How to get surrounding tiles in a tile array

Question

I've created a tiled map that holds two layers, one for the tile type, and the other for whether it is solid or not. I plan on creating very big maps, and checking every tile seems like it would slow down my game immensely.

So lets say I had an array of integers. "0" representing solid, and "1" representing not solid. How would I check only the 8 surrounding integers instead of the whole array each update.

0 0 0

0 1 0

0 0 0

Answer 1

Checking the surrounding tiles is pretty simple. You can do it pretty easily in a nested for loop:

for(int i = x-1; i <= x+1; i++) {
    for(int j = y-1; j <= y+1; j++) {
        if(i != x || j != y) { //ignore the center tile
            //process tile (i,j)
        }
    }
}

This will loop through each tile surrounding tile (x, y). You'll want to add some bounds checking as well for when you're looking at a tile on the edge of your map.

Answer 2

Another option is to hardcode the list of the surrounding tiles offsets. E.g.:

// Somewhere at init:
Object[][] xyoffsets = {{-1, -1}, {0, -1}, {1, -1}, {-1, 0}, {1, 0}, {-1, 1}, {0, 1}, {1, 1}};

// ...

// Later:
for(Object[] xyoffset : xyoffsets) {
    int i = x + xyoffset[0];
    int j = y + xyoffset[1];
    // process tile (i,j)
}

This avoids an unnecessary if and allows to easily change the order in which tiles are processed, but also adds the cost of an extra table.