index in mxSetProperty or mxGetProperty

Hi,

I'm beginning to work with MEX-C-files and I have the following question: What is the index input parameter for the mxGetProperty or mxSetProperty functions. The doc says "index of the desired element" - but the element is chosen via the parameter name, isn't it?

I have the following simple class in Matlab:

classdef testcalls

      properties
          a;
          b;
          c;
      end

      methods
          function obj = testcalls(x,y,z)
              obj.a = x+y;
              obj.b = 42;
              obj.c = rand(z, 1);
          end

          function obj = change(obj)
              obj.a = obj.b;
              obj.b = floor(obj.c(1,1) * 100);
              obj.c = obj.c';
          end

      end

end

And now I have this (very simple) mex-file:

#include "mex.h"
#include "matrix.h"

void mexFunction(int nlhs, mxArray *plhs[], int nrhs, const mxArray *prhs[]) {
  plhs[0] = mxDuplicateArray(prhs[0]);
  mxSetProperty(plhs[0], 0, "a", mxCreateDoubleScalar(192));

}

In this file, I can easily change the third argument ("a") to "b" or "c" and everything works fine. But as soon as I change the second parameter to something different than 0, it crashes. So what kind of index is this? The first paramater is a pointer to a mxArray, not to an array of mxArrays...

Answer 1

[James Tursa]

It crashes because there is only one element in the testcalls array being passed into the mex routine. I.e., I expect you are doing something like this:

x = testcalls;
y = mymexroutine(x); % 1-element array passed in

But you could add another element:

x(2) = testcalls;
y = mymexroutine(x); % 2-element array passed in

If you want to know how many elements the array being passed in has, use the mxGetNumberOfElements API function.

I will warn you that working with classdef objects inside a mex routine is very inefficient. The only routines that the API gives you, mxGetProperty and mxPutProperty, work with deep copies of the property. Not nice. To get any other behavior you have to use unofficial techniques, e.g. see this mxGetPropertyPtr function from the FEX:

http://www.mathworks.com/matlabcentral/fileexchange/30672-mxgetpropertyptr-c-mex-function