Python计数模块Counter

Python内置的Counter模块，尤其在计算对象的个数非常方便

1. 先来看看源码对这个模块的说明

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

字典的子类，用于计算hashtable，有时被称为袋子或者多类型的字典。元素作为字典的键，元素个数作为字典的值。

   >>> c = Counter('abcdeabcdabcaba')  # count elements from a string

    >>> 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
    >>> 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)]

2. 常用方法介绍
   拥有字典的访问方法,由于c是字典的子类，因此继承了字典的访问删除等方法

>>> c = Counter('abcdeabcdabcaba') 
>>>  c.items()
dict_items([('b', 4), ('c', 3), ('a', 5), ('d', 2), ('e', 1)])
>>> c['a']
5
>>> c.get("a")
5
>>> c.keys()
dict_keys(['b', 'c', 'a', 'd', 'e'])
>>> c.values()
dict_values([4, 3, 5, 2, 1])
>>> c.pop("a")
5
>>> c.clear()

3. Counter 类独有方法

>>> c.most_common(3) # 最多的前三位
[('b', 4), ('c', 3), ('d', 2)]
>>> c
 Counter({'a': 5, 'b': 4, 'c': 3, 'd': 2, 'e': 1})
>>> c.update("abc")  # 更新计数器对象c 
>>> c
Counter({'a': 6, 'b': 5, 'c': 4, 'd': 2, 'e': 1})
>>> d = Counter("abc")
>>> c.update(d) # 可以更新counter 对象
>>> c
Counter({'a': 7, 'b': 6, 'c': 5, 'd': 2, 'e': 1})
>>> c.clear()
>>> c
Counter()

# Counter 间的数学集合操作
>>> c = Counter(a=3, b=1, c=5)
>>> d = Counter(a=1, b=2, d=4)
>>> c + d                       # counter相加, 相同的key的value相加
Counter({'c': 5, 'a': 4, 'd': 4, 'b': 3})
>>> c - d                       # counter相减, 相同的key的value相减，只保留正值得value
Counter({'c': 5, 'a': 2})
>>> c & d                       # 交集:  取两者都有的key,value取小的那一个
Counter({'a': 1, 'b': 1})
>>> c | d                       # 并集:  汇聚所有的key, key相同的情况下，取大的value
Counter({'c': 5, 'd': 4, 'a': 3, 'b': 2})
常见做法:
sum(c.values())                 # 继承自字典的.values()方法返回values的列表，再求和
c.clear()                       # 继承自字典的.clear()方法，清空counter
list(c)                         # 返回key组成的list
set(c)                          # 返回key组成的set
dict(c)                         # 转化成字典
c.items()                       # 转化成(元素，计数值)组成的列表
Counter(dict(list_of_pairs))    # 从(元素，计数值)组成的列表转化成Counter
c.most_common()[:-n-1:-1]       # 最小n个计数的(元素，计数值)组成的列表

说明：因为 Counter 实现了字典的 missing 方法， 所以当访问不存在的key的时候，返回值为0