Nested cellfun in parfor loop

Hello,

I'm trying to set up a function for cells using cellfun inside a parfor loop. A simplified example seen below:

 %Main function
 function main
    M     = rand(100,100);
    N     = rand(100,100);
    const = rand(100,1); %Does not change during parfor loop

    parfor i=1:size(M,2)
       X = num2cell( M(:,i),1 );
       Y = num2cell( N(:,i),1 );
       Z = cellfun(@(x,y) nestedFunc(const,x,y), X,Y, 'uniformoutput',0 );
    end

    %Nested function
    function z = nestedFunc(const,x,y)
       %Do a bunch of stuff... 
    end

 end

However, using

 Z = cellfun(@(x,y) nestedFunc(const,x,y), X,Y, 'uniformoutput',0 );

is not allowed inside parfor: " The nested function 'nestedFunc' cannot be called within a PARFOR loop".

I've read that you can use the feval and the handle to the function to get around this, like this (from Matlab documentation):

 function A = pfeg
   function out = nfcn(in)
      out = 1 + in;
   end

   fcn = @nfcn;

   parfor idx = 1:10
      A(idx) = feval(fcn, idx); 
   end

 end

I do not manage to do this when my function is a cellfun however. I can get around the issue by defining nestedFunc in a separate .m file, but I would prefer if it could be done inside the function itself. (Also, I don't know if calling a separate function takes more time compared to a nested function?)

Can anyone please advice me? Thank you!

Answer 1

[Edric Ellis]

Here's some code that I tried combining using nested functions together with parfor and cellfun:

function out = pfeg

const = 7;
    function out = nFcn(x, offset)
        out = x + offset;
    end

fcnHandle = @(in) nFcn(in, const);

parfor idx = 1:10
    out{idx} = cellfun(fcnHandle, num2cell(1:idx));
end
end

This works as expected. Note that using nested functions inside parfor is somewhat risky - because each worker gets its own copy of "uplevel variables" - i.e. those variables shared between parent and nested function.

[Ida]

Thanks Edric,

This works as a charm and was jut what i was looking for!