Return a matrix where each row corresponds to a Twitter account, and
    each column corresponds to the number of times a term is used by that
    account.

def vectorize(json_file, vec, dofit=True):
    """ Return a matrix where each row corresponds to a Twitter account, and
    each column corresponds to the number of times a term is used by that
    account. """
    ## CountVectorizer, efficiently.
    screen_names = [x[0] for x in extract_tweets(json_file)]
    if dofit:
        X = vec.fit_transform(x[1] for x in extract_tweets(json_file))
    else:
        X = vec.transform(x[1] for x in extract_tweets(json_file))
    return screen_names, X