MATLAB: How to create a Cell Array in a MEX function which can be returned to the MATLAB workspace

arraycellMATLABmexmxcreatecellarraymxcreatecellmatrixmxsetcell

I would like to create a cell array in a MEX file which contains two character arrays as members of the cell array. I would also like to return this cell array to the base MATLAB workspace. How can this be done?

Best Answer

The 'mxCreateCellMatrix' function can be used to create a cell matrix within a MEX <file: >

https://www.mathworks.com/help/matlab/apiref/mxcreatecellmatrix.html

Note that once the mxArray corresponding to your cell array has been created, you would may need to set cell array fields using the 'mxSetCell' function:

https://www.mathworks.com/help/matlab/apiref/mxsetcell.html

An example of this workflow is provided with MATLAB. Please execute the following command on the MATLAB Command Window:

>> edit([matlabroot '/extern/examples/mx/mxcreatecellmatrix.c']);

There is another example of this workflow with this article as "cellTest.c". This file creates a cell array with "Hello" and "Goodbye" character arrays as members of the cell array.

If you would like to return a cell array created in a MEX file to the MATLAB workspace, the recommended method is to add the new 'mxArray' to the 'plhs' array within the 'mexFunction' in the MEX file. However, if this is not sufficient for your workflow, you can also use the 'mexPutVariable' function to manually send specified variables back to the base MATLAB workspace.

https://www.mathworks.com/help/matlab/apiref/mexputvariable.html

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MATLAB: How to print text to the MATLAB command window from a FORTRAN MEX file

Use mexPrintf to print to the MATLAB command window when using MEX files. Two examples are shown below.

Example 1: Print "API says hello!" to the MATLAB command window

#include "fintrf.h" 
      subroutine mexFunction(nlhs, plhs, nrhs, prhs) 
C     Specify alert to be printed 
      character (len=*), PARAMETER :: txt = "API says hello!"
C     Call the API     
      call mexPrintf(txt)
      end

Example 2: Print the number of input arguments to the MATLAB command window:

#include "fintrf.h" 
      subroutine mexFunction(nlhs, plhs, nrhs, prhs) 
        character (len=1) :: c
        character (len=50) :: txt
C     Specify alert to be printed
      write(txt,'(A)') 'Number of inputs (mexPrintf): ' 
C     Convert nrhs to char
      write(c,'(i1)') nrhs
C     Call the API
      call mexPrintf(txt)
      call mexPrintf(c)
      end

There are many FORTRAN MEX function source code examples available at:

https://www.mathworks.com/help/matlab/matlab_external/table-of-mex-file-source-code-files.html

Look in the table for examples with the .F file extension.

MATLAB: How to call mxArray with varargin in C; mxCreateCellMatrix and mxSetCell

Oyvind- it isn't clear how you are executing the above code. Is it part of a C program that you are running? Where is the function that you are providing the variable number of inputs to? How does wmlfNor relate to this function, as you seem to be just passing in strings here?

If I take the body of your above code and paste it into a MEX function (commenting out the returnval stuff and the call to the mlfNor function), the build/compile works fine, but crashes as soon as I call the MEX function. (Note that I created dummy strings in place of your input parameters.)

What seems to be happening is that the code is trying to destroy/free the same allocated memory twice at

 mxDestroyArray(pfinp);
 mxDestroyArray(pfana);
 mxDestroyArray(pfcsv);
 mxDestroyArray(pvarargin);

We destroy the mxArrays that correspond to the three strings created with mxCreateString, and then try to destroy the cell array, pvarargin, which tries to destroy/free all cells...which includes the three (string) mxArrays which we just destroyed. If you comment out these three lines and just do

 //mxDestroyArray(pfinp);
 //mxDestroyArray(pfana);
 //mxDestroyArray(pfcsv);
 mxDestroyArray(pvarargin);

it works fine: there is no crash when running the MEX function. To see the effects of this more clearly, do the following: free/destroy the memory to the three string arrays, and then set each of the three cells in pvarargin to NULL, then destroy the cell array

 mxDestroyArray(pfinp);
 mxDestroyArray(pfana);
 mxDestroyArray(pfcsv);
 mxSetCell(pvarargin,0,NULL);
 mxSetCell(pvarargin,1,NULL);
 mxSetCell(pvarargin,2,NULL);
 mxDestroyArray(pvarargin);

Again, the function can be built/compiled without any problems, and the MEX function can be run.

Try using one of the above two options in your code, and see if that prevents the crash.

Best Answer

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MATLAB: How to print text to the MATLAB command window from a FORTRAN MEX file

MATLAB: How to call mxArray with varargin in C; mxCreateCellMatrix and mxSetCell

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