Udacity.深度学习.用 Python 统计字数.2017-

用 Python 统计高频字数
python排序问题
Python标准库——collections模块的Counter类

问题
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)]

"""Count words."""

def count_words(s, n):
    """Return the n most frequently occuring words in s."""

    # TODO: Count the number of occurences of each word in s
    import collections
    top = collections.Counter(s.split()).most_common()
    # print(collections.Counter(s.split()).most_common())

    # TODO: Sort the occurences in descending order (alphabetically in case of ties)
    top.sort(key=lambda t:(-t[1], t[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()