Python Set difference to find lost element from a duplicated array

Given two arrays which are duplicates of each other except one element, that is one element from one of the array is missing, we need to find that missing element.


Input:  A = [1, 4, 5, 7, 9]
        B = [4, 5, 7, 9]
Output: [1]
1 is missing from second array.

Input: A = [2, 3, 4, 5
       B = 2, 3, 4, 5, 6]
Output: [6]
6 is missing from first array.

We have existing solution for this problem please refer Find lost element from a duplicated array. We can solve this problem quickly in python using Set difference logic. Approach is very simple, simply convert both lists in Set and perform A-B operation where len(A)>len(B).





# Function to find lost element from a duplicate
# array
def lostElement(A,B):
     # convert lists into set
     A = set(A)
     B = set(B)
     # take difference of greater set with smaller
     if len(A) > len(B):
         print (list(A-B))
         print (list(B-A))
# Driver program
if __name__ == "__main__":
    A = [1, 4, 5, 7, 9]
    B = [4, 5, 7, 9]




My Personal Notes arrow_drop_up

Experienced Software Engineer with a demonstrated history of working in the information technology and services industry Skilled in Big Data technologies, Data Structures, Algorithm, Software Development, App Development, Databases, Python, Java, C, and WordPress

If you like GeeksforGeeks and would like to contribute, you can also write an article using or mail your article to See your article appearing on the GeeksforGeeks main page and help other Geeks.

Please Improve this article if you find anything incorrect by clicking on the "Improve Article" button below.

Article Tags :
Practice Tags :

Be the First to upvote.

Please write to us at to report any issue with the above content.