Extracting data from a matlab struct in mex - matlab

Question

I'm following this example but I'm not sure what I missed. Specifically, I have this struct in MATLAB:
a = struct; a.one = 1.0; a.two = 2.0; a.three = 3.0; a.four = 4.0;
And this is my test code in MEX ---
First, I wanted to make sure that I'm passing in the right thing, so I did this check:
int nfields = mxGetNumberOfFields(prhs[0]);
mexPrintf("nfields =%i \n\n", nfields);
And it does yield 4, since I have four fields.
However, when I tried to extract the value in field three:
tmp = mxGetField(prhs[0], 0, "three");
mexPrintf("data =%f \n\n", (double *)mxGetData(tmp) );
It returns data =1.000000. I'm not sure what I did wrong. My logic is that I want to get the first element (hence index is 0) of the field three, so I expected data =3.00000.
Can I get a pointer or a hint?

Answer 1

EDITED
Ok, since you didn't provide your full code but you are working on a test, let's try to make a new one from scratch.
On Matlab side, use the following code:
a.one = 1;
a.two = 2;
a.three = 3;
a.four = 4;
read_struct(a);
Now, create and compile the MEX read_struct function as follows:
#include "mex.h"
void read_struct(int nlhs, mxArray *plhs[], int nrhs, const mxArray *prhs[])
{
if (nrhs != 1)
mexErrMsgTxt("One input argument required.");
/* Let's check if the input is a struct... */
if (!mxIsStruct(prhs[0]))
mexErrMsgTxt("The input must be a structure.");
int ne = mxGetNumberOfElements(prhs[0]);
int nf = mxGetNumberOfFields(prhs[0]);
mexPrintf("The structure contains %i elements and %i fields.\n", ne, nf);
mwIndex i;
mwIndex j;
mxArray *mxValue;
double *value;
for (i = 0; i < nf; ++i)
{
for (j = 0; j < ne; ++j)
{
mxValue = mxGetFieldByNumber(prhs[0], j, i);
value = mxGetPr(mxValue);
mexPrintf("Field %s(%d) = %.1f\n", mxGetFieldNameByNumber(prhs[0],i), j, value[0]);
}
}
return;
}
Does this correctly prints your structure?