How to import the C++array into matlab workspace by matlab engine?

Currently I am using the Matlab Engine in visual studio and there is a two dimensional array in my cpp file.The question is how I can import the array to matlab workspace by the function of 'engPutVariable'.

Answer 1

[Avni Agrawal]

Hi @QiQin Zhan,

I understand that you are trying to use the `engPutVariable` function to transfer a two-dimensional array from your C++ application to the MATLAB workspace, you need to follow these steps:

1. Initialize the MATLAB Engine: Make sure that the MATLAB Engine is properly initialized in your C++ application.

2. Create a mxArray: Convert your C++ two-dimensional array into a `mxArray`, which is the data type MATLAB uses for arrays.

3. Use engPutVariable: Transfer the `mxArray` to the MATLAB workspace using `engPutVariable`.

Here is a step-by-step example to illustrate the process:

1. Include Necessary Headers:

Make sure to include the MATLAB Engine and Matrix API headers in your C++ file.

#include "engine.h"
#include "matrix.h"

2. Initialize the MATLAB Engine:

Start the MATLAB Engine session.

Engine *ep;
if (!(ep = engOpen(NULL))) {
    fprintf(stderr, "\nCan't start MATLAB engine\n");
    return EXIT_FAILURE;
    }

3. Create a Two-Dimensional Array:

Suppose you have a 2D array in C++:

    double myArray[2][3] = { {1.0, 2.0, 3.0}, {4.0, 5.0, 6.0} };

4. Convert to mxArray:

You need to convert this C++ array to a `mxArray`.

mxArray *matlabArray = mxCreateDoubleMatrix(2, 3, mxREAL);

memcpy(mxGetPr(matlabArray), myArray, 2 * 3 * sizeof(double));

5. Transfer to MATLAB Workspace:

Use `engPutVariable` to put the `mxArray` into the MATLAB workspace.

if (engPutVariable(ep, "myMatlabArray", matlabArray)) {

fprintf(stderr, "\nFailed to put variable into MATLAB workspace\n");

return EXIT_FAILURE;

}

6. Clean Up:

After you're done, make sure to clean up and close the MATLAB Engine session.

mxDestroyArray(matlabArray);

engClose(ep);

• mxCreateDoubleMatrix: This function creates a 2D array (matrix) in MATLAB. The dimensions (2, 3) correspond to the dimensions of your C++ array.
• memcpy: This is used to copy the C++ array data into the `mxArray`.
• engPutVariable: This function sends the `mxArray` to the MATLAB workspace with the name `"myMatlabArray"`.

Make sure to link against the MATLAB Engine and Matrix libraries in your Visual Studio project settings to ensure successful compilation and linking.

I hope this helps!

[James Tursa]

This answer neglects the fact that C/C++ 2D arrays are row major, but MATLAB 2D arrays are column major. So the code above will copy the elements in a jumbled state. There are various ways to deal with this. One way is to copy into a transposed array (3x2 in this case) and then transpose the result to a 2x3. E.g.,

mxArray *matlabArray = mxCreateDoubleMatrix(3, 2, mxREAL);

To transpose the result in the Engine workspace after the transfer, simply call:

engEvalString(ep, "myMatlabArray = myMatlabArray';");