【python cookbook】【数据结构与算法】12.找出序列中出现次数最多的元素

问题：找出一个元素序列中出现次数最多的元素是什么

解决方案：collections模块中的Counter类正是为此类问题所设计的。它的一个非常方便的most_common()方法直接告诉你答案。

# Determine the most common words in a list

words = [
   ‘look‘, ‘into‘, ‘my‘, ‘eyes‘, ‘look‘, ‘into‘, ‘my‘, ‘eyes‘,
   ‘the‘, ‘eyes‘, ‘the‘, ‘eyes‘, ‘the‘, ‘eyes‘, ‘not‘, ‘around‘, ‘the‘,
   ‘eyes‘, "don‘t", ‘look‘, ‘around‘, ‘the‘, ‘eyes‘, ‘look‘, ‘into‘,
   ‘my‘, ‘eyes‘, "you‘re", ‘under‘
]

from collections import Counter
word_counts = Counter(words)
top_three = word_counts.most_common(3)
print(top_three)
# outputs [(‘eyes‘, 8), (‘the‘, 5), (‘look‘, 4)]

# Example of merging in more words

morewords = [‘why‘,‘are‘,‘you‘,‘not‘,‘looking‘,‘in‘,‘my‘,‘eyes‘]
word_counts.update(morewords)  #使用update()增加计数
print(word_counts.most_common(3))

>>> ================================ RESTART ================================
>>>
[(‘eyes‘, 8), (‘the‘, 5), (‘look‘, 4)]
[(‘eyes‘, 9), (‘the‘, 5), (‘my‘, 4)]
>>> 

在底层实现中，Counter是一个字典，在元素和它们出现的次数间做了映射。

>>> word_counts
Counter({‘eyes‘: 9, ‘the‘: 5, ‘my‘: 4, ‘look‘: 4, ‘into‘: 3, ‘around‘: 2, ‘not‘: 2, "don‘t": 1, ‘under‘: 1, ‘are‘: 1, ‘looking‘: 1, "you‘re": 1, ‘you‘: 1, ‘why‘: 1, ‘in‘: 1})
>>> word_counts.most_common(3)  #top_three
[(‘eyes‘, 9), (‘the‘, 5), (‘my‘, 4)]
>>> word_counts[‘not‘]
2
>>> word_counts[‘eyes‘]
9
>>> word_counts[‘eyes‘]+1
10
>>> word_counts
Counter({‘eyes‘: 9, ‘the‘: 5, ‘my‘: 4, ‘look‘: 4, ‘into‘: 3, ‘around‘: 2, ‘not‘: 2, "don‘t": 1, ‘under‘: 1, ‘are‘: 1, ‘looking‘: 1, "you‘re": 1, ‘you‘: 1, ‘why‘: 1, ‘in‘: 1})
>>> word_counts[‘eyes‘]=word_counts[‘eyes‘]+1  #手动增加元素计数
>>> word_counts
Counter({‘eyes‘: 10, ‘the‘: 5, ‘my‘: 4, ‘look‘: 4, ‘into‘: 3, ‘around‘: 2, ‘not‘: 2, "don‘t": 1, ‘under‘: 1, ‘are‘: 1, ‘looking‘: 1, "you‘re": 1, ‘you‘: 1, ‘why‘: 1, ‘in‘: 1})
>>> 

增加元素出现次数可以通过手动进行增加，也可以借助update()方法；

另外，Counter对象另一个特性是它们可以同各种数学运算操作结合起来使用：

>>> a=Counter(words)
>>> a
Counter({‘eyes‘: 8, ‘the‘: 5, ‘look‘: 4, ‘my‘: 3, ‘into‘: 3, ‘around‘: 2, ‘under‘: 1, "you‘re": 1, ‘not‘: 1, "don‘t": 1})
>>> b=Counter(morewords)
>>> b
Counter({‘not‘: 1, ‘my‘: 1, ‘in‘: 1, ‘you‘: 1, ‘looking‘: 1, ‘are‘: 1, ‘eyes‘: 1, ‘why‘: 1})
>>> c=a+b
>>> c
Counter({‘eyes‘: 9, ‘the‘: 5, ‘my‘: 4, ‘look‘: 4, ‘into‘: 3, ‘around‘: 2, ‘not‘: 2, "don‘t": 1, ‘under‘: 1, ‘are‘: 1, ‘looking‘: 1, "you‘re": 1, ‘you‘: 1, ‘in‘: 1, ‘why‘: 1})
>>> # substract counts
>>> d=a-b
>>> d
Counter({‘eyes‘: 7, ‘the‘: 5, ‘look‘: 4, ‘into‘: 3, ‘my‘: 2, ‘around‘: 2, ‘under‘: 1, "you‘re": 1, "don‘t": 1})
>>> 

当面对任何需要对数据制表或计数的问题时，Counter对象都是你手边的得力工具。比起利用字典自己手写算法，更应采用该方式完成任务。