How to Find Intersection of Two Sets in C++?

In C++, sets are containers that store unique elements following a specific order. The intersection of two datasets includes all the elements that are common in both sets. In this article, we will learn how to find the intersection of two sets in C++ STL.

Example:

Input:
mySet1 = {10,20,30,40,50,60}
mySet2 =  {10,30,50,40,60,80}

Output:
Intersection of two sets: 10 30 40 50 60 

Find the Intersection of Two STL Sets in C++

To find the intersection of two sets in C++, we can use the std::set_intersection() function provided in the STL <algorithm> library that is used to find the intersection of two sorted ranges and then use the inserter to insert the result after the intersection in a new set.

Syntax of std::set_intersection

set_intersection (first1, last1, first2, last2, result); 

Here,

• first1: Iterator to the beginning of the first range.
• last1: Iterator to the last of the first range.
• first2: Iterator to the beginning of the second range.
• last2: Iterator to the last of the second range.
• result: Iterator to the beginning of the resultant data container.

C++ Program to Find the Intersection of Two Sets

The below example demonstrates how we can find the intersection of the given two sets in C++ STL.

C++

// C++ Program to illustrate how to find the intersection of
// two sets
#include <algorithm>
#include <iostream>
#include <set>
using namespace std;

int main()
{
    // creating two sets
    set<int> set1 = { 10, 20, 30, 40, 50, 60 };
    set<int> set2 = { 10, 30, 50, 40, 60, 80 };

    // Declare a set to store the result of the intersection
    set<int> result;

    // Call the set_intersection(), which computes the
    // intersection of set1 and set2 and
    // inserts the result into the 'result' set
    set_intersection(set1.begin(), set1.end(), set2.begin(),
                     set2.end(),
                     inserter(result, result.begin()));

    // Print the result
    cout << "Intersection of two sets: ";
    for (int i : result)
        cout << i << " ";

    return 0;
}

Output

Intersection of two sets: 10 30 40 50 60 

Time Complexity: O(N + M), where N and M are the sizes of the set.
Auxilliary Space: O(N + M)