2D Arrays in Small-C

In a recent article in The C Users Journal, Don Lang discusses how to implement 2D arrays on C. His article covers code that when added to the original Small-C source, allows Small-C to support 2D arrays.

Since the source code for the QL’s Small-C compiler is not available, we could not use the code he provides. But he does discuss a not-so-elegant way of implementing 2D arrays. Since memory is really a 1D array, the computer figures out where in memory an element of an array is. This can be easily done in Small-C.

Given an array of size MAX_X, and elements starting at 0, then (x,y) is equal to (( x * MAX_X ) + y). Below is a sample Small-C program that demonstrates this:

/*  Two Dimensional Arrays in Small-C

      For an array of [10][5] let MAX_X = 10
      
      Assumptions:  Arrays start at 0.  For an
         array of size 10, range is 0..9.
	 
	 Way to address the cell at x,y:
	     temp = array[(x*MAX_X)+y];
	    
*/

#define    MAX_X   10
	    	    
main() {

  int array[100];  /* 10x10 array */
  int x,y;

  for (x = 0; x <= 9; x++) {
      for (y = 0; y <= 9; y++) {
         array[(x*MAX_X)+y] = x*y;
	 printf("%d ",array[(x*MAX_X)+y]);
      }
      printf("\n");
  }

}