Task

Produce a spiral array.



A   spiral array   is a square arrangement of the first    N2   natural numbers,   where the

numbers increase sequentially as you go around the edges of the array spiraling inwards.



For example, given   5,   produce this array:

 0  1  2  3  4
15 16 17 18  5
14 23 24 19  6
13 22 21 20  7
12 11 10  9  8



Related tasks

   Zig-zag matrix 
   Identity_matrix
   Ulam_spiral_(for_primes)


PROGRAM SPIRAL
 
  IMPLICIT NONE
 
  INTEGER, PARAMETER :: size = 5
  INTEGER :: i, x = 0, y = 1, count = size, n = 0
  INTEGER :: array(size,size)
 
  DO i = 1, count
    x = x + 1 
      array(x,y) = n
    n = n + 1
  END DO
 
  DO
    count = count  - 1
      DO i = 1, count
        y = y + 1
        array(x,y) = n
        n = n + 1
      END DO
      DO i = 1, count
        x = x - 1
        array(x,y) = n
        n = n + 1
      END DO
      IF (n > size*size-1) EXIT
      count = count - 1
      DO i = 1, count
        y = y - 1
        array(x,y) = n
        n = n + 1
      END DO
      DO i = 1, count
        x = x + 1
        array(x,y) = n
        n = n + 1
      END DO	
      IF (n > size*size-1) EXIT
  END DO
 
  DO y = 1, size
    DO x = 1, size
      WRITE (*, "(I4)", ADVANCE="NO") array (x, y)
    END DO
    WRITE (*,*)
  END DO
 
END PROGRAM SPIRAL