Understanding nested function reference in parfor loop

I came accross this sentence in MATLAB doc:

The body of a parfor-loop cannot make reference to a nested function. However, it can call a nested function by means of a function handle.

Can someone please explain what this means?

[Jason Stockton]

Yes, Please. I am having an issue with it now. Thanks.

[Jonathan Chin]

Look at this examples where I am creating a function handle for my nested function.

function out=parforTest(in)
 out = zeros(1,4);
 tmp=@(x)nestedFunc(in,x);
 parfor ii=1:4
    out(ii)=feval(tmp,ii)
 end
 function outv= nestedFunc(in,var)
        outv = in+var;
    end
end

Answer 1

[Himanshu]

Hey,

The sentence mentioned by you from the documentaiton points out a limitation of the 'parfor' loop, which is that it cannot directly reference nested functions within its loop body. Instead, you can call a nested function using a function handle, which is like a reference or pointer to a function. For example:

function mainFunction()
    % Nested function
    function result = nestedFunction(x)
        result = x^2;
    end

    % Create a function handle
    nestedHandle = @nestedFunction;

    % Use parfor with the function handle
    parfor i = 1:5
        results(i) = nestedHandle(i);
    end

    disp(results);
end

In the code above, inside the parfor loop, you call the nested function using the handle nestedHandle(i). This approach bypasses the restriction, allowing the nested function to be used in parallel computations.

In simple terms, while parfor can't directly use nested functions, creating a function handle allows you to call them indirectly, enabling parallel execution.

Hope this help!