algorithmmatrixloopsspiral

Looping in a spiral

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 3x3 matrix, the output should be:

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

3x3 matrix

Furthermore, the algorithm should support non-square matrices, so for example for a 5x3 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)

5x3 matrix

Solution

  • 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