Find neighbours outside of 2d grid which is reduce

Question

I have a two dimensional grid where width and height are always the same.

[0][1][2]
[3][4][5]
[6][7][8]

I reduced it's data source into a one-dimensional array.

[0][1][2][3][4][5][6][7][8]

Access of elements works, but here comes the tricky part: How to know, whether a neighbour of a cell is outside the grid when still processing the one-dimensional array?

For example the upper right neighbour of [5] is out of the grid but using a calculcated offset index, I will get [3].

Anyone with experience in this field?

Answer 1

Well assuming you know (i) the indices of the cell and (2) the dimensions of the grid (sensible right?), and that its index in the 1D array is k, and in the 2D array i, j.

Then k = i * width + j. Thus i = k / width, j = k % width. (/ is integer division, % is modulus).

Once you get these two indices, you know the neighboring cell's indices, and you can check those against the boundaries as normal (I assume you know how).