用 Python 统计高频字数

转载自http://blog.csdn.net/cxsydjn/article/details/70991846

问题

(来自Udacity机器学习工程师纳米学位预览课程)

用 Python 实现函数 count_words(),该函数输入字符串 s 和数字 n,返回 s 中 n 个出现频率最高的单词。返回值是一个元组列表,包含出现次数最高的 n 个单词及其次数,即 [(<单词1>, <次数1>), (<单词2>, <次数2>), ... ],按出现次数降序排列。

可以假设所有输入都是小写形式,并且不含标点符号或其他字符(只包含字母和单个空格)。如果出现次数相同,则按字母顺序排列。

例如:

print count_words("betty bought a bit of butter but the butter was bitter",3)
输出:
[('butter', 2), ('a', 1), ('betty', 1)]

解法:

"""Count words."""

def count_words(s, n):
    """Return the n most frequently occuring words in s."""
    w = {}
    sp = s.split()
    # TODO: Count the number of occurences of each word in s
    for i in sp:
        if i not in w:
            w[i] = 1
        else:
            w[i] += 1

    # TODO: Sort the occurences in descending order (alphabetically in case of ties)
    top = sorted(w.items(), key=lambda item:(-item[1], item[0]))
    top_n = top[:n]
    # TODO: Return the top n most frequent words.
    return top_n


def test_run():
    """Test count_words() with some inputs."""
    print count_words("cat bat mat cat bat cat", 3)
    print count_words("betty bought a bit of butter but the butter was bitter", 3)


if __name__ == '__main__':
    test_run()

小结

主要两个小技巧:

  1. split()将输入字符串按空格分开;
  2. sorted()函数对字典 先按值,再按键 进行排序,尤其是item:(-item[1], item[0])) 代表先对item的第二个元素 降序 排列(item 之前用了-),然后对第一个元素 升序 排列。多个元素的元组亦然。

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