How to iterate neighbors of a grid efficiently and concisely.

grid_neighbors.cpp

#include <algorithm>

template <typename I>
void rotate_2d(I& a, I& b) {
        std::swap(a, b);
        b = -b;
}

int main() {
        // Suppose we want to iterate over the neighbors of a cell in  a 2D grid:

        // * * *
        // * - *
        // * * *

        // This can be done with the following loop:
        printf("diagonal\n");
        for (int i = -1; i <= 1; ++i) {
                for (int j = -1; j <= 1; ++j) {
                        if (i == j) continue;

                        printf("%d, %d\n", i, j);
                }
        }

        // But what if just want the directly adjacent cells and ignore diagonal cells?
        //   *
        // * - *
        //   *

        // This is a little tricker, but the idea is just to
        // rotate one neighbor around the center cell, 4 times.

        printf("square\n");
        {
                int i = 1;
                int j = 0;
                for (int k = 0; k < 4; ++k) {
                        printf("%d, %d\n", i, j);
                        rotate_2d(i, j);
                }
        }
}