Question

For some reason I think I have allRates is x*y*z length pointer to doubles and rates is essentially a 3dimensional array. But I can’t recall exactly how this works.

// Allocate 3D Rate Array  
 double *allRates = malloc( x*y*z*sizeof(double) );
  if (!allRates) exit(1);
 double ***rates = malloc( x*sizeof(double **) );
  if (!rates) exit(1);

 for(i=0; i<x; i++) {
  rates[i] = malloc(y * sizeof(double *));
// Check rates[i] allocation?
  for(j=0; j<y; j++) {
   rates[i][j] = allRates + (i*y*z) + (j*z);
  }
 }

It runs properly… I’m just working on documentation and haven’t worked on this part of the code since March.

Answer 1

// Allocate 3D Rate Array  
 double *allRates = malloc( x*y*z*sizeof(double) );
  if (!allRates) exit(1);

allocates a block of memory large enough to hold x*y*z values of type double (if the dimensions are small enough; if the mathematical result of the product isn’t representable as a size_t, it allocates the remainder of that modulo SIZE_MAX + 1).

 double ***rates = malloc( x*sizeof(double **) );
  if (!rates) exit(1);

allocates a block of memory large enough to hold x values of type double** (again, if x is small enough). Those are used as the indices for the first dimension.

 for(i=0; i<x; i++) {
  rates[i] = malloc(y * sizeof(double *));
// Check rates[i] allocation?

Checking the allocation is definitely advisable. If nothing fails, each rates[i] is made to point to a block of memory large enough to hold y pointers to double.

  for(j=0; j<y; j++) {
   rates[i][j] = allRates + (i*y*z) + (j*z);
  }
 }

Each of the double*s rates[i][j] is made to point into the block allocated to allRates, at an offset of i*(y*z) + j*z elements, i times the size of an y×z-plane plus j times the length of a z-element row, so that rates[i][j] points to the first element of row j in plane i.

If C99 is available, or y and z are compile-time constants, that would be simpler achieved by allocating

double (*rates)[y][z] = malloc(x * sizeof *rates);

with fewer indirections when addressing.