// C++ program to store duplicate 
// elements in ordered set
#include <bits/stdc++.h>
using namespace std;
  
// Header files to implement ordered set
// Common file
#include <ext/pb_ds/assoc_container.hpp> 
#include <ext/pb_ds/tree_policy.hpp>
  
// namespace necessary for GNU based 
// policy based data structures
using namespace __gnu_pbds;
  
  
// Declaring ordered_set for pair<int,int>
typedef tree<pair<int,int>, null_type, 
        less<pair<int,int>>, rb_tree_tag,
        tree_order_statistics_node_update>
        ordered_set_pair;
  
// Driver code
int main()
{
      int arr[10] = {4, 7, 2, 4, 5, 
                   8, 4, 1, 3, 5};
      ordered_set_pair mySet;
    
      // Inserting elements to 
    // ordered_set_pair
      for(int i = 0; i < 10; i++)
    {
          mySet.insert({arr[i], i});
    }
  
    // count of elements less than 5
    cout << "Count of elements less than 5: " << 
             mySet.order_of_key({5, -1}) << endl;
  
    // Print 8th element in the ordered set
    // iterator to a pair
      auto itr = mySet.find_by_order(8); 
      cout << "The 8th element in the ordered set is: " << 
             (*itr).first << endl;
    return 0;
}  

Count of elements less than 5: 6
The 8th element in the ordered set is: 7