Finding neighbours in a two-dimensional array

Question

Is there an easy way of finding the neighbours (that is, the eight elements around an element) of an element in a two-dimensional array? Short of just subtracting and adding

Answer 1

I think Ben is correct in his approach, though I might reorder it, to possibly improve locality.

array[i-1][j-1]
array[i-1][j]
array[i-1][j+1]

array[i][j-1]
array[i][j+1]

array[i+1][j-1]
array[i+1][j]
array[i+1][j+1]

One trick to avoid bounds checking issues, is to make the array dimensions 2 larger than needed. So, a little matrix like this

3 1 4
1 5 9
2 6 5

is actually implemented as

0 0 0 0 0
0 3 1 4 0
0 1 5 9 0
0 2 6 5 0
0 0 0 0 0 

then while summing, I can subscript from 1 to 3 in both dimensions, and the array references above are guaranteed to be valid, and have no effect on the final sum. I am assuming c, and zero based subscripts for the example