Wrong indexing in parfor loop

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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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Adam Barber 2015-11-10

I was able to reproduce this on R2014a, but not R2015b on my system.

Andre Zeug 2015-11-10

编辑：Andre Zeug 2015-11-10

I could now reproduce this unexpected result also on R2013b.

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Answer 1

Andre Zeug 2015-11-11

编辑：Andre Zeug 2015-11-11

在 MATLAB Online 中打开

0 个投票

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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Wrong indexing in parfor loop

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