How to fill mxArray with mxGetComplexDoubles?

Should this C code work?

int main() {
  mxArray* array_ptr;
  double start_real[6] = { 1.01, 2.02, 3.03, 4.04, 5.05, 6.06 };
  double start_imag[6] = { 0.2, 0.3, 0.4, 0.5, 0.6, 0.7 };
  array_ptr = mxCreateNumericMatrix(2, 3, mxSINGLE_CLASS, mxCOMPLEX);
#ifdef MX_HAS_INTERLEAVED_COMPLEX
  mxComplexDouble* pc;
  pc = mxGetComplexDoubles(array_ptr);
  memcpy(&pc->real, start_real, 6 * sizeof(double));
  memcpy(&pc->imag, start_real, 6 * sizeof(double));
#else
  memcpy(mxGetPr(array_ptr), start_real, 6 * sizeof(double));
  memcpy(mxGetPi(array_ptr), start_imag, 6 * sizeof(double));
#endif
  return 0;
}

When I call mxGetComplexDoubles, the value returned is NULL! I think that is by design because the array is currently empty. However, how do I fill my array given the old way (with mxGetPr and mxGetPi) vs. the new way... whatever that new way is...?

I keep reading https://www.mathworks.com/help/matlab/matlab_external/upgrade-mex-files-to-use-interleaved-complex.html for help, but I don't understand why the call to mxGetComplexDoubles is NULL! Please help! Thanks!

Answer 1

[James Tursa]

No, this would not be expected to work. In the first place, you need to use mxDOUBLE_CLASS to create the mxArray, not mxSINGLE_CLASS. In the second place, for the interleaved complex model, you need to copy the data in a loop to get it into the interleaved memory. memcpy( ) can only be used to copy to/from contiguous memory, and your real and imaginary parts are not contiguous ... they are interleaved. E.g.,

#ifdef MX_HAS_INTERLEAVED_COMPLEX
  double* pc;
  int i;
  pc = (double *) mxGetData(array_ptr);
  for( i=0; i<6; i++ ) {
      pc[2*i  ] = start_real[i];
      pc[2*i+1] = start_imag[i];
  }

[Darrell Bircsak]

Thanks James! Well explained. Thanks for the example too!

[James Tursa]

P.S. If you use a loop for both memory models (i.e., assign the elements directly), then the type doesn't have to match. I.e., you can use mxSINGLE_CLASS for the mxArray and things will still work as long as you type pc as a float* type. That's because the assignment will take care of the type conversion for you.