Converting Matlab code to C++ with Matlab Coder - Cell problem

Question

I'm trying to convert a function to C++ in Matlab Coder.. There is a cell variable and while building mex it gives an error.

     R=cell(1,n);
for i=1:wi
        for j=1:hi
            if(cin(i,j)>0)
                     k=cin(i,j);
                    for x=i-2*rx+1:i+2*rx-1
                        for y=j-2*ry+1:j+2*ry-1
                                if(x>=1 && y>=1 && x<=wi && y<=hi)
                                    R{k}=[R{k}, (x-1)*wi+y];
                            end
                        end
                    end
            end
         end
end

It gives error on R{k}=[R{k}, (x-1)*wi+y]; part

'Attempt to access an element that was not defined before use.'

Anybody can help me about this?

this is not c++, is your issue related to c++ at all? if not, take the tag out so its not confusing — lionkor
– lionkor, Commented Dec 17, 2020 at 15:19

Ryan Livingston · Accepted Answer · 2020-12-18 10:10:24Z

For code generation, you have to define (i.e. assign) all cell array elements prior to their use. Here R{k} is used prior to being assigned. Namely, the R{k} on the RHS of that assignment reads that element prior to it having been assigned.

If you intend the elements to be empty matrices, [], then you can declare R like:

% Cells should be varsize to grow them later on
coder.varsize('R{:}');

% Initialize all cells to empty
R = repmat({[]}, 1, n);

for i=1:wi
        for j=1:hi
            if(cin(i,j)>0)
                     k=cin(i,j);
                    for x=i-2*rx+1:i+2*rx-1
                        for y=j-2*ry+1:j+2*ry-1
                                if(x>=1 && y>=1 && x<=wi && y<=hi)
                                    R{k}=[R{k}, (x-1)*wi+y];
                            end
                        end
                    end
            end
         end
end

More details in the MATLAB Coder doc:

https://www.mathworks.com/help/simulink/ug/cell-array-restrictions-for-code-generation.html

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