python学习——collections模块之Counter

一、Counter是用来统计个数的

• 下面是用传统方法统计个数，随机生成长度为20的字符串，用defaultdict初始化字典

• 用Counter来统计
  
• 在实例化的之后直接将待统计的可迭代对象传进去即可，注意是可迭代对象，下图是源码的一些使用方法
  
• __init__
  
• Counter有6个方法，下面咱们就讲讲这些方法的用法
  

1. most_commom

   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('abracadabra').most_common(3)
       [('a', 5), ('b', 2), ('r', 2)]
   
       '''
       # 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))
   
   

• 可以看到，Counter(iterable).most_common返回的是一个列表，列表里面是元组，元组的第一个元素是统计的值，元组的第二个元素是该值的个数，如果传入n，那么结果就是前n个元素的统计结果
  

2. elements，返回的是itertools.chain对象，可以使用list转换类型。

   def elements(self):
       '''Iterator over elements repeating each as many times as its count.
   
       >>> c = Counter('ABCABC')
       >>> sorted(c.elements())
       ['A', 'A', 'B', 'B', 'C', 'C']
   
       # Knuth's example for prime factors of 1836:  2**2 * 3**3 * 17**1
       >>> prime_factors = Counter({2: 2, 3: 3, 17: 1})
       >>> product = 1
       >>> for factor in prime_factors.elements():     # loop over factors
       ...     product *= factor                       # and multiply them
       >>> product
       1836
   
       Note, if an element's count has been set to zero or is a negative
       number, elements() will ignore it.
   
       '''
       # Emulate Bag.do from Smalltalk and Multiset.begin from C++.
       return _chain.from_iterable(_starmap(_repeat, self.items()))
   

• itertools.chain可以查看itertools.chain() --在不同的容器中进行迭代
  

3. subtract两个统计结果相减，会改变原来的数值，下图是c的结果减去a的结果
   

• 该方法传入的参数是可迭代对象，不一定是Counter实例化对象，如果c中没有该值，则该值置为0，然后个数相减
  

4. copy浅拷贝，不多赘述

   def copy(self):
       'Return a shallow copy.'
       return self.__class__(self)
   

5. fromkeys

   @classmethod
   def fromkeys(cls, iterable, v=None):
       # There is no equivalent method for counters because the semantics
       # would be ambiguous in cases such as Counter.fromkeys('aaabbc', v=2).
       # Initializing counters to zero values isn't necessary because zero
       # is already the default value for counter lookups.  Initializing
       # to one is easily accomplished with Counter(set(iterable)).  For
       # more exotic cases, create a dictionary first using a dictionary
       # comprehension or dict.fromkeys().
       raise NotImplementedError(
           'Counter.fromkeys() is undefined.  Use Counter(iterable) instead.')
   

• 该方法是抛出NotImplementedError异常的，咱们知道dict也有fromkeys方法，Counter是继承dict的，作者认为用Counter去初始化一个Counter对象是没必要的，因为初始化的时候可以直接Counter(itrable)就行了，没有必要再去使用fromkeys去初始化，所以这里选择覆盖了dict的fromkeys方法，让其抛出异常
  

6. update该方法重写了dict的update方法，如果该值存在，就累加，如果不存在就新添加进去，接受一个可迭代对象，也可以是关键字参数，或者字典

   def update(self, iterable=None, /, **kwds):
       '''Like dict.update() but add counts instead of replacing them.
   
       Source can be an iterable, a dictionary, or another Counter instance.
   
       >>> c = Counter('which')
       >>> c.update('witch')           # add elements from another iterable
       >>> d = Counter('watch')
       >>> c.update(d)                 # add elements from another counter
       >>> c['h']                      # four 'h' in which, witch, and watch
       4
   
       '''
       # The regular dict.update() operation makes no sense here because the
       # replace behavior results in the some of original untouched counts
       # being mixed-in with all of the other counts for a mismash that
       # doesn't have a straight-forward interpretation in most counting
       # contexts.  Instead, we implement straight-addition.  Both the inputs
       # and outputs are allowed to contain zero and negative counts.
   
       if iterable is not None:
           if isinstance(iterable, _collections_abc.Mapping):
               if self:
                   self_get = self.get
                   for elem, count in iterable.items():
                       self[elem] = count + self_get(elem, 0)
               else:
                   super(Counter, self).update(iterable) # fast path when counter is empty
           else:
               _count_elements(self, iterable)
       if kwds:
           self.update(kwds)
   

   
   

二、全局完！！