Question

A friend was in need of an algorithm that would let him loop through the elements of an NxM matrix (N and M are odd). I came up with a solution, but I wanted to see if my fellow SO’ers could come up with a better solution.

I’m posting my solution as an answer to this question.

Example Output:

For a 3×3 matrix, the output should be:

(0, 0) (1, 0) (1, 1) (0, 1) (-1, 1) (-1, 0) (-1, -1) (0, -1) (1, -1)

Furthermore, the algorithm should support non-square matrices, so for example for a 5×3 matrix, the output should be:

(0, 0) (1, 0) (1, 1) (0, 1) (-1, 1) (-1, 0) (-1, -1) (0, -1) (1, -1) (2, -1) (2, 0) (2, 1) (-2, 1) (-2, 0) (-2, -1)

Answer 1

Here’s my solution (in Python):

def spiral(X, Y):     x = y = 0     dx = 0     dy = -1     for i in range(max(X, Y)**2):         if (-X/2 < x <= X/2) and (-Y/2 < y <= Y/2):             print (x, y)             # DO STUFF...         if x == y or (x < 0 and x == -y) or (x > 0 and x == 1-y):             dx, dy = -dy, dx         x, y = x+dx, y+dy