Split a large string into multiple substrings containing 'n' number of words via python

Question

Source text: United States Declaration of Independence

How can one split the above source text into a number of sub-strings, containing an 'n' number of words?

I use split(' ') to extract each word, however I do not know how to do this with multiple words in one operation.

I could run through the list of words that I have, and create another by gluing together words in the first list (whilst adding spaces). However my method isn't very pythonic.

Answer 1

You're trying to create n-grams? Here's how I do it, using the NLTK.

punct = re.compile(r'^[^A-Za-z0-9]+|[^a-zA-Z0-9]+$')
is_word=re.compile(r'[a-z]', re.IGNORECASE)
sentence_tokenizer = nltk.data.load('tokenizers/punkt/english.pickle')
word_tokenizer=nltk.tokenize.punkt.PunktWordTokenizer()

def get_words(sentence):
    return [punct.sub('',word) for word in word_tokenizer.tokenize(sentence) if is_word.search(word)]

def ngrams(text, n):
    for sentence in sentence_tokenizer.tokenize(text.lower()):
        words = get_words(sentence)
        for i in range(len(words)-(n-1)):
            yield(' '.join(words[i:i+n]))

Then

for ngram in ngrams(sometext, 3):
    print ngram

Answer 2

text = """
When in the course of human Events, it becomes necessary for one People to dissolve the Political Bands which have connected them with another, and to assume among the Powers of the Earth, the separate and equal Station to which the Laws of Nature and of Nature?s God entitle them, a decent Respect to the Opinions of Mankind requires that they should declare the causes which impel them to the Separation.

We hold these Truths to be self-evident, that all Men are created equal, that they are endowed by their Creator with certain unalienable Rights, that among these are Life, Liberty, and the pursuit of Happiness?-That to secure these Rights, Governments are instituted among Men, deriving their just Powers from the Consent of the Governed, that whenever any Form of Government becomes destructive of these Ends, it is the Right of the People to alter or abolish it, and to institute a new Government, laying its Foundation on such Principles, and organizing its Powers in such Form, as to them shall seem most likely to effect their Safety and Happiness. Prudence, indeed, will dictate that Governments long established should not be changed for light and transient Causes; and accordingly all Experience hath shewn, that Mankind are more disposed to suffer, while Evils are sufferable, than to right themselves by abolishing the Forms to which they are accustomed. But when a long Train of Abuses and Usurpations, pursuing invariably the same Object, evinces a Design to reduce them under absolute Despotism, it is their Right, it is their Duty, to throw off such Government, and to provide new Guards for their future Security. Such has been the patient Sufferance of these Colonies; and such is now the Necessity which constrains them to alter their former Systems of Government. The History of the Present King of Great-Britain is a History of repeated Injuries and Usurpations, all having in direct Object the Establishment of an absolute Tyranny over these States. To prove this, let Facts be submitted to a candid World.
"""

words = text.split()
subs = []
n = 4
for i in range(0, len(words), n):
    subs.append(" ".join(words[i:i+n]))
print subs[:10]

prints:

['When in the course', 'of human Events, it', 'becomes necessary for one', 'People to dissolve the', 'Political Bands which have', 'connected them with another,', 'and to assume among', 'the Powers of the', 'Earth, the separate and', 'equal Station to which']

or, as a list comprehension:

subs = [" ".join(words[i:i+n]) for i in range(0, len(words), n)]

Answer 3

For large string, iterator is recommended for speed and low memory footprint.

import re, itertools

# Original text
text = "When in the course of human Events, it becomes necessary for one People to dissolve the Political Bands which have connected them with another, and to assume among the Powers of the Earth, the separate and equal Station to which the Laws of Nature and of Nature?s God entitle them, a decent Respect to the Opinions of Mankind requires that they should declare the causes which impel them to the Separation."
n = 10

# An iterator which will extract words one by one from text when needed
words = itertools.imap(lambda m:m.group(), re.finditer(r'\w+', text))
# The final iterator that combines words into n-length groups
word_groups = itertools.izip_longest(*(words,)*n)

for g in word_groups: print g

will get the following result:

('When', 'in', 'the', 'course', 'of', 'human', 'Events', 'it', 'becomes', 'necessary')
('for', 'one', 'People', 'to', 'dissolve', 'the', 'Political', 'Bands', 'which', 'have')
('connected', 'them', 'with', 'another', 'and', 'to', 'assume', 'among', 'the', 'Powers')
('of', 'the', 'Earth', 'the', 'separate', 'and', 'equal', 'Station', 'to', 'which')
('the', 'Laws', 'of', 'Nature', 'and', 'of', 'Nature', 's', 'God', 'entitle')
('them', 'a', 'decent', 'Respect', 'to', 'the', 'Opinions', 'of', 'Mankind', 'requires')
('that', 'they', 'should', 'declare', 'the', 'causes', 'which', 'impel', 'them', 'to')
('the', 'Separation', None, None, None, None, None, None, None, None)