C++ algorithm: std::set_union Function Template

C++ std::set_union

The algorithm std::set_union function computes the union of two sorted ranges and stores the result in a destination range. The result contains all elements from both ranges, with duplicate elements appearing only once. Both input ranges must be sorted according to the same criteria for the function to work correctly.

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Syntax of std::set_union

template <class InputIterator1, class InputIterator2, class OutputIterator>
OutputIterator set_union(InputIterator1 first1, InputIterator1 last1,
                         InputIterator2 first2, InputIterator2 last2,
                         OutputIterator result);

template <class InputIterator1, class InputIterator2, class OutputIterator, class Compare>
OutputIterator set_union(InputIterator1 first1, InputIterator1 last1,
                         InputIterator2 first2, InputIterator2 last2,
                         OutputIterator result, Compare comp);

Parameters of std::set_union

Parameter	Description
first1, last1	Input iterators defining the first sorted range.
first2, last2	Input iterators defining the second sorted range.
result	Output iterator to the beginning of the destination range where the result is stored.
comp (optional)	A binary comparison function that defines the order of elements. Defaults to <.

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Return Value of std::set_union

Returns an iterator to the end of the destination range containing the union of the two input ranges.

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Examples for std::set_union

Example 1: Basic Usage of std::set_union

This example demonstrates finding the union of two sorted vectors:

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

int main() {
    std::vector<int> set1 = {1, 3, 5, 7};
    std::vector<int> set2 = {2, 3, 6, 7};
    std::vector<int> result(set1.size() + set2.size());

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

    result.resize(it - result.begin()); // Adjust size to remove unused elements

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

    return 0;
}

Explanation:

1. Include necessary headers:
   • <iostream>: Used for input/output operations.
   • <algorithm>: Includes the std::set_union function.
   • <vector>: Used for the std::vector container.
2. Initialize two sorted sets:
   • set1: A vector containing {1, 3, 5, 7}.
   • set2: A vector containing {2, 3, 6, 7}.
   • Both vectors must be sorted in ascending order for std::set_union to work correctly.
3. Prepare a result vector:
   • A vector named result is initialized with a size large enough to hold all possible elements from both sets, which is the sum of their sizes.
4. Perform the union operation:
   • The function std::set_union merges the two sorted ranges set1 and set2 into result, ensuring no duplicate elements are included in the union.
   • It takes five arguments:
     • set1.begin(): Iterator pointing to the beginning of set1.
     • set1.end(): Iterator pointing to the end of set1.
     • set2.begin(): Iterator pointing to the beginning of set2.
     • set2.end(): Iterator pointing to the end of set2.
     • result.begin(): Iterator pointing to the beginning of the result vector, where the merged elements will be stored.
   • The function returns an iterator pointing to the end of the resulting range.
5. Resize the result vector:
   • The size of the result vector is adjusted to remove unused elements using result.resize(it - result.begin()).
6. Output the union of the sets:
   • The program uses a for loop to iterate over the elements in the result vector and prints them separated by spaces.
   • The output is: "Union of sets: 1 2 3 5 6 7".

Output:

Union of sets: 1 2 3 5 6 7

Example 2: Using a Custom Comparison Function for std::set_union

This example demonstrates finding the union of two sorted ranges in descending order using a custom comparison function:

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

bool descending(int a, int b) {
    return a > b;
}

int main() {
    std::vector<int> set1 = {7, 5, 3, 1};
    std::vector<int> set2 = {7, 6, 3, 2};
    std::vector<int> result(set1.size() + set2.size());

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

    result.resize(it - result.begin()); // Adjust size to remove unused elements

    std::cout << "Union of sets in descending order: ";
    for (int n : result) {
        std::cout << n << " ";
    }
    std::cout << std::endl;

    return 0;
}

Output:

Union of sets in descending order: 7 6 5 3 2 1

Exception Handling in std::set_union

The std::set_union function does not throw exceptions on its own. However, the comparison function passed as an argument may throw exceptions, which can be caught and handled appropriately.

Example 1: Exception in Custom Comparison Function

This example demonstrates how exceptions in a custom comparison function are handled:

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

bool faulty_compare(int a, int b) {
    if (a == 3 || b == 3) {
        throw std::runtime_error("Comparison involving 3 is not allowed.");
    }
    return a < b;
}

int main() {
    std::vector<int> set1 = {1, 3, 5, 7};
    std::vector<int> set2 = {2, 3, 6, 7};
    std::vector<int> result(set1.size() + set2.size());

    try {
        auto it = std::set_union(set1.begin(), set1.end(), set2.begin(), set2.end(), result.begin(), faulty_compare);

        result.resize(it - result.begin()); // Adjust size to remove unused elements

        std::cout << "Union of sets: ";
        for (int n : result) {
            std::cout << n << " ";
        }
        std::cout << std::endl;
    } catch (const std::exception& e) {
        std::cerr << "Exception caught: " << e.what() << std::endl;
    }

    return 0;
}

Output:

Exception caught: Comparison involving 3 is not allowed.