MATLAB Answers

1

Understanding parfor restrictions with indexing

Asked by Henry Shackleton on 17 Jun 2019
Latest activity Edited by Edric Ellis
on 18 Jun 2019
I was attempting to parallelize a for loop in MATLAB with parfor, and came across a strange restriction with regards to indexing. Specifically, the fact that you seemingly cannot use more than one different index into a variable. For example, something like
parfor ii=1:10
A(ii, 1) = 1;
A(ii, 2) = 2;
end
does not work, whereas the code
parfor ii=1:10
A(ii, :) = [1,2]
end
does. Why is this the case? I can make my code accomodate this restriction, but I can't understand why it's necessary. The two different methods seem equivalent to me with respect to being able to cause unintended side effects in parfor.

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1 Answer

Answer by Edric Ellis
on 18 Jun 2019
Edited by Edric Ellis
on 18 Jun 2019
 Accepted Answer

The indexing restrictions in parfor range from those which are required to make the loop iterations provably order-independent (well... modulo non-standard implementations of subsref or subsasgn), to those which are limitations of the implementation. In this case, you have two indexing expressions either of which on its own would constitute a valid "slicing" operation, but the current implementation cannot support that.
Also note that the following is allowed:
parfor ii=1:10
for jj = 1:2
A(ii, jj) = jj;
end
end

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