MATLAB: Question about parfor nested loop

Question

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

Answer 1

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