Why no data transfer between c++ and MATLAB Workspace?

I am trying to transfer variables from C++ to the MATLAB workspace. The program compiles successfully, and I get the correct results back, but the variables are not visible in the MATLAB workspace.

Here is my C++ code:

#include "MatlabDataArray.hpp"
#include "MatlabEngine.hpp"
#include <iostream>
#include <windows.h>

int main() {
    using namespace matlab::engine;
    // Start MATLAB engine synchronously
    std::unique_ptr<MATLABEngine> matlabPtr = startMATLAB();

    //Create MATLAB data array factory
    matlab::data::ArrayFactory factory;

    // Create variables    
    matlab::data::TypedArray<double> data = factory.createArray<double>({ 1, 10 },
        { 4, 8, 6, -1, -2, -3, -1, 3, 4, 5 });
    matlab::data::TypedArray<int32_t>  windowLength = factory.createScalar<int32_t>(3);
    matlab::data::CharArray name = factory.createCharArray("Endpoints");
    matlab::data::CharArray value = factory.createCharArray("discard");
      
    // Put variables in the MATLAB workspace
    matlabPtr->setVariable(u"data", std::move(data));
    matlabPtr->setVariable(u"w", std::move(windowLength));
    matlabPtr->setVariable(u"n", std::move(name));
    matlabPtr->setVariable(u"v", std::move(value));

    // Call the MATLAB movsum function
    matlabPtr->eval(u"A = movsum(data, w, n, v);");
    
    // Get the result
    matlab::data::TypedArray<double> const A = matlabPtr->getVariable(u"A");
      
    // Display the result
    int i = 0;
    for (auto r : A) {
        std::cout << "results[" << i << "] = " << r << std::endl;
        ++i;
    }
}

Used Software:

• VS Code 1.94.2
• Compiler: mingw81
• MATLAB: R2022a

Answer 1

[Jaimin]

Hello @Kreilinger

These steps and tips can help you troubleshoot and ensure that variables are visible in the MATLAB workspace:

• Check the MATLAB Workspace: After transferring the data, check if the variable is in the MATLAB workspace. You can use the “engEvalString” function to execute MATLAB commands from your C++ program.
• Visibility in MATLAB Workspace: If the variables are not visible in the MATLAB workspace after the program ends, it might be because MATLAB Engine sessions are separate from the MATLAB desktop session. The variables exist only for the duration of the engine session.
• Save Variables to MAT-File: If you need to retain the variables in MATLAB after the C++ program has finished executing, consider saving them to a MAT-file. You can then load this MAT-file in MATLAB to access the variables.
• Debugging: If the variables still do not appear, check for errors in data transfer and ensure that “engPutVariable” is called successfully. You can also use “engOutputBuffer” to capture any error messages from MATLAB.

Kindly refer following MathWorks Documentation for more information.

engEvalString: https://www.mathworks.com/help/matlab/apiref/engevalstring.html

engPutVariable: https://www.mathworks.com/help/matlab/apiref/engputvariable.html

engOutputBuffer: https://www.mathworks.com/help/matlab/apiref/engoutputbuffer.html

I hope this will be helpful.

[Kreilinger]

Hello Jaimin,

Thank you for your response. The issue was most likely due to the fact that MATLAB Engine sessions are separate from the MATLAB desktop session.

I couldn't save the data as a MAT-file because I needed a fast live stream of data.

I resolved the problem by coding a MATLAB function in C++ to handle the live data transfer.

Thanks again for your insights!