Python标准库学习 -- collections.Counter

Python

需求

在很多时候我们需要对一些可跌代项进行计数，统计里面各个值存在多少次，往往采用如下代码：

# 通过迭代对象获得计数
counter = {}
for i in iterable:
    if i not in counter:
        counter[i] = 1
    else:
        counter[i] += 1
top2 = sorted(counter.items(), key=lambda i:i[1], reverse=True)[:2]

一件很简单的事情却使用了这么多的代码，这个时候可以通过标准库中的collections.Counter直接解决。

Counter类

创建

counter = Counter()
counter_a = Counter(iterable)
counter_b = Counter({4:5,5:4})
counter_c= Counter(a=5,b=4)

访问

def visit():
    from collections import Counter
    #访问单个元素
    iterable = [1,2,2,3,3,4,4,4,4,4,5,5,5,5,6]
    counter_a = Counter(iterable)
    print(counter_a[4])
    print(counter_a[8])  #不存在返回0
    #访问次数最多
    print(counter_a.most_common(2)) #参数为空则返回所有

更新和减少

def update():
    from collections import Counter
    #增加
    iterable = [1,2,2,3,3,4,4,4,4,4,5,5,5,5,6]
    counter_a = Counter(iterable)
    counter_a.update(iterable)
    print(counter_a[4]) # 10
    counter_a.update(counter_a)
    print(counter_a[4]) # 20
    #减少
    counter_b = Counter(iterable)
    counter_a.subtract(counter_b)
    print(counter_a[4]) # 15

删除键

def delete():
    from collections import Counter
    #删除
    iterable = [1,2,2,3,3,4,4,4,4,4,5,5,5,5,6]
    counter_a = Counter(iterable)
    del counter_a[4]
    print(counter_a[4])

elements

用于返回含有所有元素的迭代器，其元素出现次数与Counter一致。

def elements():
    from collections import Counter
    iterable = [1,2,2,3,3,4,4,4,4,4,5,5,5,5,6]
    counter_a = Counter(iterable)
    print(list(counter_a.elements()))

移除负值和0

def deleteNegtive():
    from collections import Counter
    iterable = [1,2,2,3,3,4,4,4,4,4,5,5,5,5,6]
    counter_a = Counter(iterable)
    counter_a[9] = -1
    counter_a[10] = 0
    print(counter_a.items())
    counter_a += Counter() # 移除0和负值
    print(counter_a.items())

其他操作

由于Counter其类似于字典，因此其也满足字典的部分操作。
- +、-、&、| 用于集合操作，返回&用于返回小值，|返回大值（已经有+用于取和）
- 可迭代，用以取键
- counter.values(), counter.items()与字典类似