Thread Subject: problems getting MATLAB data in C++ program

Hi I am using the MX functions to get read MATLAB data in C and have been able to read the MATLAB data for MATLAB arrays and structures (created using the keword struct) but am having issues reading the data from a MATLAB structure array like test(1).dataelement1, test(1).dataelement2. Does anyone know what mx method to use? Also I think MATLAB must store additional information about the structure array as if I set the variable that I bring over to C to the first element in MATLAB I can read only the first value of the structure array. Any ideas?

"Paul" wrote in message <jdd3rc$km1$1@newscl01ah.mathworks.com>...
> Hi I am using the MX functions to get read MATLAB data in C and have been able to read the MATLAB data for MATLAB arrays and structures (created using the keword struct) but am having issues reading the data from a MATLAB structure array like test(1).dataelement1, test(1).dataelement2. Does anyone know what mx method to use? Also I think MATLAB must store additional information about the structure array as if I set the variable that I bring over to C to the first element in MATLAB I can read only the first value of the structure array. Any ideas?

Use mxGetField. E.g., if you pass in the structure as the first argument:

mxArray *test_dataelement1, *test_dataelement2;
    :
test_dataelement1 = mxGetField(prhs[0],0,"dataelement1");
test_dataelement2 = mxGetField(prhs[0],0,"dataelement2");

James Tursa

"James Tursa" wrote in message <jdd5ft$pvl$1@newscl01ah.mathworks.com>...
> "Paul" wrote in message <jdd3rc$km1$1@newscl01ah.mathworks.com>...
> > Hi I am using the MX functions to get read MATLAB data in C and have been able to read the MATLAB data for MATLAB arrays and structures (created using the keword struct) but am having issues reading the data from a MATLAB structure array like test(1).dataelement1, test(1).dataelement2. Does anyone know what mx method to use? Also I think MATLAB must store additional information about the structure array as if I set the variable that I bring over to C to the first element in MATLAB I can read only the first value of the structure array. Any ideas?
>
> Use mxGetField. E.g., if you pass in the structure as the first argument:
>
> mxArray *test_dataelement1, *test_dataelement2;
> :
> test_dataelement1 = mxGetField(prhs[0],0,"dataelement1");
> test_dataelement2 = mxGetField(prhs[0],0,"dataelement2");
>
> James Tursa
Hi James, thanks for the response, I was able to add the code but was not sure how to extract the data from the mxArray pointer test_dataelement1? I tried
double firstvalue = test_dataelement1[0]; but get the error invalid use of mxArray tag?

"Paul" wrote in message <jddg38$kd$1@newscl01ah.mathworks.com>...
> "James Tursa" wrote in message <jdd5ft$pvl$1@newscl01ah.mathworks.com>...
> > "Paul" wrote in message <jdd3rc$km1$1@newscl01ah.mathworks.com>...
> > > Hi I am using the MX functions to get read MATLAB data in C and have been able to read the MATLAB data for MATLAB arrays and structures (created using the keword struct) but am having issues reading the data from a MATLAB structure array like test(1).dataelement1, test(1).dataelement2. Does anyone know what mx method to use? Also I think MATLAB must store additional information about the structure array as if I set the variable that I bring over to C to the first element in MATLAB I can read only the first value of the structure array. Any ideas?
> >
> > Use mxGetField. E.g., if you pass in the structure as the first argument:
> >
> > mxArray *test_dataelement1, *test_dataelement2;
> > :
> > test_dataelement1 = mxGetField(prhs[0],0,"dataelement1");
> > test_dataelement2 = mxGetField(prhs[0],0,"dataelement2");
> >
> > James Tursa
> Hi James, thanks for the response, I was able to add the code but was not sure how to extract the data from the mxArray pointer test_dataelement1? I tried
> double firstvalue = test_dataelement1[0]; but get the error invalid use of mxArray tag? I think I am not setting prhs correctly, seems as though it would need to be linked to the test_dataelement1 pointer?

"Paul" wrote in message <jddg38$kd$1@newscl01ah.mathworks.com>...
> "James Tursa" wrote in message <jdd5ft$pvl$1@newscl01ah.mathworks.com>...
> > "Paul" wrote in message <jdd3rc$km1$1@newscl01ah.mathworks.com>...
> > > Hi I am using the MX functions to get read MATLAB data in C and have been able to read the MATLAB data for MATLAB arrays and structures (created using the keword struct) but am having issues reading the data from a MATLAB structure array like test(1).dataelement1, test(1).dataelement2. Does anyone know what mx method to use? Also I think MATLAB must store additional information about the structure array as if I set the variable that I bring over to C to the first element in MATLAB I can read only the first value of the structure array. Any ideas?
> >
> > Use mxGetField. E.g., if you pass in the structure as the first argument:
> >
> > mxArray *test_dataelement1, *test_dataelement2;
> > :
> > test_dataelement1 = mxGetField(prhs[0],0,"dataelement1");
> > test_dataelement2 = mxGetField(prhs[0],0,"dataelement2");
> >
> > James Tursa
>
> Hi James, thanks for the response, I was able to add the code but was not sure how to extract the data from the mxArray pointer test_dataelement1? I tried
> double firstvalue = test_dataelement1[0]; but get the error invalid use of mxArray tag?

You need to extract the data pointer first, then dereference that. E.g., assuming that dataelement1 and 2 contain double class arrays, you would do this:

mxArray *test_dataelement1, *test_dataelement2;
double *pr1, *pr2;
    :
test_dataelement1 = mxGetField(prhs[0],0,"dataelement1");
if( test_dataelement1 == NULL ) {
    mexErrMsgTxt("insert error message here");
}
test_dataelement2 = mxGetField(prhs[0],0,"dataelement2");
if( test_dataelement2 == NULL ) {
    mexErrMsgTxt("insert error message here");
}
pr1 = mxGetPr(test_dataelement1);
pr2 = mxGetPr(test_dataelement2);
    :

pr1[0] is the first element, pr1[1] is the second element, etc.

If the underlying data is some other class, e.g. int32, then you would use mxGetData instead of mxGetPr and cast the resulting pointer appropriate to the underlying data.

James Tursa

"James Tursa" wrote in message <jdegnk$2k3$1@newscl01ah.mathworks.com>...
> "Paul" wrote in message <jddg38$kd$1@newscl01ah.mathworks.com>...
> > "James Tursa" wrote in message <jdd5ft$pvl$1@newscl01ah.mathworks.com>...
> > > "Paul" wrote in message <jdd3rc$km1$1@newscl01ah.mathworks.com>...
> > > > Hi I am using the MX functions to get read MATLAB data in C and have been able to read the MATLAB data for MATLAB arrays and structures (created using the keword struct) but am having issues reading the data from a MATLAB structure array like test(1).dataelement1, test(1).dataelement2. Does anyone know what mx method to use? Also I think MATLAB must store additional information about the structure array as if I set the variable that I bring over to C to the first element in MATLAB I can read only the first value of the structure array. Any ideas?
> > >
> > > Use mxGetField. E.g., if you pass in the structure as the first argument:
> > >
> > > mxArray *test_dataelement1, *test_dataelement2;
> > > :
> > > test_dataelement1 = mxGetField(prhs[0],0,"dataelement1");
> > > test_dataelement2 = mxGetField(prhs[0],0,"dataelement2");
> > >
> > > James Tursa
> >
> > Hi James, thanks for the response, I was able to add the code but was not sure how to extract the data from the mxArray pointer test_dataelement1? I tried
> > double firstvalue = test_dataelement1[0]; but get the error invalid use of mxArray tag?
>
> You need to extract the data pointer first, then dereference that. E.g., assuming that dataelement1 and 2 contain double class arrays, you would do this:
>
> mxArray *test_dataelement1, *test_dataelement2;
> double *pr1, *pr2;
> :
> test_dataelement1 = mxGetField(prhs[0],0,"dataelement1");
> if( test_dataelement1 == NULL ) {
> mexErrMsgTxt("insert error message here");
> }
> test_dataelement2 = mxGetField(prhs[0],0,"dataelement2");
> if( test_dataelement2 == NULL ) {
> mexErrMsgTxt("insert error message here");
> }
> pr1 = mxGetPr(test_dataelement1);
> pr2 = mxGetPr(test_dataelement2);
> :
>
> pr1[0] is the first element, pr1[1] is the second element, etc.
>
> If the underlying data is some other class, e.g. int32, then you would use mxGetData instead of mxGetPr and cast the resulting pointer appropriate to the underlying data.
>
> James Tursa
Hi James, thanks for the additional information. I am getting the following compiler error
on line test_dataelement1=mxGetField(prhs[0],0,"dataelement1");
prhs was not declared in this scope. I tried adding const mxArray *prhs[2]; and it does get rid of the compiler error on that line but then I get a runtime error on that line. Also wondering if I am missing an include file as when I added mexErrMsgTxt I get the error was not declared in this scope. I have been able to read data from a matlab array dereferencing the pointer as you described.

"Paul" wrote in message <jdfj0f$fm0$1@newscl01ah.mathworks.com>...
>
> Hi James, thanks for the additional information. I am getting the following compiler error
> on line test_dataelement1=mxGetField(prhs[0],0,"dataelement1");
> prhs was not declared in this scope. I tried adding const mxArray *prhs[2]; and it does get rid of the compiler error on that line but then I get a runtime error on that line. Also wondering if I am missing an include file as when I added mexErrMsgTxt I get the error was not declared in this scope. I have been able to read data from a matlab array dereferencing the pointer as you described.

Please post your entire code so I can look at everything in context.

James Tursa