Moving around a grid

From cell (r, c), the neighbours are (r − 1, c), (r + 1, c), (r, c − 1) and (r, c + 1). Keep the changes in two small arrays and loop over them:

Always check a neighbour is inside the grid (0 ≤ row < R and 0 ≤ column < C) before using it. Grid problems in the Graphs topic all start like this.

1int dr[4] = {-1, 1, 0, 0};
2int dc[4] = {0, 0, -1, 1};
3for (int k = 0; k < 4; k++) {
4    int nr = r + dr[k], nc = c + dc[k];
5    if (nr >= 0 && nr < R && nc >= 0 && nc < C) {
6        // (nr, nc) is a real neighbour
7    }
8}