How to Find Union of Two Sets in C++: Simple Guide

To find the union of two sets in C++, use the std::set_union algorithm from the algorithm header. It merges two sorted ranges into a destination container, producing a sorted union without duplicates.

Syntax

The std::set_union function merges two sorted ranges into a destination range, producing their union. It requires the input ranges to be sorted.

• first1, last1: Iterators defining the first sorted range.
• first2, last2: Iterators defining the second sorted range.
• result: Iterator to the beginning of the destination range where the union will be stored.

The function returns an iterator to the end of the constructed union range.

cpp

template<class InputIt1, class InputIt2, class OutputIt>
OutputIt set_union(InputIt1 first1, InputIt1 last1,
                   InputIt2 first2, InputIt2 last2,
                   OutputIt result);

Example

This example shows how to find the union of two std::set containers using std::set_union. The result is stored in a std::vector and printed.

cpp

#include <iostream>
#include <set>
#include <vector>
#include <algorithm>

int main() {
    std::set<int> set1 = {1, 3, 5, 7};
    std::set<int> set2 = {3, 4, 5, 6};

    std::vector<int> union_result;
    union_result.resize(set1.size() + set2.size());

    auto it = std::set_union(set1.begin(), set1.end(),
                             set2.begin(), set2.end(),
                             union_result.begin());

    union_result.resize(it - union_result.begin());

    std::cout << "Union of sets: ";
    for (int num : union_result) {
        std::cout << num << " ";
    }
    std::cout << std::endl;

    return 0;
}

Output

Union of sets: 1 3 4 5 6 7 

Common Pitfalls

Common mistakes when finding the union of two sets include:

• Not ensuring the input ranges are sorted before calling std::set_union. This leads to incorrect results.
• Not resizing the destination container properly before or after the operation, which can cause undefined behavior or extra unwanted elements.
• Using containers without sorted order (like std::unordered_set) directly with std::set_union.

cpp

#include <iostream>
#include <vector>
#include <algorithm>

int main() {
    std::vector<int> v1 = {3, 1, 7}; // Not sorted
    std::vector<int> v2 = {4, 5, 6}; // Sorted

    std::vector<int> result(10);

    // Wrong: input ranges not sorted
    auto it = std::set_union(v1.begin(), v1.end(), v2.begin(), v2.end(), result.begin());

    result.resize(it - result.begin());

    std::cout << "Wrong union result: ";
    for (int n : result) std::cout << n << " ";
    std::cout << std::endl;

    // Correct: sort first
    std::sort(v1.begin(), v1.end());
    result.resize(10);
    it = std::set_union(v1.begin(), v1.end(), v2.begin(), v2.end(), result.begin());
    result.resize(it - result.begin());

    std::cout << "Correct union result: ";
    for (int n : result) std::cout << n << " ";
    std::cout << std::endl;

    return 0;
}

Output

Wrong union result: 3 1 4 5 6 
Correct union result: 1 3 4 5 6 7 

Quick Reference

• Use std::set_union for sorted containers.
• Ensure input ranges are sorted before calling.
• Resize the output container after the operation.
• Use std::set or sorted std::vector as inputs.

Key Takeaways

Use std::set_union to find the union of two sorted sets in C++.

Always ensure input ranges are sorted before using std::set_union.

Resize the output container after the union operation to fit the result.

std::set containers are naturally sorted and ideal for set operations.

Avoid using unsorted containers directly with std::set_union to prevent errors.