MATLAB: How to use mxCreateNumericArray

integersmexmxcreatenumericarray

I am building a mex function, but am having difficulty creating a vector of integers. My code throws about 10 errors, all of which around the declaration of 'gn' and plhs[0]. I'm just trying to make a xynum long vector of class int16. Why is it so difficult?
void findnegs(double *phi, int gnum, int xynum, int *gn, double *adj, double *newphi, double *dphidtSum)
{
    long   i,g;
    double u,s;
    for (i = 0;  i<xynum; i++)
    {
        s = 0;
        for (g = 0;  g<gnum;  g++)
        {
            u = dphidtSum[g+i*gnum]/phi[g+i*gnum];
            if (newphi[g+i*gnum]<0 & u<s)
            {
                s      =  u;
                gn[i]  =  g;
                adj[i] = -1/u;
            }
        }
    }
}
/* The gateway function that replaces the "main".
*plhs[] - the array of output values
*prhs[] - the array of input values
*/
void mexFunction( int nlhs, mxArray *plhs[], int nrhs, const mxArray *prhs[])
{
    /* DECLARATIONS*/
    double *phi, *newphi, *dphidtSum, *adj;
    int gnum, xynum, *gn;
    /* INPUTS */
    gnum      = mxGetScalar(prhs[0]);
    xynum     = mxGetScalar(prhs[1]);
    phi       = mxGetPr(prhs[2]);
    newphi    = mxGetPr(prhs[3]);
    dphidtSum = mxGetPr(prhs[4]);
    /* OUTPUTS */
    plhs[0]    = mxCreateNumericArray(xynum,1,mxINT16_CLASS); /*Creates a matrix*/
    plhs[1]    = mxCreateDoubleMatrix(xynum,1,mxREAL);        /*Creates a matrix*/
    gn         = mxGetInt16s(plhs[0]);                        /*Set to be the first of the output values*/
    adj        = mxGetPr(plhs[1]);                            /*Set to be the first of the output values*/
    /* CALL ROUTINE */
    findnegs(phi,gnum,xynum,gn,adj,newphi,dphidtSum);
}

Best Answer

This line is incorrect (wrong function and signature):
plhs[0]    = mxCreateNumericArray(xynum,1,mxINT16_CLASS); /*Creates a matrix*/
It should be this instead:
plhs[0]    = mxCreateNumericMatrix(xynum,1,mxINT16_CLASS,mxREAL); /*Creates a matrix*/
Also, these lines will not work:
    int gnum, xynum, *gn;
        :
    gn         = mxGetInt16s(plhs[0]);                        /*Set to be the first of the output values*/
An int on your system is almost surely a 32-bit integer, not a 16-bit integer. You need to make sure your pointer definition matches exactly what is in memory. E.g., you can probably use a short int for this:
    int gnum, xynum;
    short *gn;
        :
    gn         = mxGetInt16s(plhs[0]);                        /*Set to be the first of the output values*/
That will mean you need to change this signature:
    void findnegs(double *phi, int gnum, int xynum, int *gn, double *adj, double *newphi, double *dphidtSum)
to this instead also:
    void findnegs(double *phi, int gnum, int xynum, short *gn, double *adj, double *newphi, double *dphidtSum)
But this begs the question, are integer calculations in the findnegs function now going to overflow your 16-bit integers in gn? You may need to adjust your code to make sure this isn't a problem.
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