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Sorting Vector of Pairs in C++ | Set 1 (Sort by first and second)
• Difficulty Level : Easy
• Last Updated : 29 May, 2017

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.

 `// C++ program to demonstrate vector of pairs``#include``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);`` ` `    ``// Entering values in vector of pairs``    ``for` `(``int` `i=0; i

Output:

```10 30
20 60
5 20
40 50
```

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.

 `// C++ program to demonstrate sorting in``// vector of pair according to 1st element``// of pair``#include``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);`` ` `    ``// Entering values in vector of pairs``    ``for` `(``int` `i=0; i

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
```

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.

 `// C++ program to demonstrate sorting in vector``// of pair according to 2nd element of pair``#include``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);`` ` `    ``// Entering values in vector of pairs``    ``for` `(``int` `i=0; i

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
```

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

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