Matlab Nested parfor loop , variable cannot be classified

Question

I'm trying to run the below code but get an error message:

The variable R in a parfor cannot be classified

Any way to solve it?

R=zeros(M,N,Us,Vs,'single');
parfor indM=1:M   
    for indN=1:N
        for indv=1:Vs           
           temp=squeeze(X(indM,indN,:,indv));          
           if(sum(temp(:)~=0))
             R(indM,indN,:,indv)= FractionalFFT_mid0(temp,a);    
           end
        end
    end
end

@PavelOganesyan: I don't see a duplicate to that question as each worker only accesses the (indM,:,:,:) slice. — Daniel
– Daniel, Commented Jan 22, 2015 at 13:39
@PavelOganesyan - That is not the right duplicate. The one by Daniel is the correct one, which I have selected to be the final duplicate. — rayryeng
– rayryeng, Commented Jan 22, 2015 at 15:33
@rayryeng: I don't see the other questions as duplicates. Here, we have the very simple case of not slicing along the last dimension, whereas the other dupes were about more complex issues. Consequently, I have reopened the question. — Jonas
– Jonas, Commented Jan 22, 2015 at 15:37

Jonas · Accepted Answer · 2015-01-22 15:36:09Z

1

Older versions of Matlab require that the parfor-sliced index is the last one (newer versions, e.g. 2014b no longer have the requirements).

R=zeros(N,Us,Vs,M,'single');
parfor indM=1:M   
    for indN=1:N
        for indv=1:Vs           
           temp=squeeze(X(indM,indN,:,indv));          
           if(sum(temp(:)~=0))
             R(indN,:,indv,indM)= FractionalFFT_mid0(temp,a);    
           end
        end
    end
end

%# get R back the way you wanted originally
R = permute(R,[4 1 2 3]);

answered Jan 22, 2015 at 15:36

Jonas

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3 Comments

Edric Over a year ago

Hm, I don't think there ever was such a requirement - I just tried R2007b (the first version with parfor in mostly its current form - R2007a and prior releases had a completely different parfor...), and the following: x = zeros(4); parfor (idx = 1:4), x(idx,1) = 7, end worked as I expected. There have definitely been some changes in what's supported in terms of nested FOR loops though.

Edric Over a year ago

Having said that, I don't know why re-ordering the subscripts makes that work.

Jonas Over a year ago

@Edric: The OP's example does work (with some random data) for me in R2014b, but in the past, re-ordering of index variables has helped. I do not remember where I saw it in the documentation, or whether I found it out via trial and error; parfor did use to be a bit iffy at times.

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