49 lines
1.3 KiB
Plaintext
49 lines
1.3 KiB
Plaintext
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.. Copyright (C) 2001-2019 NLTK Project
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.. For license information, see LICENSE.TXT
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===============
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Grammar Parsing
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===============
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Grammars can be parsed from strings:
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>>> from nltk import CFG
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>>> grammar = CFG.fromstring("""
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... S -> NP VP
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... PP -> P NP
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... NP -> Det N | NP PP
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... VP -> V NP | VP PP
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... Det -> 'a' | 'the'
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... N -> 'dog' | 'cat'
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... V -> 'chased' | 'sat'
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... P -> 'on' | 'in'
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... """)
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>>> grammar
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<Grammar with 14 productions>
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>>> grammar.start()
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S
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>>> grammar.productions() # doctest: +NORMALIZE_WHITESPACE
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[S -> NP VP, PP -> P NP, NP -> Det N, NP -> NP PP, VP -> V NP, VP -> VP PP,
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Det -> 'a', Det -> 'the', N -> 'dog', N -> 'cat', V -> 'chased', V -> 'sat',
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P -> 'on', P -> 'in']
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Probabilistic CFGs:
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>>> from nltk import PCFG
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>>> toy_pcfg1 = PCFG.fromstring("""
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... S -> NP VP [1.0]
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... NP -> Det N [0.5] | NP PP [0.25] | 'John' [0.1] | 'I' [0.15]
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... Det -> 'the' [0.8] | 'my' [0.2]
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... N -> 'man' [0.5] | 'telescope' [0.5]
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... VP -> VP PP [0.1] | V NP [0.7] | V [0.2]
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... V -> 'ate' [0.35] | 'saw' [0.65]
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... PP -> P NP [1.0]
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... P -> 'with' [0.61] | 'under' [0.39]
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... """)
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Chomsky Normal Form grammar (Test for bug 474)
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>>> g = CFG.fromstring("VP^<TOP> -> VBP NP^<VP-TOP>")
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>>> g.productions()[0].lhs()
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VP^<TOP>
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