What is causing this intermittent error in parfor?

The attached script produces an intermittent error:

Increasing n in untitled_script.m appears to increase the likelihood of the error occuring. Replacing parfor with for prevents the error from occuring. Setting n=1 prevents the error from occuring. Replacing "test(ii,1)=Test(input,ii);" with "Test(input,ii);" prevents the error from occuring.

Anyone understand what is going on here?

[Chris]

Replacing "test(ii,1)=Test(input,ii);" with "test{ii}=Test(input,ii);" also appears to prevent the error from occuring.

Answer 1

[Matt J]

If you add the indicated line, your class constructor will support no-argument calls, and the error will go away:

        function obj = Test(inputArg1,ii)
            arguments
                inputArg1 
                ii = 2
            end

            if ~nargin, return; end  %<---- Add this

            display(num2str(ii))
            obj = obj.method1(inputArg1);
        end

This is necessary because the loop variable values ii in a parfor loop will not be visited in in sequential order, meaning that certain elements in the vector test will need to be populated temporarily with argument-free constructor calls.

[Matt J]

Alternattively (and perhaps preferably), pre-allocate the test array,

input.test_field=1;
input.test_field2=1;

n=3;

test(1:n,1)=Test(input);  %<--- pre-allocate

parfor ii=1:n
    test(ii,1)=Test(input,ii);
end

[Chris]

Thanks Matt!