Check how many items from one list appear in another list-CodePudding

If I have 2 lists, for an example:

list_1 = ['a', 'A']
list_2 = ['a', 'A', 'A', 'b', 'b', 'b', 'b']

How can I make a count to see how many times the items from list_1 appear in list_2?

So in this case it should return 3.

CodePudding user response：

list_1 = ['a', 'A']
list_2 = ['a', 'A', 'A', 'b', 'b', 'b', 'b']

found = 0
for x in list_1:
    for y in list_2:
        if x == y:
            found  = 1
print(found)

CodePudding user response：

An efficient O(n) method using a set as reference:

list_1 = ['a', 'A']
list_2 = ['a', 'A', 'A', 'b', 'b', 'b', 'b']

set_1 = set(list_1)

count = 0
for e in list_2:
    if e in set_1:
        counter  = 1

Output: 3

CodePudding user response：

A one liner:

sum([x == y for x in list_1 for y in list_2])

CodePudding user response：

Another solution, which could be beneficial in other cases, because the Counter()- object returns a dictionary with the already summed up elements

from collections import Counter

list_1 = ['a', 'A']
list_2 = ['a', 'A', 'A', 'b', 'b', 'b', 'b']

c = Counter(list_2)
print(sum([c[x] for x in list_1]))

CodePudding user response：

Just use count for lists:

print(list_2.count(list_1[0])   list_2.count(list_1[1]))

or in a loop:

sum_ = 0
for i in range(len(list_1)):
    sum_  = list_2.count(list_1[i])
print(sum_)