor, but doing that in Inkscape eases the computation of key positions (just move the object around and read the position values).
Various thoughts
• There is no direct way to specify a ping-pong animation in SVG, that is to tell an animation to go back in time when it reaches it's end. To implement this, I see 2 solutions:
• double the duration d of the animation (dur attribute) et apply a scale and a symetry around t=0.5 to the animation function. For example the simple linear interpolation defined by keyTimes="0; 1" and keyPoints="0;1" becomes keyTimes="0; 0.5; 1" and keyPoints="0;1;0". This doesn't work for paced animation though (which make use of an implicit, user-agent dependent animation function).
• copy the original animation (call it e.g. motion_forward) and swap the from and to values in the copy. Then specify begin="motion_forward.end" to tell the backward animation to start right at the end of the forward animation. This should work for every animation type.
The question is: what maps best to a UI ?
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Home » How to Unban Someone on Discord?
How to Unban Someone on Discord?
Do you know how to unban someone on Discord? In this article, today we will discuss all about these. Keep reading to know how to unban someone on Discord.
Discord is a great social application that allows you to participate in text, video, and voice chats across different servers to connect with people who share your interests. But, while it’s a great platform for lively discussions, you run the risk of getting into online discussions. This can get you blocked from accessing Discord.
There are two different types of bans on Discord. The first is a ban by the server administrator, which means you are banned on that server but can access all other servers. The second is an all-discord ban, where you can’t access any Discord services. But we need to know how to unban someone on discord accessing Discord.
You probably know why you have been banned. Usually, it is because you have violated Discord’s terms of use. It is important to know that not all bans are permanent. In fact, many of them are short-lived.
Fortunately, there are several ways to bypass IP blocking and access Discord. The most effective way is to use a VPN. Here’s how:
Sometimes, it can be an unfortunate necessity through banning users from your Discord server to create a friendly atmosphere for the rest members of your servers and remove offenders or the disliked people.
However, sometimes you have to be lenient and if someone has shown remorse and you believe they have mended their ways, or simply if you were too quick with the ban hammer and accidentally banned the wrong person, you can always lift the ban in retrospect.
Several administration or moderation bots can remove people more efficiently for large servers, such as the Carl bot or Mee6, but in the following steps, we will simply show you the basic method how to unban someone on discord without using a bot.
What happens if you are banned from Discord?
If you have been banned from Discord, you will no longer be able to access a particular server or all servers, depending on the type of ban you received. This means that if you log in with the same account, you will not be able to access anything on the service. So, you must know how to unban someone on discord.
Unlike other platforms, bans on Discord are not limited to blacklisting your account, as the application also applies IP blocking. This means that Discord recognizes your IP address and bans you even if you change your username or use another account. The ban also applies if you create a new Discord account, as you cannot do so because your IP address remains the same.
The main reason Discord users are banned?
• Sending unsolicited messages or creating unsolicited accounts
• Send repeated friend requests or messages
• Conduct or participate in spam raids or irregular activities.
• Sharing the author’s content in the media
• Posting hateful, suicidal, harmful or extortionate material.
• Channels cannot be marked as “Not Safe for Work” or send NSFW messages to SFW servers.
• Sharing offensive content
• Laughing at another person
• Announcing other disharmonies within a particular disharmony
• Use obscene references such as horror, terrorist attacks, etc.
• Leaking of a confidential communication
• committing other illegal acts
If you do any of these things, you risk being banned from the server or the platform.
The server ban is issued by the administrator of the server in question. This means that you are banned from that server, but you can use any other Discord server.
There are two types of server bans. If you get a kick, you will be removed from the server but not banned, so you can find that server and join again. But your account, IP address, and phone number will be debarred from the server if you get a traditional ban.
If you are banned directly from Discord, you will not be able to access your account or any Discord server. Discord usually issues a warning or temporary ban before deleting your account. If you are banned from Discord, you will receive an email notification, but the account will not be deleted for a month to give you time to resolve the situation. The results of all the reasons behind the discord ban will bring how to unban someone on discord.
Who can ban you from Discord?
You can be banned from Discord in two different ways: banned from the server and banned from the whole system.
A server ban is when the administrator or owner of a chat server has banned you from their server. When you are banned from a server, you can no longer visit a chat server, write messages, send text messages or join voice channels on the server, but you can still connect to other servers.
If Discord determines that you are in violation of its policies, you may receive a system-wide ban, which means you will not be able to connect to any of the servers available on Discord. The administrator or owner of a chat server can ban you but you need to learn how to unban someone on discord.
Why Discord might ban you?
Discord or the server administrator can deny access to Discord for several reasons. You can be banned from Discord if:
• Sending unsolicited messages or creating unsolicited accounts
• Repeated sending of messages and friend requests
• Organizing or participating in massive spam or pinging raids.
• Publication of multimedia content that infringes copyright.
• Sending hateful, self-destructive, suicidal, or blackmail messages.
• Not having flagged Non-Safe-For-Work channels or posted NSFW messages on SFW servers.
• Sharing pornographic material
• Pretending to be someone else
• Publicizing other disharmonies within the same disharmony
• Use of inappropriate references such as catastrophes, terrorist attacks, etc.
• Discovery of leaked private messages
• Engaging in other illegal activities
Prohibitions may also apply for reasons other than those mentioned above, and different servers have their own restrictions. However, you can be banned from different sites but all problems will be solved if you know how to unban someone on discord.
How to unban someone on Discord?
You can only unblock a blocked Discord user if you are the Discord server administrator.
Note: If you are not an administrator of the service, you must ask the server administrator to unlock a specific account for you.
To unban someone on Discord, open the Discord app on your device and log in to your account. Now go to the channel whose users you want to remove, press the down arrow next to the server name in the top left corner of the screen, and select Server Settings.
In the “Server Settings” window, click the “Bans” tab in the left sidebar. You can now see a list of all users you have banned on the selected server.
Select the user you want to unblock from the list (or search for a specific user using the search bar at the top) and click the Unban option in the pop-up window. You will get various ways to know how to unban someone on discord.
A quick guide to unban Discord
• Delete the Discord folder on your computer and uninstall Discord from your device.
• Open a VPN application (e.g. NordVPN) and connect to the VPN server to use the new IP address.
• Discord.
• Create a new account with a different e-mail address.
• Log in to Discord with your new account.
The following guides will help you know how to unban someone on discord quickly.
What happens if someone’s rights in Discord are revoked?
Once the block is lifted, the user can publish again on your server and connect to your voice channels. When you unban the user, he becomes visible as a member of the service and can add friends to his lists from your server.
These users will also be able to write text messages, join the voice channel, send private messages, and broadcast live games on the server. The result will come after completing all procedures of how to unban someone on discord. Users who have not been banned will also be able to apply for various member roles, including administrators, moderators, and other specific roles on the server.
Can someone be banned again after being banned?
Yes, you can ban anyone after removing them from your Discord server if you are the administrator of a particular server.
How can I remove someone from Discord?
If you want to ban someone from your Discord channel, open the Discord app or Discord.com on your device and log in to your account (if necessary). Once logged in, go to the server and select the channel you want to ban users from. Right-click on the user you want to ban (in the right sidebar or in the chat stream) and select Ban from the context menu.
You will then be asked if you wish to give a reason for the block and you can also decide how many messages to delete. Once you have given a reason (if you deem it necessary), you can proceed. If you want to add any discord channel, you just need to know how to unban someone on discord.
What happens if you are banned from a Discord server?
If you are banned from the Discord server, you will no longer be able to post messages in the server channels, send text messages, send/receive voice chats and see other users on the server. Even if you use another account with the same IP address, you will not be able to access the server because Discord not only bans your account but also your IP address. This IP block also applies if you are blocked by Discord itself and cannot connect to any server on the platform. If you fail to post messages in the server channels following the ban, you must follow the rules of how to unban someone on discord.
How can I unban myself on the Discord server?
If you have been unbanned on Discord, you can appeal to the service by providing your contact details and the problem you have encountered on the Discord “Submit an Appeal” page.
There, you will need to choose the option “Appeal the action taken by the Trust and Safety Service against my account” under “Type of message”, and fill in your problem.
The Discord security and trust team will contact you to inform you of your ban and how to avoid further bans. Otherwise, you need to follow the rules about how to unban someone on discord.
How can I get around the Discord ban?
If Discord does not offer you a way to bypass the block, you can bypass it by using one of the two methods described below. Note that both methods can be used to bypass IP blocking and that in both cases you will need to create a new account.
How can I keep track of Discord updates, issues, and patches?
If you have any questions about Discord in terms of new features, updates, problems, and how to fix them, Discord has an announcements page where you can keep track of all these things. There you can follow what Discord is adding or removing from their service and how to troubleshoot issues that arise in your Discord application. However, you can be banned from different sites but all problems will be solved if you know how to unban someone on discord.
How can I unsubscribe from the Discord server?
If you have been IP blocked on the Discord server, you can regain access by using an alternative solution, such as a VPN, or by contacting the administrators. The administrator or owner of a chat server can ban you, so you should have knowledge about how to unban someone on discord.
Here you can find out how to get back to the Discord server.
Using a VPN
Using a VPN is a very good way to avoid the Discord ban. To do this, you should clear your data and uninstall the Discord app. You should also subscribe to a reliable VPN service if you don’t already have one.
Create a new account
Creating a new Discord account may help you regain access.
Unless you use a VPN to generate a new IP address, this will not be effective if you have been disallowed due to a suspicious IP address.
However, if you have received a ban from the administrator of an individual server, creating a new account and re-registering is an easy way to lift the ban for that server. To do this you will need a dissimilar email address. We also recommend that you choose a completely different username. If the server administrator has banned the username “Donna123” on Monday, it is suspicious that “Donna1234” appears on Tuesday. Therefore, the administrator can ban you but you need to learn how to unban someone on discord.
Contact support or server administrator
If you have been banned from a server, it is often easier to contact the server administrator to have the ban lifted.
If you have been banned from the platform, you should contact customer support. This will help you understand why you have been banned and you can work with the support team to lift your ban. So, you should know how to unban someone on discord.
Using a cell phone or other network
If you have been blocked because your IP address has been deemed suspicious and you are not using a VPN, you can get around this by downloading the Discord app on your phone. You can also try using another network, which should change your IP address enough for how to unban someone on discord.
How to unban Discord on Windows with a VPN?
• Close Discord and go to the application data folder on your computer’s C:³ drive. You will find it in the “Username” folder. You may have to display hidden folders to get it. To enable hidden items, open the View tab and make sure the Hidden items checkbox is checked.
• After opening this folder, click on the “Local” folder and delete the “Discord” folder.
• Open the VPN application of your choice and connect to the VPN server to connect to the new IP address.
• Reinstall the Discord application.
• Create a new account with a different email address and/or phone number.
• Log in with your new account and join the server where you were banned.
How to unban Discord on Mac with VPN?
• Close Discord and delete the following folder: /Library/Application Support/Discord.
• Open the applications folder and drag the Discord application to the trash.
• Empty the “Trash” folder.
• Restart your Mac and reinstall the Discord application.
• Open the VPN application of your choice and connect to the VPN server to obtain a new IP address.
• Create a new account with a different email address and/or phone number.
• Log in with your new account and join the server where you were banned.
If you want to add any discord channel, you must ensure that you are aware of how to unban someone on discord.
How to unban someone on Discord app?
Removing a user from the Discord phone application is done in the same way. This is how it is done when you will learn how to unban someone on discord
• Open your server panel by swiping to the right on the chat screen and clicking on the three vertical dots next to the server name.
• Click on Configuration.
• Scroll down to the “Escapes” section.
• You will see a list of all the users you have blocked on your server. Click on the one you want to unblock.
• Click “Unban” and you are ready.
How to unban a user in Discord?
• Open the Discord application on your device.
• Go to the server or text channel whose ban you want to lift.
• Click on the down arrow at the top right of the server name.
• A drop-down menu appears.
• Select the server configuration from this list.
• Now click on “Prohibitions” at the bottom left.
• A list of all blocked users is opened.
• Select the user to unlock.
• A pop-up message appears asking you to unlock.
• Press the “Cancel” button.
• You have now successfully activated the user.
How to unban someone on Discord Mac/PC?
• Open the Discord application on your Mac/PC.
• Go to the server or text channel whose block you want to remove.
• Click on the down arrow at the top right of the server name.
• A drop-down menu appears.
• Select the server configuration from this list.
• Now click on “Prohibitions” at the bottom left.
• A list of all blocked users is opened.
• Select the user to unlock.
• A pop-up message appears asking you to unlock.
• Press the “Cancel” button.
• You have now successfully activated the user.
How to unban someone on Discord with MEE6/Carl/Dyno?
• Open the Discord application on your computer.
• Go to the server or text channel whose block you want to remove.
• Click on the down arrow at the top right of the server name.
• A drop-down menu appears.
• Select the server configuration from this list.
• Now click on “Prohibitions” at the bottom left.
• A list of all blocked users is opened.
• Select the user to unlock.
• A pop-up message appears asking you to unlock.
• Press the “Cancel” button.
• You have now successfully activated the user.
How to unban someone on Discord on Android/iOS?
• Open the Discord application on your computer.
• Go to the server or text channel whose block you want to remove.
• Click on the three dots in the upper right corner of the server name.
• Now click on the configuration icon in the drop-down menu.
• Select the server configuration from this list.
• Now click on “Prohibitions” at the bottom left.
• A list of all blocked users is opened.
• Select the user to unlock.
• A pop-up message appears asking you to unlock.
• Press the “Cancel” button.
• You have now successfully activated the user.
Frequently Asked Questions
How can I unban a Discord account?
You can unblock your Discord account by contacting customer support or the server administrator to dispute the block. You can also use a VPN or other device to regain access.
How long does the Discord lockout last?
If you’ve got received a server ban, it’s the discretion of the server administration team, therefore it is removed at any time. If you have received a platform ban, it is a permanent ban.
Does Discord prohibit IP addresses?
Yes, Discord sometimes blocks IP addresses. If you want to unblock, you need another IP address to solve this problem. You can get a new IP address through another device, a network, or a VPN.
Conclusion
So, ladies and gentlemen, you can now ban and unban people on Discord as you see fit.
If you have more questions about Discord or just want to learn more about the application and its various features, read one of the other recommended Discord articles on this page or check out our “How to use Discord” guide, which summarizes all the Discord articles we’ve written. If you want to enter any discord channel, everything you need to know that how to unban someone on discord.
Apart from it, if you are interested to know Antimalware service executable program, Groovy BotDiscord spoilerGC Invoker UtilityThe Equalizer 3how to recover deleted Facebook page, and WhatsApp business for beginners, you can follow our Tech category. Thanks for reading!
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Support » Plugin: NextGEN Gallery - WordPress Gallery Plugin » NextGen 2.0.11 – removing borders from thumbnails
• Resolved zanchit
(@zanchit)
my previous settings have been replaced during this update and I would like to remove all the borders from my thumbnails or change them to the colour of my background.
also I did have a line of text to enlarge the image under the mouse (hover) but can’t work out how to do it now
http://wordpress.org/plugins/nextgen-gallery/
Viewing 15 replies - 1 through 15 (of 51 total)
• Try this in other options->styles->css customization
/* Thumbnails */
.ngg-gallery-thumbnail img, .ngg-thumbnail img {
background: none !important;
border: none !important;
padding: 0px;
}
If you want to change color, play it with Firebug or Chrome, and obviously remove “none”. You could add a box-shadow if you want for a cool effect. Look at CSS school pages and you can do it by yourself.
This should work:
.ngg-gallery-thumbnail img {
background-color:transparent!important;
border:none!important;
border-radius:10px!important;
}
.ngg-gallery-thumbnail img:hover {
background-color:red!important;
}
oops tizz was faster.
The! “!important” is important. 😉
thank you both sooooo much. Could you possibly advise on a line of code so that the image enlarges when the mouse hovers over?
Try this
However, they could result pixelated. Replace the class name with those of nextgen.
Thanks fizz
should i use the css or the html option?
and when you say replace the class name with those of nextgen, what code would I be looking for ? not quite sure what i put, or where!
CSS, .ngg-gallery-thumbnail img:hover
play it with Firebug or Chrome
when I try that it does enlarge to twice the size but at least half of it is behind the other images. is there a way to do it so it comes up in front of
the surrounding images?
Not with CSS, but with javascript e.g.
What you want is similar to lightbox, so can I ask you why don’t you use the Nextgen default shutter effect, that opens the image in a box in front of the others?
don’t know anything about nextgen default shutter effect. Am a total newbie. So sorry if being thick. website is http://www.zanchit.co.uk and the galleries are all the same: http://www.zanchit.co.uk/adults would be a sample. Any advice on how to program so could have enlargement on mouseover would be fantastic. But you might have to speak slowly (blonde!)
Please try this:
.ngg-gallery-thumbnail-box {
-moz-transition:-moz-transform 0.1s ease-in;
-webkit-transition:-webkit-transform 0.1s ease-in;
-o-transition:-o-transform 0.1s ease-in;
}
.ngg-gallery-thumbnail-box:hover {
-moz-transform:scale(2);
-webkit-transform:scale(2);
-o-transform:scale(2);
z-index:10000;position:relative;
}
that’s definitely better, but the enlarged image is blurred. also it just zooms which means it crops the image. i have done it and it is live on http://www.zanchit.co.uk/adults can you see what I mean?
i think what i would prefer, it it were possible would be for the full sized image to show on mouseover. is that possible?
Actually, I think the problem is that the zoom is hindered by the edges of the central column. ie the top row end up with the top edge cropped, the bottom row with the bottom edge and the sides with the relative side cropped.
@w.bear You posted the content of the page I’ve linked up above.
zanchit has already try it and from there started his subsequent questions.
@zanchit I’m talking about something that you already have. Have you ever clicked on your images? It’s not a mouseover, but that’s how the majority of galleries work, with that lightbox.
Enlargement on mouseover, in the way you want now, it’s not two lines of code, you need a script or a plugin.
I think I may well have to make do with a minimal zoom on hover, i.e. just a few percent increase. What would I put for the code to make that happen?
Viewing 15 replies - 1 through 15 (of 51 total)
• The topic ‘NextGen 2.0.11 – removing borders from thumbnails’ is closed to new replies.
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IPython 0.0.1, a simple script to be loaded as $PYTHONSTARTUP: of historical interest only...
#!/usr/bin/env python
"""
Interactive execution with automatic history, tries to mimic Mathematica's
prompt system. This environment's main features are:
- Numbered prompts (In/Out) similar to Mathematica. Only actions that produce
output (NOT assingments, for example) affect the counter and cache.
- The following GLOBAL variables always exist (so don't overwrite them!):
_p: stores previous result which generated printable output.
_pp: next previous
_ppp: next-next previous
_cache[]: cache of all previous output, indexed by prompt counter.
_pc: short alias for _cache
- Global variables named _p<n> are dynamically created (<n> being the
prompt counter), such that the following is always true:
_p<n> == _pc[<n>]
E.g. the 4th result is always available as either _pc[4] or as _p4.
Use help() to open Python's builtin help system, which can also be
called with optional keywords as arguments.
If you are running inside an Emacs buffer, use doc() as dumb-terminal
simpler version of help() [doesn't do all that help() does, though].
Current configuration (set _load.. variables to 0 to turn off):
Control loading of Numerical Python modules: _load_Numeric = %(_load_Numeric)s
Gnuplot Globals GP and gp (module/instance): _load_Gnuplot = %(_load_Gnuplot)s
Load gracePlot (2D-plotting only) : _load_gracePlot = %(_load_gracePlot)s
Globals for SI units (including g=9.8) : _load_units = %(_load_units)s
Starting number for prompt counter : _prompt_ini = %(_prompt_ini)s
Number of history items to store in cache : _cache_size = %(_cache_size)s """
#****************************************************************************
# Configure here
_load_Numeric = 1
_load_Gnuplot = 1
_load_gracePlot = 1
_load_units = 1
_cache_size = 1000
_prompt_ini = 1
# *** Don't modify below unless you know what you're doing. ***
# Crude first version, with minimal object structure. This could be done much
# better, by defining a Cache class (probably using weak references or
# generators). But it seems to work ok. Haven't checked for memory circularity
# problems, though.
#****************************************************************************
# Copyright (C) 2001 Fernando Pérez. <[email protected]>
#
# Distributed under the terms of the GNU General Public License.
#
# The full text of the GPL is available at:
#
# http://www.gnu.org/copyleft/gpl.html
#****************************************************************************
__author__ = 'Fernando Pérez. <[email protected]>'
__version__= '0.1'
#****************************************************************************
# Class definitions
class _HistPrompt1:
"""Simple interactive prompt like Mathematica's."""
def __str__(self):
return '\nIn['+`_prompt_count`+']:= '
class _HistPrompt2:
"""Simple interactive continuation prompt."""
def __str__(self):
return '...'+' '*(len('In['+`_prompt_count`+']:= ')-3)
#****************************************************************************
# Function definitions
def _history_print(arg):
"""Printing with history cache management.
This is invoked everytime the interpreter needs to print, and is activated
by setting the variable sys.displayhook to it."""
global _p,_pp,_ppp,_cache,_prompt_count
# cleanup cache if needed
if _prompt_count > _cache_size:
print '\nWARNING: History cache size limit (currently '+\
`_cache_size`+') hit.'
print 'Flushing cache and resetting history counter...'
print 'The only history variable available will be _p with current result.'
print 'Increase the variable _cache_size if you need a larger cache.'
_prompt_count = _prompt_ini
_cache = []
_p,_pp,_ppp = '','',''
# fill bottom of cache if initial offset is not zero
for n in range(_prompt_ini): _cache.append(None)
# delete auto-generated vars from global namespace
for n in range(_prompt_ini,_cache_size):
exec 'del _p'+`n` in globals()
# update cache
_cache.append(arg)
_ppp = _pp
_pp = _p
_p = arg
exec '_p'+`_prompt_count`+'=_cache[-1]' in globals()
print '\nOut['+`_prompt_count`+']=',arg
_prompt_count += 1
# end of _history_print()
#----------------------------------------------------------------------------
def intro():
"""Show global docstring and config info."""
print __doc__ % globals()
#----------------------------------------------------------------------------
def dgrep(pat,*opts):
"""Return grep() on dir()+dir(__builtins__).
A very common use of grep() when working interactively."""
exec 'data = dir()+dir(__builtins__)' in globals()
return grep(pat,data,*opts)
# end of dgrep()
#----------------------------------------------------------------------------
def idgrep(pat):
"""Case-insensitive dgrep()"""
exec 'data = dir()+dir(__builtins__)' in globals()
return grep(pat,data,case=0)
# end of dgrep()
#-----------------------------------------------------------------------------
def Gnuplot_setup():
"""Setup some convenient globals for Gnuplot plotting, including a global
gp Gnuplot object, ready for use."""
global GP,gp
GP = Gnuplot
gp = GP.Gnuplot() # global plotting process
gp.logx = 'set logscale x'
gp.logy = 'set logscale y'
gp.nolog = 'set nologscale'
gp.points = 'set data style points'
gp.lines = 'set data style lines'
gp.linesp = 'set data style linespoints'
gp.errorbars = 'set data style errorbars'
# end of Gnuplot_setup()
#-----------------------------------------------------------------------------
def SI_Units():
"""Define some globals for physical units and constants.
The following become available:
g,mm,cm,km,gm,s,sec,hr,day,year,deg,ft,lb,inch
These can be overwritten at any time. Calling SI_Units() again will reset
their values if needed. """
global g,mm,cm,km,gm,s,sec,hr,day,year,deg,ft,lb,inch
# physical constants
g = 9.8
# units
mm = 0.001 # length
cm = 0.01
km = 1000.
gm = 0.001 # mass
s,sec = 1.,1. # time
hr = 3600.
day = 24*hr
year = 365*day
deg = pi/180. # angles
inch = 0.0254 # british
ft = 12*inch
lb = 0.454
# end of SI_Units()
#-----------------------------------------------------------------------------
def import_fail_info(mod_name):
"""Inform load failure for a module."""
print """
WARNING: Loading of %(mod_name)s-related modules failed.
Fix your configuration or set _load_%(mod_name)s to 0 to prevent
seeing this message every time.
""" % vars()
#****************************************************************************
# Setup everything at global level for history system to work
# Python standard modules
import sys
from math import *
# Other modules
from Itpl import Itpl # available at http://web.lfw.org/python/Itpl15.py
from genutils import *
# User-controlled modules and globals-affecting functions
if _load_Numeric:
try:
from Numeric import * # Numeric *must* come after math.
from numutils import *
# useful for testing infinities in results of array divisions
# (which don't raise an exception)
infty = Infinity = (array([1])/0.0)[0]
except:
import_fail_info('Numeric')
if _load_Gnuplot:
try:
import Gnuplot
Gnuplot_setup()
except:
import_fail_info('Gnuplot')
if _load_gracePlot:
try:
from gracePlot import gracePlot
except:
import_fail_info('gracePlot')
if _load_units:
SI_Units()
# pydoc doesn't exist in Python 2.0 (I think):
try:
from pydoc import help
except:
print "Module pydoc not found. help() function unavailable."
# Initialize cache
_p,_pp,_ppp = '','',''
_prompt_count = _prompt_ini
_pc = _cache = [] # _pc is just an alias
# fill bottom of cache if initial offset is not zero
for n in range(_prompt_ini): _cache.append(None)
# cleanup global namespace a bit
del import_fail_info,n
# Set in/out prompts and printing system
sys.displayhook = _history_print
sys.ps1 = _HistPrompt1()
sys.ps2 = _HistPrompt2()
# Startup info
print '\nPython',sys.version.split('\n')[0]
print '\nInteractive Python -- Type intro() for a brief explanation.'
#************************* end of file <ipython.py> ***********************
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|
__label__pos
| 0.991956 |
Ways to Tell if Something Is a Function
••• Valeriya/iStock/GettyImages
Functions are relations that derive one output for each input, or one y-value for any x-value inserted into the equation. For example, the equations:
y = x + 3 \text{ and } y = x^3 - 1
are functions because every x-value produces a different y-value. In graphical terms, a function is a relation where the first numbers in the ordered pair have one and only one value as its second number, the other part of the ordered pair.
Examining Ordered Pairs
An ordered pair is a point on an x-y coordinate graph with an x and y-value. For example, (2, −2) is an ordered pair with 2 as the x-value and −2 as the y-value. When given a set of ordered pairs, ensure that no x-value has more than one y-value paired to it. When given the set of ordered pairs [(2, −2), (4, −5), (6, −8), (2, 0)], you know that this is not a function because an x-value – in this case – 2, has more than one y-value. However, this set of ordered pairs [( −2, 4), ( −1, 1), (0, 0), (1, 1), (2, 4)] is a function because a y-value is allowed to have more than one corresponding x-value.
Solving for Y
It is relatively easy to determine whether an equation is a function by solving for y. When you are given an equation and a specific value for x, there should only be one corresponding y-value for that x-value. For example
y = x + 1
is a function because y will always be one greater than x. Equations with exponents can also be functions. For example:
y = x^2 - 1
is a function; although x-values of 1 and −1 give the same y-value (0), that is the only possible y-value for each of those x-values. However:
y^2 = x + 5
is not a function; if you assume that x = 4, then
y^2 = 4 + 5 = 9 \\ y^2 = 9
has two possible answers (3 and −3).
Vertical Line Test
Determining whether a relation is a function on a graph is relatively easy by using the vertical line test. If a vertical line crosses the relation on the graph only once in all locations, the relation is a function. However, if a vertical line crosses the relation more than once, the relation is not a function. Using the vertical line test, all lines except for vertical lines are functions. Circles, squares and other closed shapes are not functions, but parabolic and exponential curves are functions.
Using an Input-Output Chart
An input-output chart displays the output, or result, for each input, or original value. Any input-output chart where an input has two or more different outputs is not a function. For example, if you see the number 6 in two different input spaces, and the output is 3 in one case and 9 in another, the relation is not a function. However, if two different inputs have the same output, it is still possible that the relation is a function, especially if squared numbers are involved.
Related Articles
What is Function Notation?
What Makes a Relation a Function?
How to Find Vertical & Horizontal Asymptotes
What is an Inverse Function?
How to Find the Inverse of a Function
How to Explain Input & Output Tables in Algebra
How to Differentiate a Function
Differences Between Absolute Value & Linear Equations
Facts About Functions for Algebra 1
What is the Vertical Line Test?
How to Write an Equation for a Function
How to Differentiate Negative Exponentials
How to Determine If an Equation Is a Linear Function...
How to Calculate the Wronskian
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Dont Go!
We Have More Great Sciencing Articles!
|
__label__pos
| 0.918322 |
18.4.0 released Sep 25, 2023
Vely statements
Vely statements (such as write-file or run-query etc.) are programming statements written within C code in files with .vely extension. They are pre-processed by vv into C code, and then compiled into a native executable. A statement does something useful, for example runs a database query, searches a string with regex, calls a web service, parses JSON, creates a unique file, etc. (see documentation).
Statements vs API
Writing a single Vely statement instead of multiple C API calls is easier to read and maintain, less error prone, safer at run-time and more productive. The functionality, scope and default behavior of each statement are chosen to reflect those benefits for typical practical needs of application development. They are meant to be building blocks that enable productivity and collaboration.
A statement is not a macro or a simple substitution for one or more API calls. A statement can perform multiple functions that are logically connected, but would require use of different API calls and in different configurations to achieve the functionality. For example run-query can perform a simple query with no input or output parameters, or a complex one with them, and with or without a number of other options. Achieving these functionalities with APIs would require different ones used in different ways, increasing chances of human error. Note that sorting out how to do things like this is performed at compile time - no performance is lost at run-time figuring out the best way to achieve Vely statement's stated goal.
Statement name is always in the form of two or three words with a hyphen in between:
along with the clauses supplying the rest of the predicate, such as subject or subordinate clause(s). The three-word model is chosen for simplicity in order to avoid unwieldy do-this-that-way-along-with-something-else or such kind of statements, which unfortunately sometimes proliferate in APIs.
This design mimics natural speech and is typical of declarative languages, as it is easier to write and read than APIs. Just like in a natural language, the action asked in a single statement can be very simple, moderate or involved depending on the need, and the "sentence" used to carry it out is easy to comprehend at a glance, and easy to construct based on "what" needs to be done rather than "how". Even so, the C code generated is still done with performance in mind first and foremost, and the statement design is ultimately always guided by that goal.
Vely statements are the core of the language functionality. Here's a formal description of what they are.
Statement structure
Vely statements generally have three components separated by space(s):
A statement starts with a name, which designates its main purpose. An object argument denotes the object of the purpose stated in the name. Each clause consist of a clause name, which specifies some aspect of the statement's purpose and it may be followed by no additional data, or it may be accompanied with one or more data arguments. A clause may have subclauses, which follow the same structure and are associated with the clause by appearing immediately after it. Most clauses are separated by space(s), however some (like "=" or "@") may not need space(s) before any data; the statement's documentation would clearly specify this.
An object argument must immediately follow the statement's name, while clauses may be specified in any order.
For example, in the following Vely code:
encrypt-data orig_data input-length 6 output-length define encrypted_len password "mypass" salt newsalt to define res binary
encrypt-data is the statement's name, and "orig_data" is its object argument. The clauses are:
The clauses can be in any order, so the above can be restated as:
encrypt-data orig_data to define res password "mypass" salt newsalt output-length define encrypted_len binary input-length 6
Vely documentation provides a concise BNF-like notation of how each statement works, which in case of encrypt-data is (backslash simply allows continuing to multiple lines):
encrypt-data <data> to [ define ] <result> \
[ input-length <input length> ] \
[ output-length [ define ] <output length> ] \
[ binary [ <binary> ] ] \
( password <password> \
[ salt <salt> [ salt-length <salt length> ] ] \
[ iterations <iterations> ] \
[ cipher <cipher algorithm> ] \
[ digest <digest algorithm> ]
[ cache ]
[ clear-cache <clear cache> ) \
[ init-vector <init vector> ]
Optional clauses are enclosed with angle brackets (i.e between "[" and "]"), and data arguments (in general C expressions) are stated between "<" and ">". If only one of a number of clauses may appear, such clauses are separated by "|", and each clause possibly enclosed with "(" and ")" if it consists of more than one keywords or arguments. Generally a clause continues until the next clause, which means until all subclauses and arguments are exhausted.
The most common subclause is an optional "define", which always precedes an output variable, i.e. a variable that stores (one of) the results of the statements. If used, it creates such variable within the statement. It is commonly used to shorten the code written.
Keywords (other than statement names such as encrypt-data above) are generally specific to each statement (or a group of statements in which they are used). So, keyword "salt", for example, has meaning only within encrypt-data and a few other related statements, where it is used to specify the data for the "salt" clause. In order to have the complete freedom to choose your variable names so they don't clash with keywords, you can simply surround them (or the expressions in which they appear) in parenthesis (i.e. "(" and ")") and use any names you want, without worrying about keywords, for example:
const char *password = "some password";
const char *salt = "0123456789012345";
encrypt-data "some data" password (password) salt (salt) to define (define)
p-out define
In this example, keywords "password", "salt" and "define" are used as variable names as well; and in p-out statement, variable named "define" is used freely, even if it is a keyword for other statements - but it is not for the p-out statement.
It is recommended to use supplied Vely statements over your C code for the same functionality.
Note that while you can use tab characters at the beginning of the line (such as for indentation), as well as in string literals, do not use tabs in Vely code as they are not supported for lack of readability - use plain spaces.
Look and feel
Vely statements have decidedly non-C look and feel, unlike what's common with typical API interface. This is by design. They stand out when reading code in a way that clearly communicates their purpose, with the intent of increased readability and more expressive and condensed functionality. On the other hand, Vely statements are decidedly C, as they are completely integrated with C code and translate to pure C in the end.
Constructs in code blocks
Note that, Vely statements, after translated into C code by vv, are generally made of multiple C statements, hence Vely statements can never be treated as single-line statements. Thus, for example, Vely will emit an error if you write:
if (some condition) vely-statement
if (some condition)
vely-statement
Instead, write:
if (some condition) {
vely-statement
}
Integration with C
Vely is integrated with C to a certain point, with the integration points chosen for maximum practical benefit. For the most part, it does not need to parse C to work, mostly because that does not present significant benefits and it would slow down code generation.
Vely does, however, process certain C elements. It processes strings in C expressions used in Vely statements, as well as parenthesis to make sure that statement clauses are parsed properly. Comments (both C and C++ style) are processed as well.
Probably the most important aspect of integrating with C is diagnostics. Line numbers and error messages reported by gcc and gdb are referring to source Vely files, making it easy to find the exact source code line of any issue. Also, diagnostic messages are additionaly processed to provide better reporting, including displaying and highlighting both Vely and generated C code; see diagnostic-messages.
Splitting statement into multiple lines
To split a statement into multiple lines (including string continuations), use a backslash (\), for instance:
encrypt-data orig_data input-length 6 \
output-length define encrypted_len \
password "my\
pass" salt \
newsalt to define res binary
Note that all statements are always left-trimmed for whitespace. Thus the resulting string literal in the above example is "mypass", and not "my pass", as the whitespaces prior to line starting with "pass" are trimmed first. Also, all statements are right-trimmed for white space, except if backslash is used at the end, in which case any spaces prior to backslash are conserved. For that reason, in the above example there is a space prior to a backslash where clauses need to be separated.
Comments
You can use both C style (i.e. /* ... */) and C++ style (i.e. //) comments with Vely statements, including within statements, for example:
run-query @db = \
"select firstName, lastName from employee where yearOfHire>='%s'" \
output /* comment within */ firstName, lastName : "2015" // other comment
Error handling
A statement that fails for reasons that are generally irrecoverable will fail, for example out of memory or disk space, bad input parameters etc. Vely philosophy is to minimize the need to check for such conditions by preventing the program from continuing. This is preferable, as forgetting to check usually results in unforeseen bugs and safety issues, and the program should have stopped anyway.
Errors that are correctable programmatically are reported and you can check them, for example when opening a file that may or may not exist.
Overall, the goal is to stop execution when necessary and to offer the ability to handle an issue when warranted, in order to increase run-time safety and provide instant clues about conditions that must be corrected.
Code processing, error reporting, debugging
A statement is pre-processed into C code, which is then compiled into a native executable using gcc compiler. The final C code is a mix of your own code and generated Vely code.
Error reporting by default shows line numbers in your .vely source code files as well as the generated C code (see diagnostic-messages), which generally makes it easy to pinpoint an error. If you want to see generated C code and make error reporting refer to it, use "--c-lines" option of vv utility; this is useful if the error refers to details of generated code. You can also obtain the line number from .vely source file, and then examine generated source code file by looking for #line directives. Generated final C file is located in:
/var/lib/vv/bld/<app name>/__<source file base name>.o.c
For instance if application name is "myapp" and source file is "mycode.vely", then generated code is in file:
/var/lib/vv/bld/myapp/__mycode.o.c
If an error is reported as being on line 43 of file "mycode.vely", then look for lines in the above .c file that look like:
#line 43 "mycode.vely"
The code adjacent to those lines is the generated code for the Vely statement at line 43 in "mycode.vely".
Vely will perform many sanity checks when possible while preprocessing your .vely files, such as enforce the presence and count of required arguments in clauses, check if conflicting clauses are used, report imbalance of opening/closing clauses in statements that use them (such as for example write-string, read-line or with database-queries), reject unknown statements, report incorrect usage of statements and similar. However, most of the checking of your code is ultimately done by gcc (Linux C compiler), such as typing, expression syntax, C language correctness, linkage issues and others; in doing so, gcc will report the correct line number, as stated above (either a line number in your .vely source file, or a generated C code file).
When debugging (such as with using gdb), stepping through your code is similar to error reporting: by default gdb will treat Vely statement as a "single-statement" and step over it as such. If you use "--c-lines" option, then you will be able to step through final C code.
See also
Language
dot
inline-code
statement-APIs
syntax-highlighting
unused-var
See all
documentation
You are free to copy, redistribute and adapt this web page (even commercially), as long as you give credit and provide a link back to this page (dofollow) - see full license at CC-BY-4.0. Copyright (c) 2019-2023 Dasoftver LLC. Vely and elephant logo are trademarks of Dasoftver LLC. The software and information on this web site are provided "AS IS" and without any warranties or guarantees of any kind.
|
__label__pos
| 0.922052 |
Passing Hidden Value in Jquery to .cfc through Submit
I would like to pass the value of issendnow = -1 when the user clicks "Make Recurring". This is what I have so far.
My form info: (issendnow will be 1 unless 'Make Recurring' is clicked)
<form class="smart-form" id="group-form" name="group-form" action="#event.buildLink('MassEmail.SendMassMessage')#" method="post" enctype="multipart/form-data">
<input type="hidden" value="1" name="sendto" id="sendTo" />
<input type="hidden" name="issendnow" id="issendnow" value="1" />
Open in new window
My Submit (I have more than one, thus have to do it this way):
<button type="submit" class="btn btn-warning" onclick="updateRecurring();"> Make Recurring </button>
Open in new window
My Jquery:
// recurring message
function updateRecurring(){
$("##group-form").issendnow.value='-1';
return true;
}
Open in new window
What's interesting - and a bit confusing to me - is my .cfc isn't reading issendnow either way (and it's a hidden field with a value. I keep getting "key [ISSENDNOW] doesn't exist" when I get to this line
<cfif rc.issendnow EQ 1>
Open in new window
Perhaps I'm doing something wrong there?
Any help would be greatly appreciated!
tryportAsked:
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I wear a lot of hats...
"The solutions and answers provided on Experts Exchange have been extremely helpful to me over the last few years. I wear a lot of hats - Developer, Database Administrator, Help Desk, etc., so I know a lot of things but not a lot about one thing. Experts Exchange gives me answers from people who do know a lot about one thing, in a easy to use platform." -Todd S.
Hans LangerCommented:
Instead of a "submit button"
<button type="submit" class="btn btn-warning" onclick="updateRecurring();"> Make Recurring </button>
Open in new window
use just a "button"
<button type="button" class="btn btn-warning" onclick="updateRecurring();"> Make Recurring </button>
Open in new window
and submit the form through javascript
function updateRecurring(){
$("#group-form #issendnow").val('-1');
$("#group-form").submit()
}
Open in new window
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tryportAuthor Commented:
For some reason, this keeps telling me "ReferenceError: updateRecurring is not defined." ?
Thanks for your help!
tryportAuthor Commented:
Does anyone have any suggestions?
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_agx_Commented:
Since you're using jQuery you could also use a bind instead of onClick. As GERENTE said, switch to type "button", just be sure to remove the onClick call:
HTML
<form id="group-form">
<input id="issendnow" type="hidden" name="issendnow" value="1" />
<button id="recurringButton" type="button" class="btn btn-warning"> Make Recurring </button>
</form>
Open in new window
JavaScript
$('#recurringButton').click(function(){
$("#issendnow").val('-1');
// debugging only
console.log("#issendnow = "+ $("#issendnow").val());
$("#group-form").submit()
});
Open in new window
tryportAuthor Commented:
_agx_
I just can't seem to get anything to work. I wonder if it's something in my other forms on the page... ? Here's the form in question:
<form class="smart-form" id="individualform" name="individualform" action="index.cfm/MassEmail/SendMassMessage" method="post" enctype="multipart/form-data">
<input type="hidden" value="3" name="sendto" id="sendTo" />
<input type="hidden" name="issendnow" id="issendnow" value="" />
<fieldset>
<legend><strong>Send To:</strong> Individuals</legend>
<section>
<label class="label">Individual(s)</label>
<label class="select">
<select multiple style="width:100%" name="userIDs" id="userIDs">
<option value="6201">Account, Demo</option>
<option value="14543">Adams, Jefferson</option>
</select>
</label>
<div class="note"> Choose one or more individuals to send this message to </div>
</section>
<section>
<label class="label">Carbon Copy (CC)</label>
<label class="select">
<select multiple style="width:100%" name="ccUserIDs" id="ccUserIDs">
<option value="6201">Account, Demo</option>
<option value="14543">Adams, Jefferson</option>
<option value="6222">Anderson, Jonathan</option>
</select>
</label>
<div class="note"> Choose one or more individuals to carbon copy (cc) this message to</div>
</section>
<section>
<label class="label">Blind Carbon Copy (BCC)</label>
<label class="select">
<select multiple style="width:100%" name="bccUserIDs" id="bccUserIDs">
<option value="6201">Account, Demo</option>
<option value="14543">Adams, Jefferson</option>
<option value="6222">Anderson, Jonathan</option>
</select>
</label>
<div class="note"> Choose one or more individuals to blind carbon copy (bcc) this message to</div>
</section>
<section>
<label class="label">Subject</label>
<label class="input">
<input type="text" name="subject" id="subject" maxlength="80" value="">
</label>
<div class="note"> Similar to an email on your computer; provide a brief overview of this email</div>
</section>
<section>
<label class="label">Message</label>
<label class="textarea textarea-resizable">
<textarea rows="12" name="msg3" id="msg3"></textarea>
</label>
</section>
<section>
<label class="label">Attachments</label>
<div class="input input-file"> <span class="button">
<input type="file" id="emailAttach" name="emailAttach" onchange="document.getElementById('emailattachlabel').value = this.value">
Browse</span>
<input type="text" id="emailattachlabel" placeholder="Include some files" readonly>
</div>
<div class="input input-file"> <span class="button">
<input type="file" id="emailAttach2" name="emailAttach2" onchange="document.getElementById('emailAttach2label').value = this.value">
Browse</span>
<input type="text" id="emailAttach2label" placeholder="Include some files" readonly>
</div>
<div class="input input-file"> <span class="button">
<input type="file" id="emailAttach3" name="emailAttach3" onchange="document.getElementById('emailAttach3label').value = this.value">
Browse</span>
<input type="text" id="emailAttach3label" placeholder="Include some files" readonly>
</div>
</section>
<section>
<label class="label">Also Deliver Message To</label>
<div class="inline-group">
<label class="checkbox">
<input type="checkbox" id="personalemail" name="personalemail">
<i></i>Personal Emails</label>
<label class="checkbox">
<input type="checkbox" id="eBox" name="eBox">
<i></i>eBox Message</label>
</div>
</section>
</fieldset>
<footer>
<button id="recurringButton" type="button" class="btn btn-warning"> Make Recurring </button>
<button type="submit" class="btn btn-primary">Send Now </button>
<button type="button" class="btn btn-default" onclick="window.history.back();"> Back </button>
</footer>
</form>
</div>
<script type="text/javascript">
$(document).ready(function() {
$('#recurringButton').click(function(){
$("#issendnow").val('-1');
// debugging only
console.log("#issendnow = "+ $("#issendnow").val());
$("#individualform").submit()
});
$(function() {
// Validation
$("#groupform").validate({
// Rules for form validation
rules : {
groupID : {
required : true
},
subject : {
required : true,
minlength : 3,
maxlength : 80
}
},
// Messages for form validation
messages : {
groupID : {
required : 'At least one group is required'
},
subject : {
required : 'Please provide a subject for this email message'
},
},
// Do not change code below
errorPlacement : function(error, element) {
error.insertAfter(element.parent());
}
});
});
$(function() {
// Validation
$("#individualform").validate({
// Rules for form validation
rules : {
userIDs : {
required : true
},
subject : {
required : true,
minlength : 3,
maxlength : 80
}
},
// Messages for form validation
messages : {
userIDs : {
required : 'At least one one individual is required'
},
subject : {
required : 'Please provide a subject for this email message'
},
},
// Do not change code below
errorPlacement : function(error, element) {
error.insertAfter(element.parent());
}
});
});
$(function() {
// Validation
$("#smartFilterform").validate({
// Rules for form validation
rules : {
locationID : {
required : true
},
deptID : {
required : true
},
userClassID : {
required : true
},
workStatusID : {
required : true
},
subject : {
required : true,
minlength : 3,
maxlength : 80
}
},
// Messages for form validation
messages : {
locationID : {
required : 'At least one one location is required; select All Locations to include all'
},
deptID : {
required : 'At least one one department is required; select All Departments to include all'
},
userClassID : {
required : 'At least one one user class/rank is required; select All User Classes to include all'
},
workStatusID : {
required : 'At least one one work status is required; select All Work Statuses to include all'
},
subject : {
required : 'Please provide a subject for this email message'
},
},
// Do not change code below
errorPlacement : function(error, element) {
error.insertAfter(element.parent());
}
});
});
$('select').select2();
})
</script>
Open in new window
_agx_Commented:
EDIT:
> $(function() {
> $(document).ready(function() {
Usually I place document.ready() stuff in the header. Also, I'm not sure you can have both of those. Try putting all of the "on load" stuff inside a single $(document).ready() call.
Is the form even submitting? I'd tackle it in steps, to figure out which step is causing the problem.
1) Check the js console for errors and verify it even reaches the button click
2) If so, temporarily submit the form to a test .cfm page. On that page, dump out the FORM/URL scopes so you can see what's actually submitted. Verify the hidden form field value is there. If yes, it suggests the problem occurs w/the CFC part, so...
3) Switch back to the CFC and check the network tab of your JS console. What data/fields are actually submitted?
Might also be a problem with the cfc signature (or the framework you're using). How are you submitting to the CFC and what does your function looks like?
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jQuery
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Entreotrascosas.es
La importancia de una experiencia de usuario efectiva para tu web
Categoría:
Diseño web
La experiencia de usuario es un factor clave para el éxito de cualquier sitio web. Los usuarios buscan una navegación fácil y una interacción intuitiva con el contenido. En este artículo, exploraremos la importancia de una experiencia de usuario efectiva y cómo puede mejorar la satisfacción del usuario y el rendimiento del sitio web.
En primer lugar, es importante entender que una experiencia de usuario efectiva no solo se trata de un diseño visualmente atractivo. El diseño debe ser funcional y facilitar la navegación del usuario. Para lograr esto, es esencial que el sitio web tenga una estructura clara y fácil de seguir, así como una arquitectura de información bien pensada.
Otro aspecto importante de una experiencia de usuario efectiva es la velocidad de carga del sitio web. Los usuarios no tienen paciencia para esperar a que se cargue un sitio web lento. Por lo tanto, es crucial que se optimice la velocidad de carga del sitio web.
Además, la experiencia de usuario también está relacionada con el contenido que se ofrece. El contenido debe ser relevante, de calidad y fácil de encontrar. Si el usuario tiene dificultades para encontrar lo que busca, es probable que abandone el sitio web.
Por último, una experiencia de usuario efectiva también se relaciona con la capacidad de respuesta del sitio web. Los usuarios acceden a los sitios web desde diferentes dispositivos y pantallas, por lo que es importante que el sitio web se adapte a cualquier tamaño de pantalla y dispositivo para ofrecer una experiencia de usuario uniforme.
Una experiencia de usuario efectiva es fundamental para el éxito de cualquier sitio web. Para lograrlo, es necesario tener un diseño funcional, una velocidad de carga óptima, contenido de calidad y una capacidad de respuesta adecuada. Al centrarse en estos aspectos, se puede mejorar la satisfacción del usuario y el rendimiento del sitio web.
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Categorías
¿Tienes alguna duda?
Estaremos encantados de hablar sobre tu proyecto
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| 0.550265 |
Coins vs Tokens
Coins vs Tokens
Coins vs Tokens
SecuX wallets support over 1,000 different types of coins and tokens on many different chains. But did you know there are differences between the two?
Coins
Coins refer to the native cryptocurrencies built on their independent blockchain network. For example, Bitcoin is powered by its native blockchain network. Similarly, Litecoin (LTC) and Ethereum (ETH) function on their respective blockchains. These blockchains may differ in their size, rules, miners, performance, etc. A coin is issued directly by the blockchain protocol on which it runs. In many cases, coins are not only used to pay transaction fees on the network, but are also used to incentivize users to keep the cryptocurrency’s network secure.
Some of the popular coins are Bitcoin (BTC), Ethereum (ETH), Binance (BNB), Dogecoin (DOGE), and Litecoin (LTC).
Token
Tokens, on the other hand, are units of value that blockchain-based organizations or projects that don’t have a blockchain network of their own but instead, develop on top of existing blockchain networks. While they often share deep compatibility with the coins of that network, they are a wholly different digital asset class.
Most tokens exist to be used with decentralized applications, or dApps. When developers are creating their token, they can decide how many units they want to make and where these new tokens will be sent when they are created. They will pay some of the native cryptocurrency on the blockchain they are creating the token on at this point.
Once created, tokens are often used to activate features of the application they were designed for. For example, Musicoin is a token that allows users to access different features of the Musicoin platform. This could be watching a music video or streaming a song. Binance (the exchange) also has its own token. When users trade with BNB (Binance coin), their fees are 50% less. Some tokens are created for a whole different kind of purpose: to represent a physical thing.
Examples
For instance, the Ethereum blockchain’s native token is ether, also called ETH. While ether is the coin native to the Ethereum blockchain, there are many other different tokens that also utilize the Ethereum blockchain. Tokens like Tether USD (USDT), Shiba Inu (SHIB), Uniswap (UNI), and ChainLink (LINK) are among the popular tokens on the Ethereum network. Collectively, all of the tokens that operate on the Ethereum network are called ERC-20 tokens.
Another famous and well-known example is the Binance Smart Chain. Binance coin (BNB) is BSC’s native coin and there are many tokens that operate on BSC. Tokens like Binance USD (BUSD), PancakeSwap (CAKE), BakeryToken (BAKE), and Safemoon are among the popular tokens on the Binance Smart Chain network. Collectively, all of the tokens that operate on the Binance Smart Chain network are called BEP-20 tokens.
Smart Contract and NFTs
Smart Contracts
A “smart contract” is simply a program that runs on the Ethereum blockchain. It’s a collection of code and data that resides at a specific address on the Ethereum blockchain.
Smart contracts are a type of Ethereum account. This means they have a balance and they can send transactions over the network. However they’re not controlled by a user, instead they are deployed to the network and run as programmed. User accounts can then interact with a smart contract by submitting transactions that execute a function defined on the smart contract. Smart contracts can define rules, like a regular contract, and automatically enforce them via the code. Smart contracts can not be deleted by default, and interactions with them are irreversible.
Anyone can write a smart contract and deploy it to the network. You just need to learn how to code in a smart contract language, and have enough ETH to deploy your contract. Deploying a smart contract is technically a transaction, so you need to pay your Gas in the same way that you need to pay gas for a simple ETH transfer. Gas costs for contract deployment are far higher, however.
Essentially, Ethereum tokens are smart contracts that make use of the Ethereum blockchain.
Non-Fungible Tokens (NFTs)
Another type of token that also uses smart contracts is called non-fungible tokens, or also known as NFTs. NFTs are tokens that we can use to represent ownership of unique items. They let us tokenize things like art, collectibles, even real estate. They can only have one official owner at a time and they’re secured by the Ethereum blockchain – no one can modify the record of ownership or copy/paste a new NFT into existence.
An NFT is digitalized and coded the same way a token is. An NFT is also not duplicable and retains all of its transaction histories. You can tell who has owned that specific NFT and how many times it has been traded before. Because of that, you can easily prove if you created the NFT or if you are the current owner of the NFT. NFT can also be set up with royalties. If you are the creator of an NFT, you can receive royalties every time it gets traded in the future.
Benefits of a token
Since the developer of a dApp and token doesn’t have to create their own blockchain, it saves them time and resources. They can use the features of cryptocurrency with their application while benefiting from the security of the native blockchain.
Time isn’t the only thing it saves them. If they created their own blockchain and coin instead of a dApp and token, they would need to find miners to verify their transactions, too. It takes a lot of miners to create a strong blockchain that can’t be attacked. It makes much more sense for many computers to work on one shared blockchain that several applications can run on rather than there being thousands of weak, mostly-centralized blockchains. It’s a much longer, much more expensive process.
To see the complete list of coins and tokens supported by SecuX, please visit
//secuxtech.com/supported-coins-tokens
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| 0.64046 |
Wink24News
Get updates for world's latest news with Wink24news headlines.
This is add
How To Fix Issue of a Hacked Facebook Account?
How to Change Password of Hacked Facebook Account
Nowadays there are numerous cases of hacking facebook accounts. Every morning, it is common to hear, “My account is hacked”. Let’s discuss some preventive steps about how to fix a hacked facebook account i.e. profile, personal information, email address, password, posting on the wall (without permission), photos and other kinds of details. The below given steps will help in making the security harder for hackers break security wall.
What is a Facebook Hack?
When any person illegally wants to theft personal information of account like email id, account password, profile information, photos, posts, statuses, a list of friends, make app purchases from an account or delete stolen account permanently, it is called as Facebook hacking. A hacker can do all illegal or embarrassing things about account holder as he can spam friends and steal their personal information also. There are steps to prevent hacking and already hacked an account.
How to Prevent a Hack on Your Facebook Account:
Hackers are very intelligent and well-educated people and they know very well how to hack the account and steal information. Pay attention to following instructions to prevent an account from hacking.
1. Create a strong password with the help of capital case alphabets, lowercase alphabets, and numbers. Better to make unusual and unreal spell word so that no one can imagine even about it. Together strings of words also helpful, for example, write ‘ilikeapple’ instead of ‘I like apple’. In short, write without space provided.
2. Never pass facebook password to anyone. In a case of emergency if you have to give it to anyone, immediately change the password.
3. Confirm log out of account if you are in a public setting like library, gardens, malls, or other. Never save the password in a browser, phone or any diary.
4. Always secure computer with latest version antivirus software or spyware.
5. Always make sure that you visit and log in at Facebook.com only and not in any other account.
How to Fix a Hacked Facebook Account:
Whenever it comes to know that account is hacked, immediately report it at Facebook page and ask to secure the account. It is recommended to check any charges on the account because sometimes after hacking, they use the account for purchasing things. Check purchase history and verify that all purchase details are real. If any posted photo or status or comment is not added by an account holder, first secure account and then delete a post, photo or comment and send a note about that.
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| 0.618081 |
The Future Of Software Engineers In The Age Of AI
· 929 words · 5 minute read
What are we still good for? Finding our place amidst machines
The rise of artificial intelligence (AI) has been nothing short of a technological revolution. As AI continues to evolve, questions arise about its impact on various sectors. Including the world of software engineering. Will AI replace us? If not, where will we find work in an increasingly automated landscape?
The machine takeover 🔗
Imagine this: It’s a brisk morning in 2035, and you, a software engineer, are sipping your coffee, reviewing code. Except it’s not your code. It’s the code written by an AI assistant overnight. An AI tool can now do in hours what might have taken days or weeks for a (human) developer. In such a world, where does the human developer fit in? AI systems today are already able to code basic software applications. Is that the end of the road for human developers? Far from it.
The human touch: Beyond code 🔗
According to a survey featured on IEEE Xplore1, AI will influence occupations across various sectors. Among these, software engineering is at the forefront. The role of software engineers is evolving, necessitating a shift in their duties and responsibilities. However, the narrative that AI will replace software engineers is oversimplified. A more nuanced perspective suggests that AI is likely to augment, rather than replace, human capabilities2. In other words, the future of software engineering isn’t about humans versus machines. It is about humans working with machines.
AI excels at repetitive tasks and pattern recognition. But human developers still hold the edge in creative problem-solving and nuanced understanding. After all, who teaches these AIs? We do. And who integrates the myriad of AI modules into a cohesive, user-friendly application? That’s us.
User-centric software design requires a deep understanding of cultural, psychological, and societal nuances. An AI can provide solutions based on data, but human intuition and empathy, for now, remain unmatched.
The new role of software engineers 🔗
Software engineers are no longer just coders or troubleshooters. They are architects of complex systems, designers of user experiences, and custodians of data integrity3. This shift is driven by the increasing integration of AI into various aspects of software development.
For instance, in the realm of design, AI can automate routine tasks, freeing up software engineers to focus on more complex and creative aspects of the job3. Similarly, in database management, AI tools can handle data crunching, allowing engineers to concentrate on strategic decisions4.
Areas of Opportunity: Where humans shine 🔗
So, where can software engineers find work in this AI-driven landscape?
AI training and maintenance 🔗
Consider the art of training an AI. It’s akin to teaching a child5. And just like children, no two AIs learn the same way. Who better than human engineers to guide, tweak, and refine these learning processes? The creation and maintenance of AI systems themselves present significant opportunities. These systems require sophisticated programming, continuous tweaking, and rigorous testing6. Tasks that are currently beyond the capabilities of AI.
Interdisciplinary integration 🔗
Ever tried explaining the concept of ‘happiness’ or ‘sorrow’ to a machine? Complex concepts, like art, humanities, or social sciences, still need a human touch. Especially when integrating software into these domains. As AI applications grow more complex and widespread, there will be a heightened need for professionals, who can understand these systems and communicate their workings to non-technical stakeholders7. This opens up roles for software engineers as AI translators or liaisons.
Ethical considerations 🔗
AI development is fraught with ethical dilemmas. Decisions about data privacy, fairness, and more need a human moral compass. Can you trust an AI to make value judgments? The ethical and legal implications of AI introduce another domain where human expertise is indispensable8. Software engineers with a strong understanding of AI can contribute to the development of guidelines and regulations for AI use.
The human-AI collaboration: A symphony 🔗
Remember when calculators were thought to spell the end for mathematicians? Instead, they became tools that amplified human capabilities. Similarly, AIs can be our partners, not replacements. AI doesn’t replace you. It can complement you. We still need humans who can read and understand code, maybe more than ever. Who can even begin to reason what a complex system does, if not for software engineers?
The road ahead: Continuous learning 🔗
To remain relevant, what should today’s developers focus on? Here are some thoughts:
1. Soft Skills: Building relationships, understanding user needs, and effective communication are as crucial as ever.
2. Specialization: Dive deep into niche areas where AI is yet to make significant inroads.
3. AI Literacy: If you can’t beat them, join them. Understand AI, work with it, and harness its strengths. AI won’t replace you, but a person using AI probably will.
Conclusion 🔗
AI is undeniably transforming the landscape of software engineering. But it is not signaling the end for human software engineers. Instead, it’s reshaping their roles and presenting new opportunities. The future isn’t about choosing between humans and AI—it’s about harmonizing the strengths of both. As we navigate this exciting frontier, the question we should ask isn’t whether AI will replace us, but rather, how can we adapt and thrive9? The next time someone says AI will replace software developers, pose this question: Who dreams? Machines or us? The beauty of software development isn’t just in writing code but in the vision, dreams, and aspirations behind that code. We will not lose our jobs, but in a couple of years they will not look the same. As software engineers we are used to change and trying to keep up with it. We just have to adapt, again. After all, it’s part of the job.
1. https://ieeexplore.ieee.org/abstract/document/9171118/ ↩︎
2. https://books.google.com/books?hl=en&lr=&id=wpY4DwAAQBAJ&oi=fnd&pg=PT14&dq=future+of+software+engineers+in+the+age+of+AI&ots=KufwrUaOGP&sig=Fr58Qik-TrYM8Uwvjj7d7oVB9NE ↩︎
3. https://onlinelibrary.wiley.com/doi/abs/10.1111/jpim.12523 ↩︎
4. https://books.google.com/books?hl=en&lr=&id=pmYyDwAAQBAJ&oi=fnd&pg=PR5&dq=future+of+software+engineers+in+the+age+of+AI&ots=oeckqAdVSy&sig=u-lb9uu9HK2xLWN_k2F4_m4gWIU ↩︎
5. https://towardsdatascience.com/raising-a-child-vs-training-a-machine-9b33a5f4cc2a ↩︎
6. https://journals.sfu.ca/ijitls/index.php/ijitls/article/download/23/pdf/0 ↩︎
7. https://arxiv.org/abs/2304.14597 ↩︎
8. https://dl.acm.org/doi/abs/10.1145/3278721.3278737 ↩︎
9. https://books.google.com/books?hl=en&lr=&id=YHxnDwAAQBAJ&oi=fnd&pg=PR11&dq=future+of+software+engineers+in+the+age+of+AI&ots=tV2VT7b3D_&sig=Mso-38DNhIB92jeRakyHi-IwnUc ↩︎
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5. Detailed description of the Actions > 5.2. Input Actions > 5.2.22. Join Input
5.2.22. Join Input
<< Click to Display Table of Contents >>
Navigation: 5. Detailed description of the Actions > 5.2. Input Actions >
5.2.22. Join Input
Icon: ANATEL~2_img393
Function: JoinInput
Property window:
ANATEL~2_img395ANATEL~2_img394
Short description:
Left-Join several tables into one gigantic table.
Long description:
ANATEL~2_img8
Pre-requisite
All the input tables must be sorted on the "Key" columns using the SAME sorting algorithm (all "numeric sort" or all "alpha-numeric sort").
The first step before creating any predictive model is to build a learning dataset that contains as many “good” features as possible (this is named “feature engineering”). Typically, each group of features is computed using one different Anatella graph. To gain time, you’ll usually run all the required “feature engineering” graphs in parallel (using the ANATEL~2_img397 ParallelRun Action from section 5.3.3. or using the ANATEL~2_img398 loopAnatellaGraphAdv Action from section 5.20.6 or using the ANATEL~2_img399 “Loop Jenkins” action from section 5.21.1). Each of these “feature engineering” graph delivers, as output, one small .gel_anatella file that contains some more features. The final step of the construction of your learning dataset is to “assemble” all these small .gel_anatella files into one large unique .gel_anatella file (that is your learning dataset). This “final assembly” might look like this:
ANATEL~2_img400
The above (real-life) example illustrates that, for a large number of different features, the construction of this “final assembly” might rapidly become an un-manageable graph with too many arrows and boxes (that looks like a plate of spaghetti! ). At that point, you quickly understand why the ANATEL~2_img393 JoinInput Action is really interesting!
To illustrate how to the ANATEL~2_img393 JoinInput Action works, let’s look at a first example: The objective of this example is to add to a “customer.gel_anatella” table some additional features that are (initially) saved inside the 3 “slave” tables that are named “a.gel_anatella”, “b.gel_anatella” and “c.gel_anatella”. To make this assembly, we can use any of these two Anatella graphs:
clip0034
At first sight, these 2 graphs do look pretty similar in complexity. The main difference occurs when you increase the number of “slave” tables (that contains all the computed features) inside the Join (the above example only contains 3 “slave” tables: “a.gel_anatella”, “b.gel_anatella” and “c.gel_anatella”): With the ANATEL~2_img393 JoinInput Action, you can easily have thousands of Slave tables, while the (other) approach based on the simple ANATEL~2_img408 Join Action becomes un-manageable pretty quickly: Already with as little as 100 “Slave” tables, the Anatella graph is becoming a total mess to maintain&support: Just think about the number of boxes! (it will work but it will definitively be less manageable than the ANATEL~2_img393 JoinInput Action).
ANATEL~2_img8
The maximum number of “slave” tables inside the ANATEL~2_img393 JoinInput Action is around 20000. This means that Anatella can effectively compute a Left join between 20000 tables! (as a comparison, the best databases are limited to 32 tables in a join)
Let’s now assume that you need to create a learning/scoring dataset that has a very large number of rows. Each row represents one customer. To divide the computing time by 2, you decided to store the raw data collected on your customers on two different servers (the data from the customers with an odd primary key is stored on the first server and the data from the customers with an even primary key is stored on the second server). Then, when you are computing your features, you use both servers simultaneously (e.g. using the ANATEL~2_img399 “Loop Jenkins” action from section 5.21.1). At the end of the feature computation, the first server contains all the features for half your customers and the second server contains all the features for the other half of your custromer. Then, you need to do the “final assembly”: To make this assembly, we can use any of these two Anatella graphs:
clip0035
What happens if:
…instead of using 2 servers, we use 3 servers?
…instead of computing only the “a” features, we also compute the “b” and “c” feature sets?
To make the “final assembly”, we can use any of these two Anatella graphs:
clip0036
Inside the above example, all the partitions used for the ANATEL~2_img393 JoinInput Action are composed of 3 files. In practice, each partition can have a different number of files: No worries! clip0037
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__label__pos
| 0.983348 |
BI, Database Management System
(a) Why did SAP introduce the extended star schema?
Posted Date: 1/17/2015 10:55:21 PM | Location :
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__label__pos
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Working with big data#
Added in version 1.2.
HyperSpy makes it possible to analyse data larger than the available memory by providing “lazy” versions of most of its signals and functions. In most cases the syntax remains the same. This chapter describes how to work with data larger than memory using the LazySignal class and its derivatives.
Creating Lazy Signals#
Lazy Signals from external data#
If the data is large and not loaded by HyperSpy (for example a hdf5.Dataset or similar), first wrap it in dask.array.Array as shown here and then pass it as normal and call as_lazy():
>>> import h5py
>>> f = h5py.File("myfile.hdf5")
>>> data = f['/data/path']
Wrap the data in dask and chunk as appropriate
>>> import dask.array as da
>>> x = da.from_array(data, chunks=(1000, 100))
Create the lazy signal
>>> s = hs.signals.Signal1D(x).as_lazy()
Loading lazily#
To load the data lazily, pass the keyword lazy=True. As an example, loading a 34.9 GB .blo file on a regular laptop might look like:
>>> s = hs.load("shish26.02-6.blo", lazy=True)
>>> s
<LazySignal2D, title: , dimensions: (400, 333|512, 512)>
>>> s.data
dask.array<array-e..., shape=(333, 400, 512, 512), dtype=uint8, chunksize=(20, 12, 512, 512)>
>>> print(s.data.dtype, s.data.nbytes / 1e9)
uint8 34.9175808
Change dtype to perform decomposition, etc.
>>> s.change_dtype("float")
>>> print(s.data.dtype, s.data.nbytes / 1e9)
float64 279.3406464
Loading the dataset in the original unsigned integer format would require around 35GB of memory. To store it in a floating-point format one would need almost 280GB of memory. However, with the lazy processing both of these steps are near-instantaneous and require very little computational resources.
Added in version 1.4: close_file()
Currently when loading an hdf5 file lazily the file remains open at least while the signal exists. In order to close it explicitly, use the close_file() method. Alternatively, you could close it on calling compute() by passing the keyword argument close_file=True e.g.:
>>> s = hs.load("file.hspy", lazy=True)
>>> ssum = s.sum(axis=0)
Close the file
>>> ssum.compute(close_file=True)
Lazy stacking#
Occasionally the full dataset consists of many smaller files. To combine them into a one large LazySignal, we can stack them lazily (both when loading or afterwards):
>>> siglist = hs.load("*.hdf5")
>>> s = hs.stack(siglist, lazy=True)
Or load lazily and stack afterwards:
>>> siglist = hs.load("*.hdf5", lazy=True)
Make a stack, no need to pass 'lazy', as signals are already lazy
>>> s = hs.stack(siglist)
Or do everything in one go:
>>> s = hs.load("*.hdf5", lazy=True, stack=True)
Casting signals as lazy#
To convert a regular HyperSpy signal to a lazy one such that any future operations are only performed lazily, use the as_lazy() method:
>>> s = hs.signals.Signal1D(np.arange(150.).reshape((3, 50)))
>>> s
<Signal1D, title: , dimensions: (3|50)>
>>> sl = s.as_lazy()
>>> sl
<LazySignal1D, title: , dimensions: (3|50)>
Machine learning#
Warning
The machine learning features are in beta state.
Although most of them work as described, their operation may not always be optimal, well-documented and/or consistent with their in-memory counterparts.
Decomposition algorithms for machine learning often perform large matrix manipulations, requiring significantly more memory than the data size. To perform decomposition operation lazily, HyperSpy provides access to several “online” algorithms as well as dask’s lazy SVD algorithm. Online algorithms perform the decomposition by operating serially on chunks of data, enabling the lazy decomposition of large datasets. In line with the standard HyperSpy signals, lazy decomposition() offers the following online algorithms:
Available lazy decomposition algorithms in HyperSpy#
Algorithm
Method
“SVD” (default)
dask.array.linalg.svd()
“PCA”
sklearn.decomposition.IncrementalPCA
“ORPCA”
ORPCA
“ORNMF”
ORNMF
See also
decomposition() for more details on decomposition with non-lazy signals.
GPU support#
Lazy data processing on GPUs requires explicitly transferring the data to the GPU.
On linux, it is recommended to use the dask_cuda library (not supported on windows) to manage the dask scheduler. As for CPU lazy processing, if the dask scheduler is not specified, the default scheduler will be used.
>>> from dask_cuda import LocalCUDACluster
>>> from dask.distributed import Client
>>> cluster = LocalCUDACluster()
>>> client = Client(cluster)
>>> import cupy as cp
>>> import dask.array as da
Create a dask array
>>> data = da.random.random(size=(20, 20, 100, 100))
>>> data
dask.array<random_sample, shape=(20, 20, 100, 100), dtype=float64, chunksize=(20, 20, 100, 100), chunktype=numpy.ndarray>
Convert the dask chunks from numpy array to cupy array
>>> data = data.map_blocks(cp.asarray)
>>> data
dask.array<random_sample, shape=(20, 20, 100, 100), dtype=float64, chunksize=(20, 20, 100, 100), chunktype=cupy.ndarray>
Create the signal
>>> s = hs.signals.Signal2D(data).as_lazy()
Note
See the dask blog on Richardson Lucy (RL) deconvolution for an example of lazy processing on GPUs using dask and cupy
Model fitting#
Most curve-fitting functionality will automatically work on models created from lazily loaded signals. HyperSpy extracts the relevant chunk from the signal and fits to that.
The linear 'lstsq' optimizer supports fitting the entire dataset in a vectorised manner using dask.array.linalg.lstsq(). This can give potentially enormous performance benefits over fitting with a nonlinear optimizer, but comes with the restrictions explained in the linear fitting section.
Practical tips#
Despite the limitations detailed below, most HyperSpy operations can be performed lazily. Important points are:
Chunking#
Data saved in the HDF5 format is typically divided into smaller chunks which can be loaded separately into memory, allowing lazy loading. Chunk size can dramatically affect the speed of various HyperSpy algorithms, so chunk size is worth careful consideration when saving a signal. HyperSpy’s default chunking sizes are probably not optimal for a given data analysis technique. For more comprehensible documentation on chunking, see the dask array chunks and best practices docs. The chunks saved into HDF5 will match the dask array chunks in s.data.chunks when lazy loading. Chunk shape should follow the axes order of the numpy shape (s.data.shape), not the hyperspy shape. The following example shows how to chunk one of the two navigation dimensions into smaller chunks:
>>> import dask.array as da
>>> data = da.random.random((10, 200, 300))
>>> data.chunksize
(10, 200, 300)
>>> s = hs.signals.Signal1D(data).as_lazy()
Note the reversed order of navigation dimensions
>>> s
<LazySignal1D, title: , dimensions: (200, 10|300)>
Save data with chunking first hyperspy dimension (second array dimension)
>>> s.save('chunked_signal.zspy', chunks=(10, 100, 300))
>>> s2 = hs.load('chunked_signal.zspy', lazy=True)
>>> s2.data.chunksize
(10, 100, 300)
To get the chunk size of given axes, the get_chunk_size() method can be used:
>>> import dask.array as da
>>> data = da.random.random((10, 200, 300))
>>> data.chunksize
(10, 200, 300)
>>> s = hs.signals.Signal1D(data).as_lazy()
>>> s.get_chunk_size() # All navigation axes
((10,), (200,))
>>> s.get_chunk_size(0) # The first navigation axis
((200,),)
Added in version 2.0.0.
Starting in version 2.0.0 HyperSpy does not automatically rechunk datasets as this can lead to reduced performance. The rechunk or optimize keyword argument can be set to True to let HyperSpy automatically change the chunking which could potentially speed up operations.
Added in version 1.7.0.
For more recent versions of dask (dask>2021.11) when using hyperspy in a jupyter notebook a helpful html representation is available.
>>> import dask.array as da
>>> data = da.zeros((20, 20, 10, 10, 10))
>>> s = hs.signals.Signal2D(data).as_lazy()
>>> s
../_images/chunks.png
This helps to visualize the chunk structure and identify axes where the chunk spans the entire axis (bolded axes).
Computing lazy signals#
Upon saving lazy signals, the result of computations is stored on disk.
In order to store the lazy signal in memory (i.e. make it a normal HyperSpy signal) it has a compute() method:
>>> s
<LazySignal2D, title: , dimensions: (10, 20, 20|10, 10)>
>>> s.compute()
[########################################] | 100% Completed | 0.1s
>>> s
<Signal2D, title: , dimensions: (10, 20, 20|10, 10)>
Lazy operations that affect the axes#
When using lazy signals the computation of the data is delayed until requested. However, the changes to the axes properties are performed when running a given function that modfies them i.e. they are not performed lazily. This can lead to hard to debug issues when the result of a given function that is computed lazily depends on the value of the axes parameters that may have changed before the computation is requested. Therefore, in order to avoid such issues, it is reccomended to explicitly compute the result of all functions that are affected by the axes parameters. This is the reason why e.g. the result of shift1D() is not lazy.
Dask Backends#
Dask is a flexible library for parallel computing in Python. All of the lazy operations in hyperspy run through dask. Dask can be used to run computations on a single machine or scaled to a cluster. The following example shows how to use dask to run computations on a variety of different hardware:
Single Threaded Scheduler#
The single threaded scheduler in dask is useful for debugging and testing. It is not recommended for general use.
>>> import dask
>>> import hyperspy.api as hs
>>> import numpy as np
>>> import dask.array as da
Set the scheduler to single-threaded globally
>>> dask.config.set(scheduler='single-threaded')
Alternatively, you can set the scheduler to single-threaded for a single function call by setting the scheduler keyword argument to 'single-threaded'.
Or for something like plotting you can set the scheduler to single-threaded for the duration of the plotting call by using the with dask.config.set context manager.
>>> s.compute(scheduler="single-threaded")
>>> with dask.config.set(scheduler='single-threaded'):
... s.plot()
Single Machine Schedulers#
Dask has two schedulers available for single machines.
1. Threaded Scheduler:
Fastest to set up but only provides parallelism through threads so only non python functions will be parallelized. This is good if you have largely numpy code and not too many cores.
2. Processes Scheduler:
Each task (and all of the necessary dependencies) are shipped to different processes. As such it has a larger set up time. This preforms well for python dominated code.
>>> import dask
>>> dask.config.set(scheduler='processes')
Any hyperspy code will now use the multiprocessing scheduler
>>> s.compute()
Change to threaded Scheduler, overwrite default
>>> dask.config.set(scheduler='threads')
>>> s.compute()
Distributed Scheduler#
Warning
Distributed computing is not supported for all file formats.
Distributed computing is limited to a few file formats, see the list of supported file format in RosettaSciIO documentation.
The recommended way to use dask is with the distributed scheduler. This allows you to scale your computations to a cluster of machines. The distributed scheduler can be used on a single machine as well. dask-distributed also gives you access to the dask dashboard which allows you to monitor your computations.
Some operations such as the matrix decomposition algorithms in hyperspy don’t currently work with the distributed scheduler.
>>> from dask.distributed import Client
>>> from dask.distributed import LocalCluster
>>> import dask.array as da
>>> import hyperspy.api as hs
>>> cluster = LocalCluster()
>>> client = Client(cluster)
>>> client
Any calculation will now use the distributed scheduler
>>> s
>>> s.plot()
>>> s.compute()
Running computation on remote cluster can be done easily using dask_jobqueue
>>> from dask_jobqueue import SLURMCluster
>>> from dask.distributed import Client
>>> cluster = SLURMCluster(cores=48,
... memory='120Gb',
... walltime="01:00:00",
... queue='research')
Get 3 nodes
>>> cluster.scale(jobs=3)
>>> client = Client(cluster)
>>> client
Any calculation will now use the distributed scheduler
>>> s = hs.data.two_gaussians()
>>> repeated_data = da.repeat(da.array(s.data[np.newaxis, :]),10, axis=0)
>>> s = hs.signals.Signal1D(repeated_data).as_lazy()
>>> summed = s.map(np.sum, inplace=False)
>>> s.compute()
Limitations#
Most operations can be performed lazily. However, lazy operations come with a few limitations and constraints that we detail below.
Immutable signals#
An important limitation when using LazySignal is the inability to modify existing data (immutability). This is a logical consequence of the DAG (tree structure, explained in Behind the scenes – technical details), where a complete history of the processing has to be stored to traverse later.
In fact, lazy evaluation removes the need for such operation, since only additional tree branches are added, requiring very little resources. In practical terms the following fails with lazy signals:
>>> s = hs.signals.BaseSignal([0]).as_lazy()
>>> s += 1
Traceback (most recent call last):
File "<ipython-input-6-1bd1db4187be>", line 1, in <module>
s += 1
File "<string>", line 2, in __iadd__
File "/home/fjd29/Python/hyperspy3/hyperspy/signal.py", line 1591, in _binary_operator_ruler
getattr(self.data, op_name)(other)
AttributeError: 'Array' object has no attribute '__iadd__'
However, when operating lazily there is no clear benefit to using in-place operations. So, the operation above could be rewritten as follows:
>>> s = hs.signals.BaseSignal([0]).as_lazy()
>>> s = s + 1
Or even better:
>>> s = hs.signals.BaseSignal([0]).as_lazy()
>>> s1 = s + 1
Other minor differences#
• Histograms for a LazySignal do not support knuth and blocks binning algorithms.
• CircleROI sets the elements outside the ROI to np.nan instead of using a masked array, because dask does not support masking. As a convenience, nansum, nanmean and other nan* signal methods were added to mimic the workflow as closely as possible.
Saving Big Data#
The most efficient format supported by HyperSpy to write data is the ZSpy format, mainly because it supports writing concurrently from multiple threads or processes. This also allows for smooth interaction with dask-distributed for efficient scaling.
Behind the scenes – technical details#
Standard HyperSpy signals load the data into memory for fast access and processing. While this behaviour gives good performance in terms of speed, it obviously requires at least as much computer memory as the dataset, and often twice that to store the results of subsequent computations. This can become a significant problem when processing very large datasets on consumer-oriented hardware.
HyperSpy offers a solution for this problem by including LazySignal and its derivatives. The main idea of these classes is to perform any operation (as the name suggests) lazily (delaying the execution until the result is requested (e.g. saved, plotted)) and in a blocked fashion. This is achieved by building a “history tree” (formally called a Directed Acyclic Graph (DAG)) of the computations, where the original data is at the root, and any further operations branch from it. Only when a certain branch result is requested, the way to the root is found and evaluated in the correct sequence on the correct blocks.
The “magic” is performed by (for the sake of simplicity) storing the data not as numpy.ndarray, but dask.array.Array (see the dask documentation). dask offers a couple of advantages:
• Arbitrary-sized data processing is possible. By only loading a couple of chunks at a time, theoretically any signal can be processed, albeit slower. In practice, this may be limited: (i) some operations may require certain chunking pattern, which may still saturate memory; (ii) many chunks should fit into the computer memory comfortably at the same time.
• Loading only the required data. If a certain part (chunk) of the data is not required for the final result, it will not be loaded at all, saving time and resources.
• Able to extend to a distributed computing environment (clusters). :dask.distributed (see the dask documentation) offers a straightforward way to expand the effective memory for computations to that of a cluster, which allows performing the operations significantly faster than on a single machine.
|
__label__pos
| 0.662911 |
import string #Bi-directional BFS + memoization, Time O(s*d), Space O(s*d), #s is string length, d is number of words in steps def wordLadder(s, t, wordList) : if t not in wordList: return 0 dict = set(wordList) start = set() end = set() visited = set(); #works as memoization start.add(s) end.add(t) step = 1; letters = list(string.ascii_lowercase) while len(start)>0 and len(end)>0 : if len(start) > len(end): #start always has smaller set tmp = start start = end end = tmp tmp = set() for word in start: ch = list(word) for i in range(0, len(ch)): for c in letters : #constant time origC = ch[i]; ch[i] = c #replace c with all other letters in alphabet next = ''.join(ch) if next in end: return step + 1 if next not in visited and next in dict: tmp.add(next) visited.add(next) ch[i] = origC print(tmp) start = tmp step +=1 return 0 words = ["cat", "bat", "bet", "bot", "bog", "dog" ] s = "cat" t = "dog" print( words) print("Start: '" + s + "', end: '" + t + "'") print("Length of word ladder: " + str(wordLadder(s, t, words)))
|
__label__pos
| 0.999649 |
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