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Counter是dict的子类，所以它其实也是字典。只不过它的键对应的值都是计数，值可以是任意整数。下面是四种创建Counter实例的例子：
>>> c = Counter() # a new, empty counter
>>> c = Counter(‘gallahad‘) # a new counter from an iterable
>>> c = Counter({‘red‘: 4, ‘blue‘: 2}) # a new counter from a mapping
>>> c = Counter(cats=4, dogs=8) # a new counter from keyword args
以第二种为例，看下效果
c = Counter(‘gallahad‘) print(c)
输出就是一个字典
Counter({‘a‘: 3, ‘l‘: 2, ‘g‘: 1, ‘d‘: 1, ‘h‘: 1})
Counter能自动对字符串，列表等可迭代的对象里面的元素计数并转换成字典，非常好用
下面是一些Counter对象的常用函数
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 regular dictionary 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:-1] # n least common elements +c # remove zero and negative counts
并且Counter对象还可以做加法，如下
>>> c = Counter(a=3, b=1)
>>> d = Counter(a=1, b=2)
>>> c + d # add two counters together: c[x] + d[x]
Counter({‘a‘: 4, ‘b‘: 3})
>>> c - d # subtract (keeping only positive 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})
注：例子全部来自python3.5的官方文档
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原文地址：http://www.cnblogs.com/duyang/p/5071050.html
