Suscripted Assignment in to Mxarray is not supported for code generation

Gamma=zeros(n_y*Np,n);
cont=1;
for k=1:n_y:n_y*Np
    row_i=k;
    row_f=k+n_y-1;
    
    Gamma(row_i:row_f,:)=C*A^(cont);
    cont=cont+1;
end

outputs.

Error:Subscripted assignment into an mxArray is not supported for code generation.

Function 'MATLAB Function' (#484.1885.1905), line 85, column 5:

"Gamma(row_i:row_f,:)"

Launch diagnostic report.

[Shivam Gothi]

Hello @Joseph,

Can you please share the complete code so that I can reproduce the error from my end and try to get a possible work around for it?

[Walter Roberson]

I notice that each iteration of the loop, it sets n_y rows to a value, and the next iteration overwrites all but one of the rows. This feels like a waste of time.

Answer 1

[MULI]

Hi Joseph,

I understand that you are facing an error when trying to convert MATLAB code to C/C++ using MATLAB Coder.

This error is related to dynamic indexing that MATLAB Coder cannot handle due to its requirement for fixed-size arrays due to the following reasons:

Dynamic Indexing: Even though `Gamma` is preallocated with a fixed size, the assignment the line “Gamma(row_i:row_f, :) = C * A^(cont)” involves dynamic indexing, which MATLAB Coder cannot handle.

Matrix Operation Compatibility: The result of “C * A^con” must match the size of the submatrix “Gamma(row_i:row_f, :)”. If the dimensions do not match, it leads to an error.

To resolve this issue, you may follow these steps:

• Make sure that the matrices `C` and `A` have fixed sizes. For instance, if `C` is `(n_y, m)` and `A` is `(m, m)`, the result `C * A^cont` should be `(n_y, m)`.
• Ensure that `m` equals `n` so that the result of the multiplication fits into the submatrix of `Gamma`.

By ensuring that all arrays and matrix operations have fixed and compatible sizes, you can avoid the error during code generation.

You can refer to these links where similar query is answered

https://www.mathworks.com/matlabcentral/answers/13297-subscripting-into-an-mxarray

https://www.mathworks.com/matlabcentral/answers/122167-subscripting-into-an-mxarray-is-not-supported