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- # -*- coding: utf-8 -*-
- 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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