1.百科

• TF-IDF

2.源代码

• 系统环境

python 3.6
scikit-learn==0.19.1

# utf-8

import os
import math
import numpy as np

'''
不使用NLTK和Scikits-Learn包，构建文本向量空间模型

reference：
https://mp.weixin.qq.com/s/DisMF8frY2pkpGMfrWk4Wg
'''


def load_doc_list(file):
    with open(file, 'r') as f:
        return f.read().splitlines()


'''
第一步:Basic term frequencies

frequencies:计算文本各行的单词频度（出现次数）,存在问题，文本空间大小不一样
build_lexicon:创建词汇表，以便构造相同文本空间向量（特征向量）
tf:调用freq，计算单词在文档中出现的次数
'''


def frequencies(doc_list):
    from collections import Counter
    counters=[]
    for doc in doc_list:
        c=Counter()
        for word in doc.split():
            c[word]+=1
        counters.append(c)
    return counters


def build_lexicon(corpus):
    lexicon=set()
    for doc in corpus:
        lexicon.update([w for w in doc.split()])
    return lexicon

def tf(term,doc):
    return freq(term,doc)

def freq(term,doc):
    return doc.split().count(term)


'''
第二步:Normalizing vectors to L2 Norm = 1
如果有些单词在一个单一的文件中过于频繁地出现，它们将扰乱我们的分析。
我们想要对每一个词频向量进行比例缩放，使其变得更具有代表性。换句话说，我们需要进行向量标准化，需要确保每个向量的L2范数等于1
l2_normalizer


'''

def l2_normalizer(vec):
    denom = np.sum([el**2 for el in vec])
    return [(el / math.sqrt(denom)) for el in vec]

'''
第三步:逆向文件频率(IDF)频率加权
利用反文档词频（IDF）调整每一个单词权重
对于词汇中的每一个词，我们都有一个常规意义上的信息值，用于解释他们在整个语料库中的相对频率。
回想一下，这个信息值是一个“逆”！即信息值越小的词，它在语料库中出现的越频繁。
为了得到TF-IDF加权词向量，你必须做一个简单的计算：tf * idf。
numDocsContaining
idf
build_idf_matrix
'''
def numDocsContaining(word, doclist):
    doccount = 0
    for doc in doclist:
        if freq(word, doc) > 0:
            doccount +=1
    return doccount


def idf(word, doclist):
    n_samples = len(doclist)
    df = numDocsContaining(word, doclist)
    return math.log(n_samples / 1+df)


def build_idf_matrix(idf_vector):
    '''
    将IDF向量转化为BxB的矩阵了，矩阵的对角线就是IDF向量，这样就可以用反文档词频矩阵乘以每一个词频向量
    '''
    idf_mat = np.zeros((len(idf_vector), len(idf_vector)))
    np.fill_diagonal(idf_mat, idf_vector)
    return idf_mat



if __name__ == '__main__':
    os.chdir('./python')
    file="vector.txt"
    doc_list=load_doc_list(file)
    
    # counters=frequencies(doc_list)
    # for counter in counters:
    #     print(counter)
    
    vocabulary=build_lexicon(doc_list)
    # print(vocabulary)
    doc_term_matrix =[]
    for doc in doc_list:
        # print("doc = ",doc)
        doc_term_vector = [tf(word, doc) for word in vocabulary]
        # print("vec = ",doc_term_vector)
        doc_term_matrix.append(doc_term_vector)
    # print(doc_term_matrix)
    doc_term_matrix_l2 = []
    for vec in doc_term_matrix:
        doc_term_matrix_l2.append(l2_normalizer(vec))
    # print('A regular old document term matrix: ')
    # print(np.matrix(doc_term_matrix))
    # print('A document term matrix with row-wise L2 norms of 1:')
    # print(np.matrix(doc_term_matrix_l2))

    idf_vector = [idf(word, doc_list) for word in vocabulary]
    print('Our vocabulary vector is [' + ', '.join(list(vocabulary)) + ']')
    print('The inverse document frequency vector is [' + ', '.join(
        format(freq, 'f') for freq in idf_vector) + ']')
    
    idf_matrix = build_idf_matrix(idf_vector)
    # print(idf_matrix)

    #tf*idf
    doc_term_matrix_tfidf = []
    #performing tf-idf matrix multiplication
    for tf_vector in doc_term_matrix:
        doc_term_matrix_tfidf.append(np.dot(tf_vector, idf_matrix))

    #normalizing
    doc_term_matrix_tfidf_l2 = []
    for tf_vector in doc_term_matrix_tfidf:
        doc_term_matrix_tfidf_l2.append(l2_normalizer(tf_vector))
    print(vocabulary)
    print(np.matrix(doc_term_matrix_tfidf_l2)) # np.matrix() just to make it easier to look at

3.参考：

1. http://python.jobbole.com/81311/
2. http://stanford.edu/~rjweiss/public_html/IRiSS2013/text2/notebooks/tfidf.html