python中的Collections模块之Counter

 1 class Counter(dict):
 2     '''Dict subclass for counting hashable items.  Sometimes called a bag
 3     or multiset.  Elements are stored as dictionary keys and their counts
 4     are stored as dictionary values.'''

import collections
ret=collections.Counter("abbbbbccdeeeeeeeeeeeee")
ret
Out[12]: Counter({'a': 1, 'b': 5, 'c': 2, 'd': 1, 'e': 13})

    >>> c = Counter('abcdeabcdabcaba')  # count elements from a string
        # 返回最多的3个key和value
    >>> c.most_common(3)                # three most common elements
    [('a', 5), ('b', 4), ('c', 3)]
　　　　
    >>> sorted(c)                       # list all unique elements
    ['a', 'b', 'c', 'd', 'e']
    >>> ''.join(sorted(c.elements()))   # list elements with repetitions
    'aaaaabbbbcccdde'
    >>> sum(c.values())                 # total of all counts
    15

    >>> c['a']                          # count of letter 'a'
    5

　　#对元素的更新
    >>> for elem in 'shazam':           # update counts from an iterable
    ...     c[elem] += 1                # by adding 1 to each element's count
    >>> c['a']                          # now there are seven 'a'
    7
    >>> del c['b']                      # remove all 'b'
    >>> c['b']                          # now there are zero 'b'
    0

    >>> d = Counter('simsalabim')       # make another counter
    >>> c.update(d)                     # add in the second counter
    >>> c['a']                          # now there are nine 'a'
    9

    >>> c.clear()                       # empty the counter
    >>> c
    Counter()

    Note:  If a count is set to zero or reduced to zero, it will remain
    in the counter until the entry is deleted or the counter is cleared:

    >>> c = Counter('aaabbc')
    >>> c['b'] -= 2                     # reduce the count of 'b' by two
    >>> c.most_common()                 # 'b' is still in, but its count is zero
    [('a', 3), ('c', 1), ('b', 0)]

    def most_common(self, n=None):
        '''List the n most common elements and their counts from the most
        common to the least.  If n is None, then list all element counts.

        >>> Counter('abcdeabcdabcaba').most_common(3)
        [('a', 5), ('b', 4), ('c', 3)]

        '''
        # Emulate Bag.sortedByCount from Smalltalk
        if n is None:
            return sorted(self.items(), key=_itemgetter(1), reverse=True)
        return _heapq.nlargest(n, self.items(), key=_itemgetter(1))

>>> c = Counter('ABCABC')
>>> c.elements() ==》<itertools.chain at 0x5f10828>
>>> list(c.elements())
>>> ['A', 'A', 'B', 'B', 'C', 'C']  #这里不是排序，是你恰好参数顺序是ABC，官方给的这例子容易误导人

d=Counter('ACBCDEFT')
list(d.elements())
['A','C','B','D','E','F','T']

所以，要想带顺序，自己在调用sorted一下
sorted(d.elements()) => ['A','B','C','D','E','F','T']

    def subtract(*args, **kwds):
        '''Like dict.update() but subtracts counts instead of replacing them.
        Counts can be reduced below zero.  Both the inputs and outputs are
        allowed to contain zero and negative counts.

        Source can be an iterable, a dictionary, or another Counter instance.

c=Counter("which")  
out:>> Counter({'w': 1, 'h': 2, 'i': 1, 'c': 1})
c.subtract('witch') #传入一个迭代对象，对迭代对象的每一个元素，对原对象进行减法，注意，t是原对象没有的
out:>> Counter({'w': 0, 'h': 1, 'i': 0, 'c': 0, 't': -1}) 

c.subtract(Counter('watch')) #传入另一个Counter对象
out:>> Counter({'w': -1, 'h': 0, 'i': 0, 'c': -1, 't': -2, 'a': -1})

c.subtract({'h':3,'q':5}) #传入一个字典，value是个数 也就是减去多少个key
out:>> Counter({'w': -1, 'h': -3, 'i': 0, 'c': -1, 't': -2, 'a': -1, 'q': -5})