How to use a matrix in parfor?

Hello,

I have troubles with using parfor. I have program similar to this (heat transfer):

    A=rand(20,20,2);
    d=rand(20);
    parfor i=2:19   
         for j=2:19
            A(i,j,2)=d(i)*A(i+1,j,1)+d(i+1)*A(i-1,j,1)+d(i-1)*A(i,j+1,1)+A(i,j-1,1);       
         end
    end

There is always error like: The PARFOR loop cannot run due to the way variable 'A' is used .

tank you very much Lukas

[Juraj Hliník]

From Troubleshoot Variables in parfor-Loops Documentation

If you run into variable classification problems, consider these approaches before you resort to the more difficult method of converting the body of a parfor-loop into a function.

If you use a nested for-loop to index into a sliced array, you cannot use that array elsewhere in the parfor-loop.

The code on the top does not work because A is sliced and indexed inside the nested for-loop.

The code on the bottom works because v is assigned to A outside the nested loop. You can compute an entire row, and then perform a single assignment into the sliced output.

Invalid

 A = zeros(4, 10); 
 parfor i = 1:4
    for j = 1:10
        A(i, j) = i + j;
    end
    disp(A(i, 1))
end

Valid

 A = zeros(4, 10);
parfor i = 1:4
    v = zeros(1, 10);
    for j = 1:10
        v(j) = i + j;
    end
    disp(v(1))
    A(i, :) = v;
end

[ahmad eldeeb]

@Juraj Hliník

What if I want to calculate A_new=A_old+v?

[Matt J]

I'm skeptical that parfor is going to make things faster here. Try the following simpler test. The times shown are what I get with a 12 worker default local profile.

N=2000;

A=rand(N,N);

d=rand(N,1);

B=A(:,:,1);

C=B;

tic

for j=2:N-1

for i=2:N-1

C(i,j)=d(i)*B(i,j-1);

end

end

toc %Elapsed time is 0.014457 seconds.

tic

parfor j=2:N-1

for i=2:1999

C(i,j)=d(i)*B(i,j-1);

end

end

toc %Elapsed time is 1.889115 seconds.

Answer 1

[Namnendra]

The different iterations in a parfor loop work independently. So, if "i" is the iterating variable in a parfor loop, you can't use A[i]=A[i+1], because the order of different iterations of loop can vary. You can call a function instead to perform the necessary operation.

[Rik]

Comment posted as flag by @ahmad eldeeb:

Could you give example?