Spiral Matrix

Have you ever tried to assign integer values to a square matrix in spiral order? Eg. for a 5x5 matrix this would look as follows:

1	2	3	4	5
16	17	18	19	6
15	24	25	20	7
14	23	22	21	8
13	12	11	10	9

If this is what you have been lying wake at night for, here comes an iterative solution using Python/Numpy:

#!/usr/bin/env python

""" create a spiral array """

import numpy

def spiral(n):

     mat = numpy.zeros([n,n])

    dirs = [

        numpy.array([0,1]),  #right

        numpy.array([1,0]),  #down

        numpy.array([0,-1]), #left

        numpy.array([-1,0]), #up

    ]

    ptr = numpy.array([0,-1])

    ptr_prime = ptr

    cur_dir_ind = 0

    for i in range(1,n*n+1):

        #adjust direction if necessary

        while(True):

                ptr_prime = ptr + dirs[cur_dir_ind]

                if ptr_prime[0]<0 or ptr_prime[0]>=n or ptr_prime[1]<0 \\ 

                   or ptr_prime[1] >= n:

                        cur_dir_ind = (cur_dir_ind + 1) % len(dirs)

                elif mat[ptr_prime[0],ptr_prime[1]] > 0.0:

                        cur_dir_ind = (cur_dir_ind + 1) % len(dirs)

                else:

                    break

        #update ptr;

        ptr = ptr_prime

        mat[ptr[0],ptr[1]] = i

    

    return mat