MATLAB: How to call a structure whose name is assigned to another variable??

MATLABstructure

Hello all,

I am trying to get the data from a structure but I the problem I am facing is that I am not able to call the Structure though a variable which cointains the structure name. When I am writng the name of the structure specifically then its fine but not otherwise. Can anyone be able to help.

clear all
name='Test_4';
load(name);
Nr_Variabel=length(name.Y);
for k=1:Nr_Variabel
    Tem=name.Y(1,k).Data;
    Variabel_name=name.Y(1,k).Name;
    plot(Tem);
    xlabel(Variabel_name);  
end

Best Answer

I recommend that you call LOAD with an output argument.

 data = load('mymatfile.mat');

Now you can access, for example, the variable named x that was stored in mymatfile.mat as:

 data.x

This avoids a problem where your MAT-file contains a variable with the same name as a function [often ALPHA] -- this is known as "poofing" on the MATLAB newsgroup and can be difficult to diagnose.

Related Solutions

MATLAB: HELP….mxCallMATLAB output assignment to variable??

Try this. It works for me with some simple test code for FILLAB, my_matlab_solver, and plot_variable. I added some comments where you have memory leaks and type issues, and put in some code for variable class and size checks.

        /* Did you include string.h? (for memcpy) */
      void mexFunction(int nlhs, mxArray *plhs[], 
                       int nrhs, const mxArray *prhs[])
      {
        static double *A,*B;
        mwSize dims[2] = {5,10}; // Use mwSize here, not int.
        static double *U,*V;
        mxArray *APPLE[1];
        mxArray *Uin[2],*Uout[2];
      if( nlhs < 2 ) {
            mexErrMsgTxt("Not enough outputs.");
      }
        A   = mxGetPr(mxCreateNumericArray(2,dims,mxDOUBLE_CLASS,mxREAL)); // MEMORY LEAK!
        B   = mxGetPr(mxCreateNumericArray(2,dims,mxDOUBLE_CLASS,mxREAL)); // MEMORY LEAK!
        // In the above two lines, you lose the pointers to the mxArrays! Bad practice!
        /* C code which fills A and B */
        FILLAB(A,B);
        Uin[0] = mxCreateNumericArray(2,dims,mxDOUBLE_CLASS,mxREAL);
        Uin[1] = mxCreateNumericArray(2,dims,mxDOUBLE_CLASS,mxREAL);
        memcpy(mxGetPr(Uin[0]),A,5*10*sizeof(double));
        memcpy(mxGetPr(Uin[1]),B,5*10*sizeof(double));
        Uout[0] = mxCreateNumericArray(2,dims,mxDOUBLE_CLASS,mxREAL); // MEMORY LEAK!
        Uout[1] = mxCreateNumericArray(2,dims,mxDOUBLE_CLASS,mxREAL); // MEMORY LEAK!
        // In the above two lines, the mxArrays created are lost by the subsequent
        // call to mexCallMATLAB, which OVERWRITES the pointers in Uout[0] and Uout[1].
        // This is not good programming practice! Just DELETE these two lines entirely!
        mexCallMATLAB(2,Uout,2,Uin,"my_matlab_solver"); // This CREATES new outputs in Uout.
        // Note, you could use plhs in the 2nd argument above and skipped the following
        // two lines assigning Uout to plhs.
        plhs[0] = Uout[0];
        plhs[1] = Uout[1]; // Need nlhs >= 2 in order for this to work.
        U  = mxGetPr(plhs[0]);
        V  = mxGetPr(plhs[1]); // Need nlhs >= 2 in order for this to work.
        APPLE[0] = mxCreateNumericArray(2,dims,mxDOUBLE_CLASS,mxREAL);
      if( !mxIsDouble(plhs[1]) || mxIsSparse(plhs[1]) || mxGetNumberOfElements(plhs[1]) < 50 ) {
            mexErrMsgTxt("plhs[1] is not full double class with 50 or more elements");
      }
      if( !mxIsDouble(plhs[0]) || mxIsSparse(plhs[0]) || mxGetNumberOfElements(plhs[0]) < 50 ) {
            mexErrMsgTxt("plhs[0] is not full double class with 50 or more elements");
      }
        memcpy(mxGetPr(APPLE[0]),V,5*10*sizeof(double));
        /*APPLE[0] = plhs[1]; */
        mexCallMATLAB(0,NULL,1,&APPLE[0],"plot_variable");
        // Note, you can just use APPLE in the 3rd argument above. No need to use &APPLE[0].
    }

MATLAB: Assign values to a new variable via a function

"% Below line is the line that doesn't work"

namesNew = namesOld;

It doesn't work because you are trying to magically access variable names using strings, which is certainly possible, but it is one way that beginners force themselves into writing slow, complex, buggy code that is hard to debug. Read this to know why:

https://www.mathworks.com/matlabcentral/answers/304528-tutorial-why-variables-should-not-be-named-dynamically-eval

Writing better (simpler, more robust, less buggy) code makes this task easier. For example, you should always load a .mat file into an output variable (which is a structure):

S = load(...);

Working with fields of a structure is much easier than magically accessing variable names, so lets try that:

S = load(namesOld);
C = struct2cell(S);
T = cell2struct(C,namesNew,1);
save(namesNew,'-struct','T')

Best Answer

Related Solutions

MATLAB: HELP….mxCallMATLAB output assignment to variable??

MATLAB: Assign values to a new variable via a function

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