Python | Print all the common elements of two lists

Given two lists, print all the common elements of two lists. 

Examples:

Input : list1 = [1, 2, 3, 4, 5] 
        list2 = [5, 6, 7, 8, 9]
Output : {5}
Explanation: The common element of the lists is 5. 

Input : list1 = [1, 2, 3, 4, 5] 
        list2 = [6, 7, 8, 9]
Output : No common elements 
Explanation: They do not have any 
elements in common in between them

Method 1:Using Set’s & property

Convert the lists to sets and then print set1&set2. set1&set2 returns the common elements set, where set1 is the list1 and set2 is the list2. 
Below is the Python3 implementation of the above approach: 

Python3

# Python program to find the common elements
# in two lists
def common_member(a, b):
    a_set = set(a)
    b_set = set(b)

    if (a_set & b_set):
        print(a_set & b_set)
    else:
        print("No common elements")


a = [1, 2, 3, 4, 5]
b = [5, 6, 7, 8, 9]
common_member(a, b)

a = [1, 2, 3, 4, 5]
b = [6, 7, 8, 9]
common_member(a, b)

Output:

{5}
No common elements

Method 2:Using Set’s intersection property

Convert the list to set by conversion. Use the intersection function to check if both sets have any elements in common. If they have many elements in common, then print the intersection of both sets. 
Below is the Python3 implementation of the above approach: 

Python3

# Python program to find common elements in
# both sets using intersection function in
# sets


# function
def common_member(a, b):
    a_set = set(a)
    b_set = set(b)

    # check length
    if len(a_set.intersection(b_set)) > 0:
        return(a_set.intersection(b_set))
    else:
        return("no common elements")


a = [1, 2, 3, 4, 5]
b = [5, 6, 7, 8, 9]
print(common_member(a, b))

a =[1, 2, 3, 4, 5]
b =[6, 7, 8, 9]
print(common_member(a, b))

Output:

{5}
No common elements

Method 3 : Using for loop

Python

def common_member(a, b):
    result = [i for i in a if i in b]
    return result

a = [1, 2, 3, 4, 5]
b = [5, 6, 7, 8, 9]

print("The common elements in the two lists are: ")
print(common_member(a, b))

Output:

The common elements in the two lists are: 
[5]

Method 4: Using collections

This code uses the collections module to create two Counter objects from the two lists, a and b. The & operator is then used to return the common elements from the two lists. The result is then printed out.

Python3

import collections

def common_member(a, b):
    result = collections.Counter(a) & collections.Counter(b)
    return result.keys()

a = [1, 2, 3, 4, 5]
b = [5, 6, 7, 8, 9]

print("The common elements in the two lists are: ")
print(common_member(a, b))

Output

The common elements in the two lists are: 
dict_keys([5])

Method 5: Using operator.countOf()

Python3

import operator as op
def common_member(a, b):
    result = [i for i in a if op.countOf(b,i)>0 ]
    return result

a = [1, 2, 3, 4, 5]
b = [5, 6, 7, 8, 9]

print("The common elements in the two lists are: ")
print(common_member(a, b))

Output

The common elements in the two lists are: 
[5]

Time Complexity: O(N)

Auxiliary Space : O(N)