Error in using mldivide in mex

I am trying to use mldivide inside mex. The below code is simply trying to solve Ax=b, for r=5, where A is 5*5 Identity and b=0:1:4. So mldivide should output x=0:1:4. Instead I am getting all 0's as output vector. Can anyone see the mistake in the code?

Thanks

A=lsm, b=lsv and x=lss in the code below.

#include "mex.h"

void mexFunction( int nlhs, mxArray *plhs[], int nrhs, const mxArray *prhs[])

{
   mwIndex l, k;
    double  *lsm, *lsv, *lss;
    mxArray *Lsmatrix[2];
    int r;     
    r=*((double *)mxGetPr(prhs[0]));

    plhs[0]=mxCreateDoubleMatrix(r, 1, mxREAL);
    lss=mxGetPr(plhs[0]);

    Lsmatrix[0]=mxCreateDoubleMatrix(r, r, mxREAL);
    Lsmatrix[1]=mxCreateDoubleMatrix(r, 1, mxREAL);
    lsm=mxGetPr(Lsmatrix[0]);
    lsv=mxGetPr(Lsmatrix[1]);

    for(l=0; l<r;l++){
        for(k=0;k<r;k++){
            lsm[k+l*r]=0;
            if(l==k){
                lsm[k+l*r]=1;
            }
        }
        lsv[l]=l;
    }
    mexCallMATLAB(1, lss, 2, &Lsmatrix , "mldivide");

}

Answer 1

[James Tursa]

You don't need to create plhs[0] explicitly since the mexCallMATLAB call will do that for you. Also, you don't need to explicitly fill in the 0's in the off diagonals of the r x r matrix since mxCreateDoubleMatrix will automatically do that for you as well. And you don't need to cast the result of mxGetPr with a (double *) since that is the return type of the function ... also a simpler way of doing this is mxGetScalar anyway. Finally, you can't pass a (double *) to mexCallMATLAB as the 2nd argument ... it is expecting an array of (mxArray *) type. And you don't need to explicitly pass the "address of" the array as the 4th argument, you can just pass the array name. So, e.g.,

#include "mex.h"
void mexFunction( int nlhs, mxArray *plhs[], int nrhs, const mxArray *prhs[])
{
    mwIndex k;
    double  *lsm, *lsv;
    mxArray *Lsmatrix[2];
    int r;     
    r = mxGetScalar(prhs[0]);

    Lsmatrix[0] = mxCreateDoubleMatrix(r, r, mxREAL);
    Lsmatrix[1] = mxCreateDoubleMatrix(r, 1, mxREAL);
    lsm = mxGetPr(Lsmatrix[0]);
    lsv = mxGetPr(Lsmatrix[1]);

    for(k=0; k<r; k++){
        *lsm = 1.0;
        lsm += r+1;
        lsv[k] = k;
    }
    mexCallMATLAB(1, plhs, 2, Lsmatrix , "mldivide");
    mxDestroyArray(Lsmatrix[1]);
    mxDestroyArray(Lsmatrix[0]);
}

[srinadh]

Hi James,

Thanks for all the great suggestions to write the code. But even with this new code I am still getting answers as all 0's.

[James Tursa]

Did you copy and paste the entire function exactly as I have written it? How are you calling the function? Here is what I get:

>> mldivide_example(5)

ans =

       0
       1
       2
       3
       4

[srinadh]

Sorry, I was compiling the wrong file. It works great now. Thanks.