Parallel Pool; variable does not exist after running

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Parsa Ghadermazi
Parsa Ghadermazi 2020-8-12
评论： Parsa Ghadermazi 2020-8-14
Hello,
When I run the following simple script there is no problem and I'll get i and e at the end of the loop
for i=1:30
e=i/2
end
However, when I want to use parallel pool for the same loop, but with parfor:
parfor i=1:30
e=i/2
end
I don't get any thing when running the loop finishes. Am I missing something here?

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回答（1 个）

Raymond Norris
Raymond Norris 2020-8-13
Hi Parsa,
A for loop runs in a serial fashion, one iteration after the other, with the same reproducable results each time (more or less). In your example, we know e will always equal 15. A parfor loop runs its iterations in any order. Therefore, the last value of i could be 14 (and not 30) and therefore, e would equal 7 (and not 15). For this reason, after a parfor, i and e are not know (more about this in a second).
MATLAB must clasify each of the variables within the body of a parfor. You can read more about classification here
https://www.mathworks.com/help/coder/ug/classification-of-variables-in-parfor-loops.html
Based on this classification, we deem i to be a loop variable and e to be a temporary variables, nether of which persist after the completion of a parfor. Sliced output and reduction variables can be assigned and persist after a parfor.
Raymond
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Raymond Norris
Raymond Norris 2020-8-13
I make brief mention at the end about sliced output and reduction variables. In the OP's example, e is neither of them, though I suspect it's a toy example and not the actual code.

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