How to find the intersection between two lists in Python

In Python, there are 4 ways to find the intersection between two lists
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Using Set Intersection

A set is an unordered collection with no duplicate elements. Basic uses include membership testing and eliminating duplicate entries. Set objects also support mathematical operations like union, intersection, difference, and symmetric difference.

#!/usr/bin/python3
# -*- coding: utf8 -*-

l1 = ["a", "b", "c"]
l2 = ["b", "a", "C"]

lst = list(set(l1) & set(l2))
print(lst)

# Sets
A = {"a", "b", "c"}
B = {"b", "a", "C"}

# A & B
print(A & B)
# B & A
print(B & A)
['b', 'a']
{'b', 'a'}
{'b', 'a'}

This will remove the duplicates in each list.

Using intersection Method

Return a new set with elements common to the set and all others. For example,

#!/usr/bin/python3
# -*- coding: utf8 -*-

l1 = ["a", "b", "c"]
l2 = ["b", "a", "C"]

lst = list(set(l1).intersection(l2))
print(lst)
['b', 'a']

Using List Comprehension

If you want to preserve order of input list, try this:

#!/usr/bin/python3
# -*- coding: utf8 -*-

a = ["a", "b", "c"]
b = ["b", "a", "C"]

# for loop
s = set(b)
lst = [x for x in a if x in s]
print(lst)
['a', 'b']

This is an O(n) solution, faster than O(n*m) performance.

Using lambda Operator

It can be achieved using filter and lambda operator. For example,

#!/usr/bin/python3
# -*- coding: utf8 -*-

a = ["a", "b", "c"]
b = ["b", "a", "C"]

# for loop
lst = list(filter(lambda x:x in a, b))
print(lst)
['b', 'a']

Related Tags

#intersection# #compare# #list#

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