# Parfor loop and variable length output

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Alessandro Masullo on 26 Feb 2015
Edited: Edric Ellis on 2 Mar 2015
Hello everyone,
I would like to speed-up a code using a parfor loop but my function is not straightforwardly suitable for it. In particular, the problem is the for loop contains a function whose output has a variable length and I need to store all the variable outputs in a final vector.
I can predict the maximum length of the final result, but most likely I will only use one tenth of this maximum.
result = zeros(1,10000);
N_result = 0;
for i = 1:N
result_i = generate_result(i); % This has a variable number of elements
result(N_result+1:numel(result_i) = result_i;
N_result = N_result+numel(result_i);
end
I would like to use the parfor instead of a for, but I don't know how merge the variable result_i in a single output result. It would be perfect if I could create a variable length vector for each worker, and then merge those vectors.
How can I do this?
Alessandro.

Edric Ellis on 26 Feb 2015
You have several options here. Probably the simplest is to return a cell array and then concatenate the contents. It might not be terribly efficient though if N is large compared to the number of outputs you're returning (this is because cell arrays where each cell contains only a small number of elements have memory and performance overhead compared to a numeric array).
Here's an example. (Note the use of feval to work around a PARFOR restriction on using function handles)
% Here's a simple function that generates a random array of a random size:
generate_result = @(x) x * rand(1, randi([0,10]))
parfor idx = 1:7
output{idx} = feval(generate_result, idx);
end
% Now, concatenate the output elements into a single array
output = [output{:}];
##### 2 CommentsShowHide 1 older comment
Edric Ellis on 2 Mar 2015
Edited: Edric Ellis on 2 Mar 2015
You don't normally need to use feval - I needed it only because generate_result is a function handle, and there's a limitation with PARFOR which means you must invoke those using feval. In your case, if generate_result is an ordinary function, you shouldn't use feval.