Thread: Looked Everywhere (Malloc)

I have been desperately scouring the net for information on how to use malloc, and I seriously still don't even understand the concept behind it. I just want to create a 3D array with dimensions [4][11][5], and my assignment requires dynamic mem allocation.

I found this code that I adapted, and I guess that allocates space for my theoretical array. Now I need to define the values for it, and I don't know how to do that without doing the other way of defining values for an array, which would defeat the purpose of malloc.


here's what I have...

Code:

int dim1 = 4, dim2 = 11, dim3 = 5;
 int i,j,k;
 
 double *** grades = (double ***)malloc(dim1*sizeof(double**));

        for (i = 0; i< dim1; i++) {

         grades[i] = (double **) malloc(dim2*sizeof(double *));

          for (j = 0; j < dim2; j++) {

              grades[i][j] = (double *)malloc(dim3*sizeof(double));
          }

        }

If you don't understand something, try simplifying it.

Do you understand the 2D case?

No, not with malloc.

I know it has something to do with a pointer to an array. and then for a 3D array it's a pointer to a pointer to an array...or something like that

you are creating an array of pointers to an array of pointers to an array of elements. that's one way to do it but complicated. if the problem allows it, a simpler way to do it is to allocate a single block of memory for the array, and then create a pointer variable of the correct type to let you do the 3d indexing into it. something like this

Code:

#include <stdio.h>
#include <stdlib.h>

int main(int argc,char *argv[])
{
        // create a POINTER to the 3d array
        // the parens are necessary to tell the compiler your are creating a POINTER to a 3d array of doubles. without
        // the parens you would be creating a 3d array of pointers to double
        double (*p)[4][3][2];

        // allocate space for the entire array
        p = malloc(sizeof(double) * 4 * 3 * 2);

        // point at the array and index into it
        // again you need the parens to dereference
        (*p)[0][0][0] = 1;

        return 0;
}