for loops nested inside parfor loops.
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Alexantrou Serb
on 16 Apr 2015
Edited: BHUSHAN MUTHIYAN
on 5 Sep 2017
Hallo everyone, I have a bit of a problem with some parfor coding. The concept is this:
parfor j = 1:1:10;
for i = 1:1:10;
Q(i) = i^2;
end
R = Q(j);
end
Of course this code is non-sensicle, but it illustrates the problem. I cannot create an array of values inside a parfor loop that I can then use throughout the parfor loop. In a practical application the expression for Q(i) will depend on j, so taking the sub-loop outside the parfor in not an option. Therefore, for now I wish to know why such a simple example can't be parallelised. As far as the algorithm is concerned it has to individually loop through i and then compute R in parallel with no obvious interdependency between the loops. Many thanks.
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Accepted Answer
Titus Edelhofer
on 16 Apr 2015
Hi,
if you add the following line before the inner loop, it should work:
Q = zeros(10, 1);
Titus
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More Answers (1)
Alexantrou Serb
on 16 Apr 2015
2 Comments
Edric Ellis
on 16 Apr 2015
No, it's because parfor cannot immediately deduce that you're not re-using values from Q in subsequent iterations of the loop. By assigning to the whole of Q, then it can correctly deduce that Q is a temporary variable. More details about parfor variable classification here.
BHUSHAN MUTHIYAN
on 5 Sep 2017
Edited: BHUSHAN MUTHIYAN
on 5 Sep 2017
Hello Edric,
If I have to change the global variable which is declared outside the parfor loop, I am getting an error. Any reasoning for this ?
My code looks something like this :
out = zeros(dimfox, dimfoy, fis(4), ins(4), 'like', sumModel);
H = vision.Convolver('OutputSize', 'Valid', 'CustomProductDataType',numerictype([],32,frac), 'CustomOutputDataType', numerictype([],32,frac));
parfor im = 1:ins(4)
carve = zeros(dimfox, dimfoy, 'like', sumModel);
for f = 1:fis(4)
for ch = 1:fis(3)
carve(:) = carve + step(H,input(:,:,ch,im),filter(:,:,ch,f));
end
carve(:) = carve + bias(1, f);
out(:,:,f,im) = carve;
end
end
I am getting error as :
The variable out in a parfor cannot be classified.
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