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I have written a program for printing array in spiral order (iterative version) in an earlier post. Here, in this post let us see how we can make this algorithm even simpler using recursive routine.

the logic behind this routine is explained in the below diagram.

in C, array is stored in row major order, we can get address of any element (arr[row][column]) using

arr + (row * sizeof(elementType)*MAX_COLS) + column

so we are taking advantage of this layout (row major order), and printing rings by rings by updating the base address and num of columns, and num of rows.

here is the routine…

#define MAX_COLS 20
 
int print_array_spirally(int arr[][MAX_COLS], int num_rows, int num_cols)
{
  int i;
 
  if ((num_rows<=0) || (num_cols<=0))
  {
    return 0;
  }
 
  if (num_rows < 2 || num_cols < 2)
  {
    //deal with 1xN, Nx1 arrays
    for(i=0; (num_cols-1) && (i<num_cols); i++)
    {
      printf("%d ", arr[0][i]);
    }
 
    for(i=0; (num_rows-1) && (i<num_rows); i++)
    {
      printf("%d ", arr[i][0]);
    }
    return 0;
  }
 
  for(i=0; i<num_cols; i++)
  {
    printf("%d ", arr[0][i]);
  }
 
  for(i=0; i<num_rows-1; i++)
  {
    //exclude top column number as it is printed part of row
    printf("%d ", arr[i+1][num_cols-1]);
  }
 
  for(i=1; i<=num_cols-1; i++)
  {
    //exclude bottom right number as it is printed part of column
    printf("%d ", arr[num_rows-1][num_cols-i-1]);
  }
 
  for(i=1; i<=num_rows-2; i++)
  {
    //exclude bottom left and top, left numbers as they are already printed
    printf("%d ", arr[num_rows-i-1][0]);
  }
 
  //array stored in row-major order
  print_array_spirally((char *)arr+(sizeof(int)*(1+MAX_COLS)), 
                       num_rows-2, 
                       num_cols-2);
 
  return 0;
}
 
#ifdef TEST
int main()
{
  int arr[][MAX_COLS] = {{1, 2, 3, 10, 17}, {4, 5, 6, 11, 18}, {7, 8, 9, 12, 19}, {13, 14, 15, 16, 20}};
 
  print_array_spirally(arr, 4, 5);
}
#endif

download full [source code]

comments are always welcome…

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Related Blog Items

• Printing linked List in Reverse without Reversing The List Actually
• Printing array spirally - c program
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• Questions asked in various job interviews (in Programming)
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This entry was written by ponnada and posted on April 22, 2007 at 2:15 pm and filed under Algorithms, C Tidbits. Bookmark the permalink. Follow any comments here with the RSS feed for this post. Post a comment or leave a trackback: Trackback URL.