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import unittest
import nltk
from nltk.grammar import CFG
class ChomskyNormalFormForCFGTest(unittest.TestCase):
def test_simple(self):
grammar = CFG.fromstring(
"""
S -> NP VP
PP -> P NP
NP -> Det N | NP PP P
VP -> V NP | VP PP
VP -> Det
Det -> 'a' | 'the'
N -> 'dog' | 'cat'
V -> 'chased' | 'sat'
P -> 'on' | 'in'
"""
)
self.assertFalse(grammar.is_flexible_chomsky_normal_form())
self.assertFalse(grammar.is_chomsky_normal_form())
grammar = grammar.chomsky_normal_form(flexible=True)
self.assertTrue(grammar.is_flexible_chomsky_normal_form())
self.assertFalse(grammar.is_chomsky_normal_form())
grammar2 = CFG.fromstring(
"""
S -> NP VP
NP -> VP N P
VP -> P
N -> 'dog' | 'cat'
P -> 'on' | 'in'
"""
)
self.assertFalse(grammar2.is_flexible_chomsky_normal_form())
self.assertFalse(grammar2.is_chomsky_normal_form())
grammar2 = grammar2.chomsky_normal_form()
self.assertTrue(grammar2.is_flexible_chomsky_normal_form())
self.assertTrue(grammar2.is_chomsky_normal_form())
def test_complex(self):
grammar = nltk.data.load("grammars/large_grammars/atis.cfg")
self.assertFalse(grammar.is_flexible_chomsky_normal_form())
self.assertFalse(grammar.is_chomsky_normal_form())
grammar = grammar.chomsky_normal_form(flexible=True)
self.assertTrue(grammar.is_flexible_chomsky_normal_form())
self.assertFalse(grammar.is_chomsky_normal_form())
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