Why do I get the error "Non-constant indexing into a heterogeneous cell array is not supported in code generation" when my function takes a cell array input with character vectors of varying length?

I use MATLAB R2023a and I have a cell array with character vectors of varying length in the base workspace:

n=1000;
cellArray = cell(1,n);
for i=1:n
    cellArray{i} = sprintf('String %d',i);
end

I am trying to read this cell array into my Simulink model using a MATLAB Function block with this code below, where the index 'idx' is a block input (runtime variable) and 'out' will be an output string signal:

function [out] = fcn(idx, cellArray)
out = string(cellArray{idx});
end

But I get this error when I try to run my model:

Non-constant indexing into a heterogeneous cell array is not supported in code generation.

The same error will occur if I try to generate code from a MATLAB function with this code using MATLAB Coder.

Answer 1

[MathWorks Support Team]

This is a current limitation of the underlying code generation functionality. Runtime indexing into a heterogeneous cell array is not supported as mentioned in the documentation below:

https://www.mathworks.com/help/coder/ug/homogeneous-vs-heterogeneous-cell-arrays.html

To work around this, create a local copy of the 'cellArray' variable as shown in the code below:

function [out] = fcn(idx, cellArray)
c = cellArray;
out = string(c{idx});
end

The function input cannot be changed from heterogeneous to homogeneous, while the local copy can.

The "loadStringsFromCellArray.zip" file attached above contains the example and solution.