C++ <algorithm> - set_union() Function

The C++ algorithm::set_union function is used to form a sorted range starting in the location pointed to by result containing elements of sorted ranges [first1, last1) and [first2, last2).

The union of two sets is formed by the elements that are present in either one of the sets, or in both. Elements of the second range with equivalent element in the first range are not copied to result.

In default version elements are compared using operator< and in custom version elements are compared using comp.

Syntax

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

//custom version  
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

`first1`	Specify initial position of the input iterator1. The range used is [first1,last1).
`last1`	Specify final position of the input iterator1. The range used is [first1,last1).
`first2`	Specify initial position of the input iterator2. The range used is [first2,last2).
`last2`	Specify final position of the input iterator2. The range used is [first2,last2).
`result`	Specify initial position of the output iterator where the result to be stored.
`comp`	Specify a binary function that accepts two elements pointed by the iterators, and returns a value convertible to bool. The returned value indicates which element will go first.

Return Value

Returns an iterator pointing to the past-the-end element of result sequence.

Time Complexity

Up to Linear i.e, Θ(n).

Example:

In the example below, the algorithm::set_union function is used to form union of two sorted arrays into a vector.

#include <iostream>
#include <algorithm>
#include <vector>
using namespace std;

int main (){
  int arr1[] = {10, 20, 30, 40, 50};
  int arr2[] = {10, 15, 20, 25, 30};
  vector<int> vec(10);
  vector<int>::iterator it;

  //constructing union of arr1 and arr2 into vec
  it = set_union(arr1, arr1+5, 
                 arr2, arr2+5, 
                 vec.begin());

  //resizing the vec
  vec.resize(it-vec.begin()); 

  cout<<"vec contains "<<vec.size()<<" elements:\n";
  for(it = vec.begin(); it != vec.end(); ++it)
    cout<<*it<<" ";

  return 0;
}

The output of the above code will be:

vec contains 7 elements:
10 15 20 25 30 40 50

❮ C++ <algorithm> Library