Argh, Tom! This thing ate my post again!
*grumble* Now, what was I saying... ah, yes.
pinastro:
Without arrays, it would be difficult. You could create additional invisible sprites with collision mask that matches adjacent areas. Just get picked squares' values and do the math with them.
However, I wouldn't recommend this if you plan to do the maths like this every tick. Go with array in that case.