Sample usage for grammar

Grammar Parsing

Grammars can be parsed from strings:

>>> from nltk import CFG
>>> grammar = CFG.fromstring("""
... S -> NP VP
... PP -> P NP
... NP -> Det N | NP PP
... VP -> V NP | VP PP
... Det -> 'a' | 'the'
... N -> 'dog' | 'cat'
... V -> 'chased' | 'sat'
... P -> 'on' | 'in'
... """)
>>> grammar
<Grammar with 14 productions>
>>> grammar.start()
S
>>> grammar.productions()
[S -> NP VP, PP -> P NP, NP -> Det N, NP -> NP PP, VP -> V NP, VP -> VP PP,
Det -> 'a', Det -> 'the', N -> 'dog', N -> 'cat', V -> 'chased', V -> 'sat',
P -> 'on', P -> 'in']

Probabilistic CFGs:

>>> from nltk import PCFG
>>> toy_pcfg1 = PCFG.fromstring("""
... S -> NP VP [1.0]
... NP -> Det N [0.5] | NP PP [0.25] | 'John' [0.1] | 'I' [0.15]
... Det -> 'the' [0.8] | 'my' [0.2]
... N -> 'man' [0.5] | 'telescope' [0.5]
... VP -> VP PP [0.1] | V NP [0.7] | V [0.2]
... V -> 'ate' [0.35] | 'saw' [0.65]
... PP -> P NP [1.0]
... P -> 'with' [0.61] | 'under' [0.39]
... """)

Chomsky Normal Form grammar (Test for bug 474)

>>> g = CFG.fromstring("VP^<TOP> -> VBP NP^<VP-TOP>")
>>> g.productions()[0].lhs()
VP^<TOP>