Dominic Bou-Samra Dominic Bou-Samra - 5 months ago 9
Python Question

Determining neighbours of cell two dimensional list

I have a list of lists, something like

[[1, 2, 3,],[4, 5, 6,],[7, 8, 9]]
.

Represented graphically as:

1 2 3
4 5 6
7 8 9


I'm looking for an elegant approach to checking the value of neighbours of a cell, horizontally, vertically and diagonally. For instance, the neighbours of [0][2] are [0][1], [1][1] and [1][2] or the numbers 2, 5, 6.

Now I realise I could just do a bruteforce attack checking every value a la:

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


But thats easy, and I figured I can learn more by seeing some more elegant approaches.

Answer
# Size of "board"
X = 10
Y = 10

neighbors = lambda x, y : [(x2, y2) for x2 in range(x-1, x+2)
                               for y2 in range(y-1, y+2)
                               if (-1 < x <= X and
                                   -1 < y <= Y and
                                   (x != x2 or y != y2) and
                                   (0 <= x2 <= X) and
                                   (0 <= y2 <= Y))]

>>> print(neighbors(5, 5))
[(4, 4), (4, 5), (4, 6), (5, 4), (5, 6), (6, 4), (6, 5), (6, 6)]

I don't know if this is considered clean, but this one-liner gives you all neighbors by iterating over them and discarding any edge cases.