How can I place a matrix of unsigned characters into a field in a MATLAB structure?

How can I place a matrix of unsigned characters into a field in a MATLAB structure?

I am having difficulty in using a MEX-file to receive a 20-by-2 matrix of unsigned character values in C++ and transfer them to the same size matrix that is part of a structure in MATLAB.

Answer 1

[MathWorks Support Team]

To receive a matrix of unsigned characters into a field in a MATLAB structure, you can use our C API routines. These routines will copy the unsigned character matrix into the field of the MATLAB structure. The logical structure of an MATLAB structure is illustrated below:

 mxArray *(structure) #created using mxCreateStructureMatrix

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       -- mxArray *(field1)  #created using mxCreate#####

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      -- mxArray *(field2)  #created using mxCreate#####

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      -- mxArray *(field3)  #created using mxCreate#####

An unsigned char is equivalent to an unsigned 8-bit integer, which you can create in your MEX-file using the mxCreateNumericArray routine:

field3 = mxCreateNumericMatrix(20, 2, mxUINT8_CLASS, mxREAL);

//The above says to create a real (as opposed to complex) 20 x 2 matrix of type "uint8".

You can now add this to the structure that was previously created with the mxCreateStructMatrix using the mxSetField routine.

 mxArray *structure;

 mxArray *field3;

 unsigned char *pdata;

 structure = mxCreateStructMatrix(1, 1, 3, {"field1", "field2","field3"});

 mxSetField(structure, 0, "field1", mxCreateDoubleScalar(20));

 mxSetField(structure, 0, "field2", mxCreateDoubleScalar(2));

 field3 = mxCreateNumericMatrix(20, 2, mxUINT8_CLASS, mxREAL);

 //returns the pointer to the data part of the MATLAB array

 pdata = mxGetData(field3);

 // Now copy actual data into the data part of field3

 for (int j = 0 ; j < 2 ; j++) {

    for (int i = 0 ; i < 20 ; i++) {

          pdata[20*j + i] = CppMatrix[i][j];

    }

 }

 mxSetField(structure, 0,"field3", field3);