MATLAB: Codegen with dynamic field names

codegenmatlab coderstructstructures

I'm trying to compile a mex-file from my m-code. I've problems in the following lines
    Atm = sys.A0;
    Btm = sys.B0;
    Ctm = sys.C0;
    Dtm = sys.D0;
    for i = 1:np
        Atm = Atm+sys.(['A',sprintf('%.0f',i)]) * p(i,t);
        Btm = Btm+sys.(['B',sprintf('%.0f',i)]) * p(i,t);
        Ctm = Ctm+sys.(['C',sprintf('%.0f',i)]) * p(i,t);
        Dtm = Dtm+sys.(['D',sprintf('%.0f',i)]) * p(i,t);
    end
which returns the error '??? Non-constant expression or empty matrix'.
The variable sys is a structure with field names sys.A0, sys.A1,…, sys.A[np], sys.B0, …, sys.D[np], sys.np. Where np may vary from case to case and the variables sys.A[i],…,sys.D[i] are 2-D matrices. The variable p is a matrix of size np X m. I compile the code for a fixed sized variable sys and p, with
codegen errAndJac -args {u,p,y,sys}
My problem seems the following. The code generation cannot handle ['A',sprintf('%.0f',i)] to locate a certain matrix. I've also tried to first pre locate a variable iA = char('A0',…,'A[np]') and then use sys.(iA(i,:)), which resulted in the same error.
An alternative and/or solution will be highly appreciated. Thanks in advanced. Attached is the function to compile and an example of my variables.

Best Answer

Pepjin - why not just combine all of your (for example) AX fields of the sys struct into one three-dimensional field? In your variableExample.mat, the A0, A1, and A2 fields are of the same 10x10 dimension, so just create A as
 sys.A = zeros(10,10,3);
 sys.A(:,:,1) = A0;
 sys.A(:,:,2) = A1;
 sys.A(:,:,3) = A2;
If you do this for B, C and D, then your above code becomes
 Atm = sys.A(:,:,1);
 Btm = sys.B(:,:,1);
 Ctm = sys.C(:,:,1);
 Dtm = sys.D(:,:,1);
 for i = 1:np
     Atm = Atm+sys.A(:,:,i+1) * p(i,t);
     Btm = Btm+sys.B(:,:,i+1) * p(i,t);
     Ctm = Ctm+sys.C(:,:,i+1) * p(i,t);
     Dtm = Dtm+sys.D(:,:,i+1) * p(i,t);
 end
This way you avoid having to access the the dynamic fields within your struct.
Related Question