Topic       : C-Language Documentation
Author      : John Kormylo
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Subject     : Documentation/C-Language
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Compiled on : Atari
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View Ref-File[ Pointer Arrays ]

FORTRAN programmers might be dismayed by the fact that C does not
support variable dimensions for 2 (or more) dimensional arrays:

  float trace(int n, float a[n][n]);  /* NOT ALLOWED!  */

However, an effective substitute is to use a pointer array.  For
example, if an array of pointers 'float *pa[]' were initialized using

  for(i=0; i<n; i++) pa[i] = a[i];  /* &(a[i][0]) */

then one could use 'pa[i][j]' in place of 'a[i][j]' in any expression,
and

  float trace(int n, float *pa[]);

is legal!  And while this technique uses more RAM, it compensates with
much faster execution speeds.

The following function will allocate space for a 2 dimensional array
and initialize a pointer array for it.  (You can copy and paste it
directly from the Help window.)

/* Allocate a 2 dimensional array and return a pointer array[n] */

  void* array_alloc(
    const int n,       /* first dimension */
    const long size)   /* sizeof(type) * second dimension */
   {
    char **a, *p;
    int i;

    a = malloc(n * (sizeof(char*) + size));
    if(!a) return(0L);  /* not enough RAM */
    p = (char*) (a + n);
    for(i=0; i<n; i++, p += size) a[i] = p;
    return(a);
   }

which can be called using

  float **a;
  ...
  a = array_alloc(n, m * sizeof(float));

  for(i=0; i<n; i++) for(j=0; j<m; j++) a[i][j] = 0.0;
  ...
  free(a);

Note: Use 'a[0]' instead of 'a' in functions which expect the array
      itself (like memset).