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# Natural Language Toolkit: Generating from a CFG
#
# Copyright (C) 2001-2012 NLTK Project
# Author: Steven Bird <sb@csse.unimelb.edu.au>
# URL: <http://www.nltk.org/>
# For license information, see LICENSE.TXT
#
from __future__ import print_function
from nltk.grammar import Nonterminal, parse_cfg
def generate(grammar, start=None):
if not start:
start = grammar.start()
return _generate_all(grammar, [start])[0]
def _generate_all(grammar, items):
frags = []
if len(items) == 1:
if isinstance(items[0], Nonterminal):
for prod in grammar.productions(lhs=items[0]):
frags.append(_generate_all(grammar, prod.rhs()))
else:
frags.append(items[0])
else:
for frag1 in _generate_all(grammar, [items[0]]):
for frag2 in _generate_all(grammar, items[1:]):
for frag in _multiply(frag1, frag2):
frags.append(frag)
return frags
def _multiply(frag1, frag2):
frags = []
if len(frag1) == 1:
frag1 = [frag1]
if len(frag2) == 1:
frag2 = [frag2]
for f1 in frag1:
for f2 in frag2:
frags.append(f1+f2)
return frags
grammar = parse_cfg("""
S -> NP VP
NP -> Det N
VP -> V NP
Det -> 'the'
Det -> 'a'
N -> 'man' | 'park' | 'dog' | 'telescope'
V -> 'saw' | 'walked'
P -> 'in' | 'with'
""")
for sent in generate(grammar):
print(' '.join(sent))
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