how do I slice a variable? parfor loop

Hello guys,

Matlab is showing me an error that involves a parfor loop. Below you can see the structure of the code and the error. It is the first time I'm using the "parfor" command.

Any ideas on how to solve this problem? Thanks

parfor counter3=1:n3

    
    
    for counter2=1:n2

        for counter1=1:n1
            for i=1:N
                for k=1:m-1
    
                quantity(k+1,1)=something;
               
                end
            Quantity_av(i,1)=sum(quantity)/m;
            end
        QT=horzcat(QT,Quantity_av);
        end
    end
end

Answer 1

[Edric Ellis]

The problem here is that the parfor machinery cannot tell that you are completely overwriting quantity and Quantity_av in your inner loops. (In fact, you might not be, it's impossible to tell from the code you posted). This leads the parfor machinery to conclude that those variables are being used in such a way as to cause a dependence between the order of iterations in the loop. (If you don't full clear out e.g. quantity, then on the second iteration of the parfor loop, the values computed in the first iteration are still present, and so might influence the result of the second iteration - that's not allowed).

Presuming you did intend to completely overwrite quantity and Quantity_av, then the fix is simple: initialise those variables like so:

parfor counter3=1:n3
    for counter2=1:n2
        for counter1=1:n1
            Quantity_av = zeros(n1, 1);
            for i=1:N
                quantity = zeros(m, 1);
                for k=1:m-1
                    quantity(k+1,1)=something;
                end
                Quantity_av(i,1)=sum(quantity)/m;
            end
            QT=horzcat(QT,Quantity_av);
        end
    end
end

This initialisation causes the parfor machinery to conclude that quantity and Quantity_av are temporary variables, and thus the loop can proceed.

[Juan Nunez]

Hello Edric,

Thank you for your answer. The problem is I need to keep all Quantity_av values. Is there a way to do this?

[Edric Ellis]

The variable QT will store all the concatenated values of Quantity_av - isn't that what you want?

[Stephen23]

Juan Nunez's "Answer" moved here:

Apparently, QT is only storing the last value. The rest of the stored values is "0"

[Edric Ellis]

I don't see that. I slightly modified the example to make it executable, and QT accumulates the result:

n1 = 3;
n2 = 4;
n3 = 5;
N  = 6;
m  = 7;
QT = [];
parfor counter3=1:n3
    for counter2=1:n2
        for counter1=1:n1
            Quantity_av = zeros(n1, 1);
            for i=1:N
                quantity = zeros(m, 1);
                for k=1:m-1
                    quantity(k+1,1)=rand();
                end
                Quantity_av(i,1)=sum(quantity)/m;
            end
            QT=horzcat(QT,Quantity_av);
        end
    end
end

[madhan ravi]

Juan Nunez's answer moved here for consistency:

Thanks Edric. Your last code was very helpful. I solved the issue.

[madhan ravi]

@Juan Nunez stop adding answers to make a comment , instead make a comment to the answer which you are responding to.

[Juan Nunez]

I'm sorry Madhan, my mistake.