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# Natural Language Toolkit: Utility functions
#
# Copyright (C) 2001-2020 NLTK Project
# Author: Steven Bird <[email protected]>
# URL: <http://nltk.org/>
# For license information, see LICENSE.TXT
from itertools import chain
def pad_sequence(
sequence,
n,
pad_left=False,
pad_right=False,
left_pad_symbol=None,
right_pad_symbol=None,
):
"""
Returns a padded sequence of items before ngram extraction.
>>> list(pad_sequence([1,2,3,4,5], 2, pad_left=True, pad_right=True, left_pad_symbol='<s>', right_pad_symbol='</s>'))
['<s>', 1, 2, 3, 4, 5, '</s>']
>>> list(pad_sequence([1,2,3,4,5], 2, pad_left=True, left_pad_symbol='<s>'))
['<s>', 1, 2, 3, 4, 5]
>>> list(pad_sequence([1,2,3,4,5], 2, pad_right=True, right_pad_symbol='</s>'))
[1, 2, 3, 4, 5, '</s>']
:param sequence: the source data to be padded
:type sequence: sequence or iter
:param n: the degree of the ngrams
:type n: int
:param pad_left: whether the ngrams should be left-padded
:type pad_left: bool
:param pad_right: whether the ngrams should be right-padded
:type pad_right: bool
:param left_pad_symbol: the symbol to use for left padding (default is None)
:type left_pad_symbol: any
:param right_pad_symbol: the symbol to use for right padding (default is None)
:type right_pad_symbol: any
:rtype: sequence or iter
"""
sequence = iter(sequence)
if pad_left:
sequence = chain((left_pad_symbol,) * (n - 1), sequence)
if pad_right:
sequence = chain(sequence, (right_pad_symbol,) * (n - 1))
return sequence
# add a flag to pad the sequence so we get peripheral ngrams?
def ngrams(
sequence,
n,
pad_left=False,
pad_right=False,
left_pad_symbol=None,
right_pad_symbol=None,
):
"""
Return the ngrams generated from a sequence of items, as an iterator.
For example:
>>> from nltk.util import ngrams
>>> list(ngrams([1,2,3,4,5], 3))
[(1, 2, 3), (2, 3, 4), (3, 4, 5)]
Wrap with list for a list version of this function. Set pad_left
or pad_right to true in order to get additional ngrams:
>>> list(ngrams([1,2,3,4,5], 2, pad_right=True))
[(1, 2), (2, 3), (3, 4), (4, 5), (5, None)]
>>> list(ngrams([1,2,3,4,5], 2, pad_right=True, right_pad_symbol='</s>'))
[(1, 2), (2, 3), (3, 4), (4, 5), (5, '</s>')]
>>> list(ngrams([1,2,3,4,5], 2, pad_left=True, left_pad_symbol='<s>'))
[('<s>', 1), (1, 2), (2, 3), (3, 4), (4, 5)]
>>> list(ngrams([1,2,3,4,5], 2, pad_left=True, pad_right=True, left_pad_symbol='<s>', right_pad_symbol='</s>'))
[('<s>', 1), (1, 2), (2, 3), (3, 4), (4, 5), (5, '</s>')]
:param sequence: the source data to be converted into ngrams
:type sequence: sequence or iter
:param n: the degree of the ngrams
:type n: int
:param pad_left: whether the ngrams should be left-padded
:type pad_left: bool
:param pad_right: whether the ngrams should be right-padded
:type pad_right: bool
:param left_pad_symbol: the symbol to use for left padding (default is None)
:type left_pad_symbol: any
:param right_pad_symbol: the symbol to use for right padding (default is None)
:type right_pad_symbol: any
:rtype: sequence or iter
"""
sequence = pad_sequence(
sequence, n, pad_left, pad_right, left_pad_symbol, right_pad_symbol
)
history = []
while n > 1:
# PEP 479, prevent RuntimeError from being raised when StopIteration bubbles out of generator
try:
next_item = next(sequence)
except StopIteration:
# no more data, terminate the generator
return
history.append(next_item)
n -= 1
for item in sequence:
history.append(item)
yield tuple(history)
del history[0] |