Parfor loops skipping an index

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Oyeniyi 2015-1-1

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https://ww2.mathworks.cn/matlabcentral/answers/168672-parfor-loops-skipping-an-index

评论： Edric Ellis 2015-1-6

When I run the piece of code below, the result shows that there is an index that is not executed...

 temp=struct('myfield1',num2cell(ones(2,6)));
 parfor i = 1:6
   temp(2,i).myfield1 = i*4;
   temp(2,i).myfield2 = i;
 end

I have run this on a few computers and one of the index is always not executed (or at least seems to). For example, I could have temp(2,6).myfield1=1 and temp(2,6).myfield2=[] instead of temp(2,6).myfield1=24 and temp(2,6).myfield2=6.

Could it be because I did not define all the fields beforehand?

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Answer 1

Edric Ellis 2015-1-6

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Yes, defining all the fields of temp beforehand works around this problem, like so:

temp=struct('myfield1',num2cell(ones(2,6)), 'myfield2', []); 
parfor i = 1:6 
  temp(2,i).myfield1 = i*4; 
  temp(2,i).myfield2 = i; 
end 
% Assertions: 
assert(isequal(4:4:24, [temp(2, :).myfield1])); 
assert(isequal(1:6, [temp(2, :).myfield2]));

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Oyeniyi 2015-1-6

I also noticed earlier that pre-defining the fields eliminates the problem but it is not really feasible to do this for my application.

Do you know why an index is skipped in the initial instance? Many thanks.

Edric Ellis 2015-1-6

I'm afraid I know of no other workaround involving structures. Could you perhaps return cell arrays of data with which you can recreate the structure outside the PARFOR loop?

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