How to Declare Comparator For Set of Pair in C++?

Prerequisites:

• Set in C++
• Pair in C++

The set in STL has the property that it stores only the distinct values in the sorted order if the datatype is an integer, in lexicographically smallest to largest if the data type is a string. If the datatype is pair the set keeps the distinct pairs only with the pairs sorted on the basis of the first element of the pair. 

The default behavior of the set of pairs can be modified or can be made custom according to our purpose by declaring the custom comparator. 

Syntax:

set<pair<data_type_1, data_type_2, comparator>> set_name

Comparator:

struct comparator {
    // operator() overloading
    bool operator() (const pair<int,int> &p1, const pair<int,int> &p2){
       // custom definition code 
    }
};

Example 1: Declaring a set of pairs with a comparator that keeps the set sorted on the 2nd element of the pair.

C++

// C++ Program to  Declare
// Comparator For Set Of Pair
#include <bits/stdc++.h>

using namespace std;

// Declaring a custom comparator
struct comp {
    // Operator() overloading
    bool operator()(const pair<int, int>& p1,
                    const pair<int, int>& p2)
    {
        // new definition
        return p1.second - p2.second;
    }
};

int main()
{
    // Declaring a set of pairs with comparator
    set<pair<int, int>, comp> s;

    // Adding pairs into the set
    s.insert({ 4, 3 });
    s.insert({ 5, 2 });
    s.insert({ 6, 1 });
    s.insert({ 7, 0 });

    for (auto i = s.begin(); i != s.end(); i++) {
        cout << i->first << " " << i->second << endl;
    }

    return 0;
}

Output

7 0
6 1
5 2
4 3

Example 2: Declaring a set of pairs with a comparator that keeps the set sorted on the basis of the difference between the first and second elements of the pair.

C++

// C++ Program to  Declare
// Comparator For Set Of Pair
#include <bits/stdc++.h>

using namespace std;

// Declaring a custom comparator
struct comp {
    // Operator() overloading
    bool operator()(const pair<int, int>& p1,
                    const pair<int, int>& p2)
    {
        // new definition
        int diff1 = p1.first - p1.second;
        int diff2 = p2.first - p2.second;
        return diff1 < diff2;
    }
};

int main()
{
    // Declaring a set of pairs with comparator
    set<pair<int, int>, comp> s;

    // Adding pairs into the set
    s.insert({ 4, 3 });
    s.insert({ 5, 2 });
    s.insert({ 6, 4 });
    s.insert({ 7, 3 });

    for (auto i = s.begin(); i != s.end(); i++) {
        cout << i->first << " " << i->second << endl;
    }

    return 0;
}

Output

4 3
6 4
5 2
7 3

Example 3: Given a set of pairs with the pair’s first element as the char and 2nd element as the rank of the character declare a custom comparator that stores the order as descending order of rank if the race between the same first element occurs.

C++

// C++ Program to  Declare
// Comparator For Set Of Pair
#include <bits/stdc++.h>

using namespace std;

// Declaring a custom comparator
struct comp {
    // Operator() overloading
    bool operator()(const pair<char, int>& p1,
                    const pair<char, int>& p2)
    {
        // new definition
        if (p1.first == p2.first) {
            return p1.second > p2.second;
        }
        return p1.first < p2.first;
    }
};

int main()
{
    // Declaring a set of pairs with comparator
    set<pair<char, int>, comp> s;

    // Adding pairs into the set
    s.insert({ 'a', 3 });
    s.insert({ 'c', 2 });
    s.insert({ 'c', 4 });
    s.insert({ 'c', 5 });
    s.insert({ 'b', 4 });
    s.insert({ 'b', 3 });

    for (auto i = s.begin(); i != s.end(); i++) {
        cout << i->first << " " << i->second << endl;
    }

    return 0;
}

Output

a 3
b 4
b 3
c 5
c 4
c 2