Map of pairs in STL

Map in STL is used to hash key and value. We generally see map being used for standard data types. We can also use map for pairs.

For example consider a simple problem, given a matrix and positions visited, print which positions are not visited.

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

map<pair<int, int>, int> vis;

// Print positions that are not marked
// as visited
void printPositions(int a[3][3])
{
    for (int i = 0; i < 3; i++)
        for (int j = 0; j < 3; j++)
            if (vis[{ i, j }] == 0)
                cout << "(" << i << ", " << j << ")"
                     << "\n";
}

int main()
{
    int mat[3][3] = { { 0, 1, 2 }, { 3, 4, 5 }, { 6, 7, 8 } };

    // Marking some positions as visited
    vis[{ 0, 0 }] = 1; // visit (0, 0)
    vis[{ 1, 0 }] = 1; // visit (1, 0)
    vis[{ 1, 1 }] = 1; // visit (1, 1)
    vis[{ 2, 2 }] = 1; // visit (2, 2)

    // print which positions in matrix are not visited by rat
    printPositions(mat);
    return 0;
}

Output:

(0, 1)
(0, 2)
(1, 2)
(2, 0)
(2, 1)