Sort Vector of Pairs in descending order in C++

We have discussed some of the cases of sorting vector of pairs in below set 1.

Sorting Vector of Pairs in C++ | Set 1 (Sort by first and second)

More cases are discussed in this article. Sometimes we require to sort the vector in reverse order. In those instances, rather than first sorting the vector and later using “reverse” function increases the time complexity of the code. Therefore, to avoid this we sort the vector in descending order directly.

Case 3 : Sorting the vector elements on the basis of first element of pairs in descending order.

This type of sorting arranges selected rows of pairs in vector in descending order. This is achieved by using “sort()” and passing iterators of 1D vector as its arguments.

C++

// C++ program to demonstrate sorting in vector of
// pair according to 1st element of pair in
// descending order

#include <bits/stdc++.h>
using namespace std;

int main()
{
    // declaring vector of pairs
    vector<pair<int, int> > vect;

    // initializing 1st and 2nd element of
    // pairs with array values
    int arr[] = { 5, 20, 10, 40 };
    int arr1[] = { 30, 60, 20, 50 };
    int n = sizeof(arr) / sizeof(arr[0]);

    // Entering values in vector of pairs
    for (int i = 0; i < n; i++)
        vect.push_back(make_pair(arr[i], arr1[i]));

    // Printing the original vector(before sort())
    cout << "The vector before applying sort is:\n";
    for (int i = 0; i < n; i++) {
        // "first" and "second" are used to access
        // 1st and 2nd element of pair respectively
        cout << vect[i].first << " " << vect[i].second
             << endl;
    }

    // using modified sort() function to sort
    sort(vect.rbegin(), vect.rend());

    // Printing the sorted vector(after using sort())
    cout << "The vector after applying sort is:\n";
    for (int i = 0; i < n; i++) {
        // "first" and "second" are used to access
        // 1st and 2nd element of pair respectively
        cout << vect[i].first << " " << vect[i].second
             << endl;
    }
    return 0;
}

C++

// C++ program to demonstrate sorting in vector of
// pair according to 1st element of pair in
// descending order using STL sort function

#include <bits/stdc++.h>
using namespace std;

int main()
{
    // declaring vector of pairs
    vector<pair<int, int> > vect;

    // initializing 1st and 2nd element of
    // pairs with array values
    int arr[] = { 5, 20, 10, 40 };
    int arr1[] = { 30, 60, 20, 50 };
    int n = sizeof(arr) / sizeof(arr[0]);

    // Entering values in vector of pairs
    for (int i = 0; i < n; i++)
        vect.push_back({arr[i], arr1[i]});

    // Printing the original vector(before sort())
    cout << "The vector before applying sort is:\n";
    for (int i = 0; i < n; i++) {
        // "first" and "second" are used to access
        // 1st and 2nd element of pair respectively
        cout << vect[i].first << " " << vect[i].second
             << endl;
    }

    // modifying sort function by adding 3rd parameter
      // to sort the vector of pair in descending order
    sort(vect.begin(), vect.end(),
         greater<pair<int, int> >());

    // Printing the sorted vector(after using sort())
    cout << "The vector after applying sort is:\n";
    for (int i = 0; i < n; i++) {
        // "first" and "second" are used to access
        // 1st and 2nd element of pair respectively
        cout << vect[i].first << " " << vect[i].second
             << endl;
    }
    return 0;
}

Output

The vector before applying sort is:
5 30
20 60
10 20
40 50
The vector after applying sort is:
40 50
20 60
10 20
5 30

Time Complexity: O(N*logN), where N is the size of the vector.
Auxiliary Space: O(N) 

Case 4 : Sorting the vector elements on the basis of second element of pairs in descending order.

These instances can also be handled by modifying “sort()” function and again passing a call to user-defined function.

CPP

// C++ program to demonstrate sorting/in vector of
// pair according to 2nd element of pair in
// descending order
#include<bits/stdc++.h>
using namespace std;

// Driver function to sort the vector elements by
// second element of pair in descending order
bool sortbysecdesc(const pair<int,int> &a,
                   const pair<int,int> &b)
{
       return a.second>b.second;
}


int main()
{
    // Declaring vector of pairs
    vector< pair <int,int> > vect;

    // Initializing 1st and 2nd element of
    // pairs with array values
    int arr[] = {5, 20, 10, 40 };
    int arr1[] = {30, 60, 20, 50};
    int n = sizeof(arr)/sizeof(arr[0]);

    // Entering values in vector of pairs
    for (int i=0; i<n; i++)
        vect.push_back( make_pair(arr[i],arr1[i]) );

    // Printing the original vector(before sort())
    cout << "The vector before sort operation is:\n" ;
    for (int i=0; i<n; i++)
    {
        // "first" and "second" are used to access
        // 1st and 2nd element of pair respectively
        cout << vect[i].first << " "
            << vect[i].second << endl;
    }

    // using modified sort() function to sort
    sort(vect.begin(), vect.end(), sortbysecdesc);

    // Printing the sorted vector(after using sort())
    cout << "The vector after applying sort operation is:\n" ;
    for (int i=0; i<n; i++)
    {
        // "first" and "second" are used to access
        // 1st and 2nd element of pair respectively
        cout << vect[i].first << " "
             << vect[i].second << endl;
    }
    return 0;
}

Output

The vector before sort operation is:
5 30
20 60
10 20
40 50
The vector after applying sort operation is:
20 60
40 50
5 30
10 20

Time Complexity: O(N*logN), where N is the size of the vector.
Auxiliary Space: O(N) 

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