Sorting Vector of Pairs in C++ | Set 2 (Sort in descending order by first and second)

1.4

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 time complexity of 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.

For these instances, we modify the sort() function and we pass a third argument, a call to an user defined explicit function in the sort() function.

// 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;

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

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.begin(), vect.end(), sortinrev);

    // 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

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.

// 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

This article is contributed by Manjeet Singh .If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

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