collections Counter

importcollections
 
printcollections.Counter(['a', 'b', 'c', 'a', 'b', 'b'])
printcollections.Counter({'a':2, 'b':3, 'c':1})
printcollections.Counter(a=2, b=3, c=1)

#./collections_counter_init.py
Counter({'b':3, 'a': 2, 'c': 1})
Counter({'b':3, 'a': 2, 'c': 1})
Counter({'b':3, 'a': 2, 'c': 1})

importcollections
 
c =collections.Counter()
print'Initial :', c
 
c.update('abcdaab')
print'Sequence:', c
 
c.update({'a':1,'d':5})
print'Dict    :', c








 
 


 
 

  
  执行结果：
 
 

 
 

  
  
#./collections_counter_update.py
Initial: Counter()
Sequence:Counter({'a': 3, 'b': 2, 'c': 1, 'd': 1})
Dict    : Counter({'d': 6, 'a': 4, 'b': 2, 'c': 1})
  
  


  
  


 
 

 
 

  
   
 
 

 
 

  
  访问计数：
 
 

 
 

  
  
importcollections
 
c =collections.Counter('abcdaab')
 
for letter in 'abcde':
    print '%s : %d' % (letter, c[letter])
  
  


  
  


 
 

 
 

  
  执行结果：
 
 

 
 

  
  
#./collections_counter_get_values.py
a :3
b :2
c :1
d :1
e :0
  
  


  
  


 
 

 
 

  
  elements可以列出所有元素：
 
 

 
 

  
  
impor tcollections
 
c =collections.Counter('extremely')
c['z']= 0
printc
printlist(c.elements())
执行结果：
#./collections_counter_elements.py
Counter({'e':3, 'm': 1, 'l': 1, 'r': 1, 't': 1, 'y': 1, 'x': 1, 'z': 0})
['e','e', 'e', 'm', 'l', 'r', 't', 'y', 'x']
  
  


  
  


 
 

 
 

  
   
 
 

 
 

  
  most_common()可以提取出最常用的。
 
 

 
 

  
  
importcollections
 
c =collections.Counter()
withopen('/usr/share/dict/words', 'rt') as f:
    for line in f:
        c.update(line.rstrip().lower())
 
print'Most common:'
forletter, count in c.most_common(3):
    print '%s: %7d' % (letter, count)
  
  


  
  


 
 

 
 

  
  执行结果：
 
 

 
 

  
  
#./collections_counter_most_common.py
Mostcommon:
e:  484673
i:  382454
a:  37803
  
  

0
 
 

 
 

  
   
 
 

 
 

  
  Counter还支持算术和集合运算，它们都只会保留数值为正整数的key。
 
 

 
 

  
  
importcollections
 
c1 =collections.Counter(['a', 'b', 'c', 'a', 'b', 'b'])
c2 =collections.Counter('alphabet')
 
print'C1:', c1
print'C2:', c2
 
print'nCombined counts:'
printc1 + c2
 
print'nSubtraction:'
printc1 - c2
 
print'nIntersection (taking positive minimums):'
printc1 & c2
 
print'nUnion (taking maximums):'
printc1 | c2
执行结果：
#./collections_counter_arithmetic.py
C1:Counter({'b': 3, 'a': 2, 'c': 1})
C2:Counter({'a': 2, 'b': 1, 'e': 1, 'h': 1, 'l': 1, 'p': 1, 't': 1})
 
 
Combinedcounts:
Counter({'a':4, 'b': 4, 'c': 1, 'e': 1, 'h': 1, 'l': 1, 'p': 1, 't': 1})
 
 
Subtraction:
Counter({'b':2, 'c': 1})
 
 
Intersection(taking positive minimums):
Counter({'a':2, 'b': 1})
 
 
Union(taking maximums):
Counter({'b':3, 'a': 2, 'c': 1, 'e': 1, 'h': 1, 'l': 1, 'p': 1, 't': 1})
上面的例子让人觉得collections只能处理单个字符。其实不是这样的，请看标准库中的实例。
>>>from collections import Counter
>>>cnt = Counter()
>>>for word in ['red', 'blue', 'red', 'green', 'blue', 'blue']:
...     cnt[word] += 1
...
>>>cnt
Counter({'blue':3, 'red': 2, 'green': 1})
 
>>>cnt = Counter(['red', 'blue', 'red', 'green', 'blue', 'blue'])
>>>cnt
Counter({'blue':3, 'red': 2, 'green': 1})
 
>>> import re
>>> words = re.findall('w+',open('/etc/ssh/sshd_config').read().lower())
>>>Counter(words).most_common(10)
[('yes', 27), ('no', 23), ('to', 12),('the', 9), ('for', 8), ('and', 8), ('protocol', 6), ('ssh', 6), ('default',6), ('this', 6)]
  
  


  
  


 
 

 
 

  
   
 
 

 
 

  
  第1段代码和第2段的代码效果式样的，后面一段代码通过Counter实现了简单的单词的统计功能。比如面试题：使用python打印出/etc/ssh/sshd_config出现次数最高的10个单词及其出现次数。
 
 

 
 

  
  下面看看Counter的相关定义：
 
 

 
 

  
        classcollections.Counter([iterable-or-mapping]) 。注意Counter是无序的字典。在key不存在的时候返回0.c['sausage'] = 0。设置值为0不会删除元素，要使用delc['sausage']。
 
 

 
 

  
  除了标准的字典方法，额外增加了：
 
 

 
 

  
  elements() ：返回一个包含所有元素的迭代器，忽略小于1的计数。
 
 

 
 

  
  most_common([n])：返回最常用的元素及其计数的列表。默认返回所有元素。
 
 

 
 

  
  subtract([iterable-or-mapping]) ：相减。
 
 

 
 

  
  >>> c =Counter(a=4, b=2, c=0, d=-2)
 
 

 
 

  
  >>> d =Counter(a=1, b=2, c=3, d=4)
 
 

 
 

  
  >>> c - d
 
 

 
 

  
  Counter({'a': 3})
 
 

 
 

  
  >>> c
 
 

 
 

  
  Counter({'a': 4,'b': 2, 'c': 0, 'd': -2})
 
 

 
 

  
  >>> d
 
 

 
 

  
  Counter({'d': 4,'c': 3, 'b': 2, 'a': 1})
 
 

 
 

  
  >>>c.subtract(d)
 
 

 
 

  
  >>> c
 
 

 
 

  
  Counter({'a': 3,'b': 0, 'c': -3, 'd': -6})
 
 

 
 

  
  >>> d
 
 

 
 

  
  Counter({'d': 4,'c': 3, 'b': 2, 'a': 1})
 
 

 
 

  
  从上面可以看出subtract会对实际的Counter产生作用，负数也会计算在里面。
 
 

 
 

  
  标准的字典方法，fromkeys在Counter中没有实现。Update被重载，实现机制不一样。
 
 

 
 

  
  常用方式：
 
 

 
 

  
  sum(c.values())                 # total of all counts
 
 

 
 

  
  c.clear()                       # reset all counts
 
 

 
 

  
  list(c)                         # list unique elements
 
 

 
 

  
  set(c)                          # convert to a set
 
 

 
 

  
  dict(c)                         # convert to a regulardictionary
 
 

 
 

  
  c.items()                       # convert to a list of(elem, cnt) pairs
 
 

 
 

  
  Counter(dict(list_of_pairs))    # convert from a list of (elem, cnt) pairs
 
 

 
 

  
  c.most_common()[:-n:-1]         # n least common elements
 
 

 
 

  
  c += Counter()                  # remove zero and negativecounts
 
 

 
 

  
          数学和交集，并集：
 
 

 
 

  
         >>> c= Counter(a=3, b=1)
 
 

 
 

  
  >>> d = Counter(a=1, b=2)
 
 

 
 

  
  >>> c + d                       # add two counterstogether:  c[x] + d[x]
 
 

 
 

  
  Counter({'a': 4, 'b': 3})
 
 

 
 

  
  >>> c - d                       # subtract (keeping onlypositive counts)
 
 

 
 

  
  Counter({'a': 2})
 
 

 
 

  
  >>> c & d                       # intersection:  min(c[x], d[x])
 
 

 
 

  
  Counter({'a': 1, 'b': 1})
 
 

 
 

  
  >>> c | d                       # union:  max(c[x], d[x])
 
 

 
 

  
  Counter({'a': 3, 'b': 2})
 
 

 
 

  
          关于运算的说明：
 
 

 
 

  
         The Counter class itself is a dictionary subclasswith no restrictions on its keys and values. The values are intended to benumbers representing counts, but you could store anything in the value field.
 
 

 
 

  
  The most_common() method requires only that the values beorderable.
 
 

 
 

  
  For in-place operations such as c[key] += 1, the valuetype need only support addition and subtraction. So fractions, floats, anddecimals would work and negative values are supported. The same is also truefor update() and subtract() which allow negative
 and zero values for bothinputs and outputs.
 
 

 
 

  
  The multiset methods are designed only for use cases withpositive values. The inputs may be negative or zero, but only outputs withpositive values are created. There are no type restrictions, but the value typeneeds to support addition, subtraction, and
 comparison.
 
 

 
 

  
  The elements() method requires integer counts. It ignoreszero and negative counts.
 
 

 
 

  
  


 
 

 
 

  
  转自：http://www.2cto.com/kf/201303/196938.html
 
 

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