Python cookbook(數(shù)據(jù)結(jié)構(gòu)與算法)找出序列中出現(xiàn)次數(shù)最多的元素算法示例

本文實(shí)例講述了Python找出序列中出現(xiàn)次數(shù)最多的元素。分享給大家供大家參考，具體如下：
問(wèn)題：找出一個(gè)元素序列中出現(xiàn)次數(shù)最多的元素是什么
解決方案：collections模塊中的Counter類(lèi)正是為此類(lèi)問(wèn)題所設(shè)計(jì)的。它的一個(gè)非常方便的most_common()方法直接告訴你答案。    
# 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()增加計(jì)數(shù)
print(word_counts.most_common(3))    
>>> ================================ RESTART ================================
>>>
[('eyes', 8), ('the', 5), ('look', 4)]
[('eyes', 9), ('the', 5), ('my', 4)]
>>>

在底層實(shí)現(xiàn)中，Counter是一個(gè)字典，在元素和它們出現(xiàn)的次數(shù)間做了映射。    
>>> 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 #手動(dòng)增加元素計(jì)數(shù)
>>> 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})
>>>

增加元素出現(xiàn)次數(shù)可以通過(guò)手動(dòng)進(jìn)行增加，也可以借助update()方法；

另外，Counter對(duì)象另一個(gè)特性是它們可以同各種數(shù)學(xué)運(yùn)算操作結(jié)合起來(lái)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})
>>>
當(dāng)面對(duì)任何需要對(duì)數(shù)據(jù)制表或計(jì)數(shù)的問(wèn)題時(shí)，Counter對(duì)象都是你手邊的得力工具。比起利用字典自己手寫(xiě)算法，更應(yīng)采用該方式完成任務(wù)。