Question

Let’s say I have 2D list and I want to do a check if previous/next element equals something. What is the best way to make sure that I will not access list[-1][-1] or list[len + 1][len + 1]?

Here’s an example of what I’m trying to do:

if list[y + 1][x] == value and list[y - 1][x] == value:
    do something
elif list[y][x + 1] == value and list[y][x - 1] == value:
    do something else
... # end so on

I can’t see any other options except for doing:

if y - 1 > 0 and y + 1 < len(list) and x - 1 > 0 and x + 1 < len(list[y]):

Which doesn’t seem right…

Answer 1

A common way to solve this is to add some “padding” around your grid that contains a sentinel value that indicates you are off the grid (such as 0 or -1 or None or something). Your valid indexes, instead of being from 0 to size-1, would be from 1 to size (with a list length of size+2).