查找两个可迭代对象共享的匹配值数量的更有效方法? [英] More efficient ways to find the number of matching values shared by two iterables?

问题描述

寻找匹配项而不是匹配项本身.无法用集合或[x for x in list1 if x in list2]类型的方式求解. list1.count(x) if x in list2可以.

Looking for the number of matches not the matches themselves. Cannot solve with sets or [x for x in list1 if x in list2] type manner. list1.count(x) if x in list2 works though.

比方说，您有两个列表，分别为list1和list2，并希望查找list1中的值与list2中的值相匹配的次数.

Let's say you have two lists, list1 and list2, and want to find the number of times a value from list1 matches a value from list2.

我使用以下代码成功完成了此任务-

I used the following code to perform this task successfully -

sum([x==y for x in list1 for y in list2])

问题是此代码无法有效处理较大的列表.有没有比"double for"循环更快，更有效的方法了，我敢说比double for循环更多的pythonic方法来解决这个问题?

The problem is this code cannot handle larger lists efficiently. Is there a faster, more efficient, dare I say more pythonic way to solve this problem than the "double for" loop?

推荐答案

计数器使用&运算符支持多集交集:

Counters support multiset intersections with the & operator:

>>> from collections import Counter
>>> list1 = list("abba")   
>>> list2 = list("bbanana") 
>>> c1 = Counter(list1)
>>> c2 = Counter(list2)
>>> sum(c1[k]*c2[k] for k in c1 & c2)  # O(n)
10
>>> sum([x==y for x in list1 for y in list2])  # O(n**2)
10

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