More efficient way to check neighbours in a two-dimensional array in Java

Question

Hey all, for a few of my college assignments I\'ve found the need to check neighbouring cells in 2-dimensional arrays (grids). The solution I\'ve used is a bit of a hack usi

Answer 1

The basic principle is not to access things that are out of bounds -- so either protect the bounds or don't go out of bounds in the first place. That is, start at a place where you won't immediately go out of bounds and stop before you get out of bounds.

for ( int row = 1; row < grid.length - 1; row++ ) {
    for ( int col = 1; col < grid.length - 1; col++ ) {
        // this section will usually be in a function
        // checks neighbours of the current "cell"
        for ( int rowMod = -1; rowMod <= 1; rowMod++ ) {
            for ( int colMod = -1; colMod <= 1; colMod++ ) {
                if ( someVar == grid[row+rowMod][col+colMod] ) {
                    // do something
                }
            }
        }
        // end checking neighbours
    }
}

Like your current code, this doesn't necessarily deal appropriately with edge conditions -- that is, it applies a 3x3 grid everywhere that the 3x3 grid fits within the matrix, but does not shrink the grid to a 2x2, 2x3 or 3x2 grid when on the edge of the matrix. It will, however, allow a method in the main body checking a 3x3 grid to observe every cell in the matrix.