Quick implementation of character n-grams for word

Question

I wrote the following code for computing character bigrams and the output is right below. My question is, how do I get an output that excludes the last character (ie t)? and

Answer 1

Try zip:

>>> def word2ngrams(text, n=3, exact=True):
...   """ Convert text into character ngrams. """
...   return ["".join(j) for j in zip(*[text[i:] for i in range(n)])]
... 
>>> word2ngrams('foobarbarblacksheep')
['foo', 'oob', 'oba', 'bar', 'arb', 'rba', 'bar', 'arb', 'rbl', 'bla', 'lac', 'ack', 'cks', 'ksh', 'she', 'hee', 'eep']

but do note that it's slower:

import string, random, time

def zip_ngrams(text, n=3, exact=True):
  return ["".join(j) for j in zip(*[text[i:] for i in range(n)])]

def nozip_ngrams(text, n=3):
    return [text[i:i+n] for i in range(len(text)-n+1)]

# Generate 10000 random strings of length 100.
words = [''.join(random.choice(string.ascii_uppercase) for j in range(100)) for i in range(10000)]

start = time.time()
x = [zip_ngrams(w) for w in words]
print time.time() - start

start = time.time()
y = [nozip_ngrams(w) for w in words]
print time.time() - start        

print x==y

[out]:

0.314492940903
0.197558879852
True