Multidimensional arrays do not work in mex functions?

I am slowly trying to integrate C/C++ functions into my MATLAB code, but I always have trouble with multidimensional arrays.. I am converting the following code to a mex function:

 function [dphidtSum] = matsum4(dphidt,po,gnum,xynum)
 dphidtSum = zeros(gnum,xynum);
 for a=1:gnum
    for b=1:gnum
        if a~=b
            ind = (po(a,:) & po(b,:));
            if a>b
                dphidtSum(a,ind) = dphidtSum(a,ind)-dphidt(ind,b,a)';
            else
                dphidtSum(a,ind) = dphidtSum(a,ind)+dphidt(ind,a,b)';
            end
        end
    end
 end

The following is my attempt:

void matsum(double *dphidt, double *po, int gnum, int xynum, double *dphidtSum)
 {
    long i;
    long a;
    long b;

    for (i = 0;  i<xynum; i++)
    {
        for (a = 0;  a<gnum;  a++)
        {
            for (b = 0;  b<gnum; b++)
            {
                if (a!=b)
                {
                    if (po[a][i] & po[b][i])
                    {
                    if (a>b)    {    dphidtSum[a][i] += -dphidt[i][b][a];   }
                    else        {    dphidtSum[a][i] +=  dphidt[i][a][b];   }
                    }
                }
            }
        }
    }
}

 void mexFunction( int nlhs, mxArray *plhs[], int nrhs, const mxArray *prhs[])
 {
    /* DECLARATIONS*/
    double *po, *dphidt, *dphidtSum;
    int gnum, xynum;

    /* INPUTS */
    gnum   = mxGetScalar(prhs[0]);
    xynum  = mxGetScalar(prhs[1]);
    po     = mxGetPr(prhs[2]);
    dphidt = mxGetPr(prhs[3]);


    /* OUTPUTS */
    plhs[0]    = mxCreateDoubleMatrix(gnum,xynum,mxREAL); /*Creates a matrix*/
    dphidtSum  = mxGetPr(plhs[0]);                        /*Set to be the first of the output values*/

    /* CALL ROUTINE */
    matsum(dphidt,po,gnum,xynum,dphidtSum);
 }

When I try to compile the mex function I get 6 "subscripted value is neither array nor pointer nor vector" errors, basically everywhere I try to access a multidimensional array. What the heck is wrong?

Answer 1

[James Tursa]

You can't use multi-level [ ][ ]... syntax with simple pointers. E.g., look at these lines:

void matsum(double *dphidt, double *po, int gnum, int xynum, double *dphidtSum)
                            :
                    if (a>b)    {    dphidtSum[a][i] += -dphidt[i][b][a];   }

dphidt is a pointer-to-double

dphidt[i] is a double

dphidt[i][b] is an error ... you can't dereference a double. That's too many levels of indirection.

Same comment for po and dphidtSum.

So how to fix this? The simplest way is to manually calculate the equivalent linear index of the 2D or 3D indexing you are trying to do and use that inside a single pair of brackets [ ]. E.g., dphidtSum is a 2D matrix of dimension gnum X xynum. So this (invalid) expression:

                        dphidtSum[a][i]

would need to be replaced with

                        dphidtSum[ a + i*gnum ]

Similar fix for po. For your 3D array behind dphidt, you need to calculate the equivalent linear index of your 3D indexing and place that inside a single pair of brackets [ ] also.

If you insist on using the multi-level bracket syntax, you would need to allocate and fill in an array of column and page pointers. It can be done, but is usually not worth the effort IMO so I am not showing that code.