Visit neighbor of a position in a 2d-array

Question

I have the following two dimensional array:

static int[,] arr = new int[5, 5] 
{ 
    { 00, 00, 00, 01, 00 },
    { 00, 00, 01, 01, 00 },
    { 00, 00, 01, 01, 00 },
    { 00, 00, 01, 01, 00 },
    { 00, 00, 00, 01, 00 },
};

I have to a implement a method called Hit(int x, int y). When we hit a 0 in the array (i.e. Hit(0, 0), Hit(1, 1), but not Hit(3, 0)) I would like all the adjacent zeros to the zero we hit to be incremented by 10. So if I call Hit(1, 1), the array should become the following.

static int[,] arr = new int[5, 5] 
{ 
    { 10, 10, 10, 01, 00 },
    { 10, 10, 01, 01, 00 },
    { 10, 10, 01, 01, 00 },
    { 10, 10, 01, 01, 00 },
    { 10, 10, 10, 01, 00 },
};

Any idea how I could implement that? It sounds to me like a Depth First Search/Recursive sort-of algorithm should do the job, but I haven't been able to implement it for an 2d array.

Thanks for the help!

Answer 1

I would say that you should be able to so this with a recursive method.

Something like

    private void Hit(int[,] arr, int x, int y)
    {
        if (    
                x < 0 ||
                x + 1 > arr.GetLongLength(0)||
                y < 0 ||
                y + 1 > arr.GetLongLength(1)
            )
            return;
        if (arr[x, y] == 0)
        {
            arr[x, y] += 10;
            Hit(arr, x, y + 1);
            Hit(arr, x + 1, y + 1);
            Hit(arr, x + 1, y);
            Hit(arr, x + 1, y - 1);
            Hit(arr, x, y - 1);
            Hit(arr, x - 1, y - 1);
            Hit(arr, x - 1, y);
        }
    }

Answer 2

What you are looking for is a Flood fill algorithm, which you can implement in various ways, from DFS (watch the stack) to BFS.

You can also tighten the code a little bit, but here's a rough sketch of what I would do (with DFS):

int[] dx = { 1, 1, 1, 0, 0, -1, -1, -1 };
int[] dy = { 1, 0, -1, 1, -1, 1, -1, 0 };
void Hit( int x, int y ) {
    if ( board[x,y] == 0 ) {
        board[x,y] = 10;
        for ( int i = 0; i < 8; i++ ) {
            nx = x + dx[i];
            ny = y + dy[i];

            // if nx, ny is in bound
            if ( nx >= 0 && nx < height && ny >= 0 && ny < height )
                Hit( nx, ny );
        }
    }
}

Answer 3

Personally I think it's odd to do this with a recursive method. I'd just treat it as an image processing task and run a filter across the array.