Check If Two Lists Have At Least One Common Element in Python

To check if two lists have at least one common element, we will loop through list1 and list2 and check for atleast one common element in both the lists. The list is the most versatile datatype available in Python, which can be written as a list of comma-separated values (items) between square brackets. Important thing about a list is that the items in a list need not be of the same type.

Let's say we have the following inputs i.e. 2 lists ?

[15, 45, 67, 30, 98]
[10, 8, 45, 48, 30]

The output should be the following i.e. two lists have atleast one common element ?

Common Element Found

Check if two lists have at least one common element using for loop

We have used nested for loops to check if the two lists entered by the user is having at least two elements or not. Both the lists are traversed and the common element is found using the if statement. The input() method is used for user input ?

Example

def commonFunc(A, B):
    c = "Common Element Not Found"

    # traverse in the 1st list
    for i in A:
		# traverse in the 2nd list
        for j in B:
			# if one common
            if i == j:
                c="Common Element Found"
                return c
    return c

# Driver code
A=list()
B=list()
n1=int(input("Enter the size of the first List ="))
print("Enter the Element of the first List =")
for i in range(int(n1)):
	k=int(input(""))
	A.append(k)
n2=int(input("Enter the size of the second List ="))
print("Enter the Element of the second List =")
for i in range(int(n2)):
	k=int(input(""))
	B.append(k)
	
print("Display Result =",commonFunc(A, B))

Output

Enter the size of the first List =5
Enter the Element of the first List =
15
45
67
30
98
Enter the size of the second List =7
Enter the Element of the second List =
10
8
45
48
30
86
67
Display Result = Common Element Found
Enter the size of the first List =5
Enter the Element of the first List =
10
20
30
40
50
Enter the size of the second List =5
Enter the Element of the second List=
23
39
45
67
89
Display Result = Common Element Not Found

Check if two lists have at least one common element using set_intersection()

We have used the set_intersection() method to check if two have lists have at least one common element. First, the lists is converted to set and then the method is applied to find the common elements ?

Example

def commonFunc(list1, list2):
	p_set = set(p)
	q_set = set(q)
	if len(p_set.intersection(q_set)) > 0:
		return True
	return False
	
# First List
p = [4, 10, 15, 20, 25, 30]
print("List1 = ",p)

# Second List
q = [12, 24, 25, 35, 45, 65]
print("List2 = ",q)

print("Are common elements in both the lists? ",commonFunc(p, q))

Output

List1 = [4, 10, 15, 20, 25, 30]
List2 = [12, 24, 25, 35, 45, 65]
Are common elements in both the lists? True

Check if two lists have at least one common element using Set

In this example, we have used the set() method to convert the lists to set and then the & operator is used to find the common elements. Therefore, we haven't used the set_intersection() method here ?

Example

def commonFunc(list1, list2):
	p_set = set(p)
	q_set = set(q)
	if (p_set & q_set):
		return True
	else:
		return False
		
# First List
p = [5, 10, 15, 20, 25, 30]
print("List1 = ",p)

# Second List
q = [12, 20, 30, 35, 40, 50]
print("List2 = ",q)

print("Are common elements in both the lists? ",commonFunc(p, q))

Output

List1 = [5, 10, 15, 20, 25, 30]
List2 = [12, 20, 30, 35, 40, 50]
Are common elements in both the lists? True

Above, we saw that True is returned, since both the lists had at least one common element. In this example, the common element was 30.