C++ Algorithm set_intersection()

C++ Algorithm set_intersection() function is used to find the intersection of two sorted ranges[first1, last1) and [first2, last2), which is formed only by the elements that are present in both sets.

Elements are compared using operator < for the first version or using the given binary comparison function comp for the second version.

Syntax

default (1)   template <class InputIterator1, class InputIterator2, class OutputIterator>
                   OutputIterator set_intersection (InputIterator1 first1, InputIterator1 last1,
                        InputIterator2 first2, InputIterator2 last2, OutputIterator result);

custom (2)   template <class InputIterator1, class InputIterator2,
                   class OutputIterator, class Compare>
                    OutputIterator set_intersection (InputIterator1 first1, InputIterator1 last1,
                InputIterator2 first2, InputIterator2 last2,OutputIterator result, Compare comp);

Parameter

first1: An input iterator pointing to the first element in the first of two sorted source sequence.

last1: An input iterator pointing to the past last element in the first of two sorted source sequence.

first2: An input iterator pointing to the first element in the second sorted source sequence.

last2: An input iterator pointing to the past last element in the second sorted source sequence.

comp: A user-defined binary predicate function that accepts two arguments and returns true if the two arguments are in order and false otherwise. It follows the strict weak ordering to order the elements.

result: An output iterator addressing to the position of the first element in the destination range .

Return value

This function returns an iterator to the end of the constructed range.

Complexity

Complexity is linear in the distance between [first1, last1) and [first2, last2): performs up to 2*(count1+count2)-1 comparisons. Where count1 = last1- first1 and count2 = last2- first2.

Data races

The object in the range [first1, last1) and [first2. last2) are accessed.

The object in the range between result and returned value are modified.

Exceptions

This function throws an exception if any of element comparisons, the element assignments or an operation on iterator throws an exception.

Note: The invalid parameters cause an undefined behavior.

Example 1

Let's see the simple example to demonstrate the use of set_intersection():

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

using namespace std;

int main()
{
  list<int> a = {1, 2, 3, 4};
  multiset<int> b = {4, 5, 6, 2};
  vector<int> result;

  set_intersection(begin(a), end(a),
                 begin(b), end(b),
                 inserter(result, end(result)));

  for_each(begin(result), end(result), [](int x) {
    cout << x << endl;
  });
  
  return 0;
}

Output:

2
4

Example 2

Let's see another simple example:

#include <iostream>     // std::cout
#include <algorithm>    // std::set_union, std::sort
#include <vector>       // std::vector

using namespace std;

int main() 
{ 
    int first[] = { 5, 10, 15, 20, 25 }; 
    int second[] = { 50, 40, 30, 20, 10 }; 
    int n = sizeof(first) / sizeof(first[0]); 
  
    // Print first array 
    cout << "First array contains :"; 
    for (int i = 0; i < n; i++) 
        cout << " " << first[i]; 
    cout << "\n"; 
  
    // Print second array 
    cout << "Second array contains :"; 
    for (int i = 0; i < n; i++) 
        cout << " " << second[i]; 
    cout << "\n\n"; 
  
    vector<int> v(10); 
    vector<int>::iterator it, st; 
  
    sort(first, first + n); 
    sort(second, second + n); 
  
    // Using default function 
    it = set_intersection(first, first + n, second, second + n, v.begin()); 
  
    cout << "The union has " << (it - v.begin()) << " elements:\n"; 
    for (st = v.begin(); st != it; ++st) 
        cout << ' ' << *st; 
    cout << '\n'; 
  
    return 0; 
}

Output:

First array contains : 5 10 15 20 25
Second array contains : 50 40 30 20 10

The union has 2 elements:
 10 20

Example 3

Let's see another simple example to find the list of all students who are attending both subjects:

#include <iostream> 
#include <algorithm> 
#include <vector> 
#include <string> 
  
using namespace std; 
  
// Driver code 
int main() 
{ 
    string first[] = { "Nikita", "Divya", "Deep", "Kashish" }; 
    string second[] = { "Aman", "Nikita", "Amita", "Deep" }; 
    int n = sizeof(first) / sizeof(first[0]); 
  
    // Print students of first list 
    cout << "Students in first subject :"; 
    for (int i = 0; i < n; i++) 
        cout << " " << first[i]; 
    cout << "\n"; 
  
    // Print students of second list 
    cout << "Students in second subject :"; 
    for (int i = 0; i < n; i++) 
        cout << " " << second[i]; 
    cout << "\n\n"; 
  
    vector<string> v(10); 
    vector<string>::iterator it, st; 
  
    // Sorting both the list 
    sort(first, first + n); 
    sort(second, second + n); 
  
    // Using default operator< 
    it = set_intersection(first, first + n, second, second + n, v.begin()); 
  
    cout << "Students attending both subjects are :\n"; 
    for (st = v.begin(); st != it; ++st) 
        cout << ' ' << *st; 
    cout << '\n'; 
  
    return 0; 
}

Output:

Students in first subject : Nikita Divya Deep Kashish
Students in second subject : Aman Nikita Amita Deep

Students attending both subjects are :
 Deep Nikita

Example 4

Let's see a simple example:

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

using namespace std;
 
int main()
{ 
    vector<char> v1 = {'A', 'B', 'C'}; 
    vector<char> v2 = {      'B', 'C', 'D', 'E', 'F'}; 
    
    vector<char> dest1;
 
    set_intersection(v1.begin(), v1.end(),
                   v2.begin(), v2.end(),                  
                   back_inserter(dest1));
 
    for (const auto &i : dest1) {
        cout << i << ' ';
    }   
    cout << '\n';
    
    return 0;
}

Output: