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Wrong indexing in parfor loop

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Andre Zeug
Andre Zeug on 10 Nov 2015
Edited: Andre Zeug on 11 Nov 2015
I tried to use parfor to change parts of a variable but receive unexpected result
d = ones(10,10);
d = repmat(d,[1 1 1 10]);
d(:,:,2,:) = .5*d(:,:,1,:);
parfor n=1:10;
d(:,:,2,n) = 2*d(:,:,2,n);
end
I expected to receive something like
d(:,:,2,:) = d(:,:,1,:);
Instead I get
d(:,:,1,:) = 2;
d(:,:,2,:) = .5;
What went wrong? Using for instead of parfor works fine.
Thanks

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Andre Zeug
Andre Zeug on 10 Nov 2015
Thanks for support.
I'm using R2014a and Win7-64bit. Get same (unexpected) result after system restart.
Adam Barber
Adam Barber on 10 Nov 2015
I was able to reproduce this on R2014a, but not R2015b on my system.
Andre Zeug
Andre Zeug on 10 Nov 2015
I could now reproduce this unexpected result also on R2013b.

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Answers (1)

Andre Zeug
Andre Zeug on 11 Nov 2015
Edited: Andre Zeug on 11 Nov 2015
Thanks to the community. Summarizing, the problem of wrong indexing in PARFOR loops appears in R2014a (and older versions?) but is not an issue in R2015a and later.
Here a simple workaround for Matlab users with releases before introduction of HG2:
d = ones(10,10,10,10);
% slice variable
dSub = d(:,:,2,:);
% if memory is an issue
d(:,:,2,:) = [];
parfor n=1:10;
% do manipulation to subset only
dSub(:,:,:,n) = 2*dSub(:,:,:,n);
end
% if memory is an issue
d(:,:,[1 3:10],:) = d;
% include the manipulated part
d(:,:,2,:) = dSub;
clear dSub

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