运行Matlab mex-算法，而不是双精度单精度

Question

I have a working Matlab C code (mex files) which currently uses double precision. Thus I replaced

double *datOut = mxGetPr(mxOut) by float *datOut = (float*)mxGetData(mxOut); ,

mxCreateDoubleMatrix by mxCreateNumericArray()

and

the datatypes of the variables double by float . The only other mex-Function which is being used is mxDuplicateArray() but nothing else. I didn't change anything to this call... Now I have a Code running which never finishes. I stripped it down quite a bit so that I hope it's short enough that somebody could help me:

float myFunc(const mxArray *point, int index)
{
    float *dat = (float*)mxGetData(point);
    return dat[index]*dat[index]*dat[index];
}

void mexFunction(int nlhs, mxArray *plhs[], int nrhs, const mxArray *prhs[] )
{
    float h, f0, df1, df2, diff;

    // Input Vars #1
    float diff  = (float)mxGetScalar(prhs[1]);
    float H = (float)mxGetScalar(prhs[2]);
    int index1 = (int)mxGetScalar(prhs[3]);
    int index2 = (int)mxGetScalar(prhs[4]);

    // Input Vars #2 -> Duplicate it
    mxArray *newPnt = mxDuplicateArray(prhs[0]);
    float *newPntDat = (float*)mxGetData(newPnt);

    // ...
    // PERHAPS SOME UNIMPORANT CODE HERE ...
    // ...
    h = H;
    f0 = myFunc(prhs[0], index1);

    newPntData[ index2 ] += h;
    df1 = (myFunc(newPnt, index1)-f0)/h;
    while(true)
    {
        h /= 2;

        newPntDat[ index2 ] -= h;
        df2 = (myFunc(newPnt, index1)-f0)/h;

        // If precision is okay
        if(abs(df2-df1) <= diff)
            break;

        // Save for next loop iteration
        df1 = df2;
    }

    // Return df2-Value to Matlab
}

somehow it's an infinite loop and I dunno why since the precision which is defined via diff should be easily reachable for the given function myFunc() . The identicall code runs fine when using double precision with the both functions double *datOut = mxGetPr(mxOut) and mxCreateDoubleMatrix . I also tried to call the mex-Function by explicitly pass the point via point = zeros(rows, 1, 'single'); .

Thanks very much for pointing me to the right direction or giving me ANY hint about it. Thanks!

Answer 1

You need to replace abs() with fabs() .

In general, in such cases, I would use mexPrintf() to print the values that affect the termination condition. Ie, if the above change does not help, try adding

mexPrintf("%g %g %g %g\n",df2,df1,diff, fabs(df2-df1));

Just to make sure the behavior is as you expected.