function with unknown number of outputs within parfor

Dear all,

I am facing this apparently simple problem: I am trying to write a parfor loop that containts a function call with an unknown number of output arguments. The way I tried to implement it is as follows

Tank=initialize_tank();
parfor ii=1:n
  [outputs{:}]=myfunction(a,b,c,d);
  Tank(:,ii)=outputs(:);
end

But due to the way variable outputs is used, I get a complaint from parfor. Does anybody know the workaround to this?

Thanks

Answer 1

[Edric Ellis]

The trick here is that (unfortunately) you need to assign the outputs of your function to a temporary variable, and convince parfor that the variable is indeed a temporary. Like so:

numOut = 3;
n = 7;
out = cell(numOut, n);
parfor idx = 1:n
    % This line is required by PARFOR to ensure that 'tmp' is treated
    % as a temporary loop variable:
    tmp = cell(1, numOut);
    % Call the function with a variable number of outputs:
    [tmp{1:numOut}] = deal(rand());
    % Assign into the output:
    out(:, idx) = tmp;
end

Without the tmp = cell(1, numOut); statement, parfor thinks you are assigning to only part of tmp in the following line, and therefore concludes that your loop is order-dependent. By assigning to tmp without any indexing, parfor realises that you are creating a whole new value.

[Patrick Mboma]

Edric,

Your solution works very well. Thanks a lot.

Answer 2

[Matt J]

    Tank=initialize_tank();
    numOutputs=size(Tank,1);

    parfor ii=1:n
      [outputs{1:numOutputs}]=myfunction(a,b,c,d);
      Tank(:,ii)=outputs(:);
    end

[Patrick Mboma]

Hi Matt,

Thanks for the answer. This is exactly what I meant to write, sorry for the typo. But this does not seem to work.

[Patrick Mboma]

I have also tried

    Tank=initialize_tank();
    numOutputs=size(Tank,1);
    parfor ii=1:n
      [Tank{1:numOutputs,ii}]=myfunction(a,b,c,d);
    end

But this does not work either

[Matt J]

Hmmmm. Well, one solution would be to return just a single argument from myfunction. I.e., have it return "outputs" already in cell array form, like below. Even if you don't want to rewrite myfunction, it should be easy to write a wrapper for it that will do this.

    n=10; 
    a=1;b=1;c=1;d=1;

     Tank=initialize_tank(n);
        numOutputs=size(Tank,1);
        parfor ii=1:n
          out=myfunction(a,b,c,d,numOutputs);
          Tank(:,ii)=out(:);
        end

    function T=initialize_tank(n)

            T=nan(4,n);

    function out=myfunction(a,b,c,d, numOutputs)

       out=cell(1,numOuputs);

[Patrick Mboma]

Matt,

This workaround using a wrapper function would work brilliantly. Edric, below, proposes an alternative solution but which also requires an extra step. Both solutions work very well. Thank you very much.