Sort Vector of Pairs in ascending order in C++

What is Vector of Pairs?
A pair is a container which stores two values mapped to each other, and a vector containing multiple number of such pairs is called a vector of pairs. 

CPP

// C++ program to demonstrate vector of pairs
#include<bits/stdc++.h>
using namespace std;

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

    // initialising 1st and 2nd element of
    // pairs with array values
    int arr[] = {10, 20, 5, 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 vector
    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:

10 30
20 60
5 20
40 50

Time Complexity: O(n), where N is the size of the Vector.
Auxiliary Space: O(1)

Case 1 : Sorting the vector elements on the basis of first element of pairs in ascending order.
This type of sorting can be achieved using simple “ sort() ” function. By default the sort function sorts the vector elements on basis of first element of pairs. 

CPP

// C++ program to demonstrate sorting in
// vector of pair according to 1st element
// of pair
#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[] = {10, 20, 5, 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 simple sort() function to sort
    sort(vect.begin(), vect.end());

     // Printing the sorted vector(after using sort())
    cout << "The vector after 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 applying sort operation is:
10 30
20 60
5 20
40 50
The vector after applying sort operation is:
5 20
10 30
20 60
40 50

Time Complexity: O(N*logN), where N is the size of the sorted vector.
Auxiliary Space: O(1)

Case 2 : Sorting the vector elements on the basis of second element of pairs in ascending order.
There are instances when we require to sort the elements of vector on the basis of second elements of pair. For that, we modify the sort() function and we pass a third argument, a call to an user defined explicit function in the sort() function. 

CPP

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

// Driver function to sort the vector elements
// by second element of pairs
bool sortbysec(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;

    // Initialising 1st and 2nd element of pairs
    // with array values
    int arr[] = {10, 20, 5, 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 sort() function to sort by 2nd element
    // of pair
    sort(vect.begin(), vect.end(), sortbysec);

    // Printing the sorted vector(after using sort())
    cout << "The vector after 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 applying sort operation is:
10 30
20 60
5 20
40 50
The vector after applying sort operation is:
5 20
10 30
40 50
20 60

Time Complexity: O(N*logN), where N is the size of the sorted vector.
Auxiliary Space: O(1)

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

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