Question about parfor nested loop
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Hi,
I have a question about the nested parfor loop. Please see the following codes:
clc
clear
parfor i=1:3
for j=1:3
if j<2
B(i,j)=j;
end
end
end
The result of this code should be B=[1;1;1], but the Matlab gives B=[1,1,1;1,1,1;1,1,1]. I have been confused by this problem for a long time and thank you in advance if you have some time to look into the problem.
Haonan
7 个评论
Walter Roberson
2021-1-1
When I test, I get [1;1;1] as you expected.
haonan He
2021-1-1
Thanks for your answer. I have tried many times and via different versions of Matlab, including 2016b and 2019a, but the results are the same [1,1,1;1,1,1;1,1,1], which is not expected. I think it is the issue with parallel programming. Please see the following picture for the Matlab result. Thanks very much.
clc
clear
parfor i=1:3
for j=1:3
if j<2
B(i,j)=j;
end
end
end
B
Can you also check this?
B = (0);
parfor ii=1:3
for jj=1:3
if jj<2
B(ii,jj)=jj;
end
end
end
disp(size(B))
haonan He
2021-1-2
haonan He
2021-1-2
采纳的回答
So maybe it is the bug of the old versions.
Yes, it appears to be version-related. I get the same thing in R2018a. Regardless, it is not good practice to use parfor to loop over sliced variables whose slices, in this case the B(i,:), are not allocated in advance. This cleaner version, for example, does not have the issue:
clc
clear
B=nan(3); %pre-allocate
parfor i=1:3
for j=1:3
if j<2
B(i,j)=j;
end
end
end
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