MATLAB: How to create a multidimensional matrix with variable dimension sizes in C for the MATLAB Compiler

ccreatecreatingdeclarationdeclareMATLABmatrixmulti-dimensional

When I try to declare a matrix in C as having dimensions specified by a variable, I get an error message from the C compiler:
 integer expression must be constant
where the integer referred to is a matrix dimension.
The problem is that I do not know the matrix dimension until run-time.
#include "mex.h"
#include <stdio.h>
/******* ROUTINE *******/
void func(int z)  
   {
      double A[z][5];    /* HERE'S THE PROBLEM: WANT TO MAKE MATRIX z-by-z, not 5-by-5 */
      printf("z=%d\n", z); 
    }  
/******* GATEWAY FUNCTION HERE TO LINK C & MATLAB **************/
void mexFunction(
      int nlhs, mxArray *plhs[],
      int nrhs, const mxArray *prhs[] )
     {  
        int z;
        /* get scalar data from Matlab input */
        z = mxGetScalar(prhs[0]);
        /* call the C subroutine */
        func(z);
      }

Best Answer

The declaration of a variable multidimensional array like the one below is illegal in C:
double A[z][5];    /* ILLEGAL */
If you wish to create a multidimensional array of variable size, you will have to declare a pointer to an array of pointers and then allocate memory dynamically, as shown below:
double** func (int x, int y)
{
        double **A;
        int i;
        /*allocates dynamic memory and returns a pointer to an array of double pointers*/
        A = (double **) malloc( x * sizeof(double *));
        /*allocates memory for each element of the array of double pointers*/
        for (i = 0; i < x ; i++ )
            A[i] = (double *) malloc( y * sizeof(double)); 
        printf("x = %d\n", x);
        printf("y = %d\n", y);
        /* deallocate the memory */
        for (i = 0; i < x ; i++ )
            free(A[i]);
        free(A);
}
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