C++ <algorithm> - set_intersection() Function

The C++ algorithm::set_intersection function is used to form a sorted range starting in the location pointed to by result with the set intersection of sorted range [first1, last1) with respect to the sorted range [first2, last2).

The intersection of two sets is formed by the elements that are present in both sets. Elements in result always comes from first range and in the same order.

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_intersection (InputIterator1 first1, 
                                   InputIterator1 last1,
                                   InputIterator2 first2, 
                                   InputIterator2 last2,
                                   OutputIterator result);

//custom version  
template <class InputIterator1, class InputIterator2,
          class OutputIterator, class Compare>
  OutputIterator set_intersection (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_intersection function is used to form intersection 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 intersection of arr1 and arr2 into vec
  it = set_intersection(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 3 elements:
10 20 30

❮ C++ <algorithm> Library