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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。
【python cookbook】【数据结构与算法】12.找出序列中出现次数最多的元素
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原文地址：http://www.cnblogs.com/apple2016/p/5747075.html
