# How to use a matrix in parfor?

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Lukas Kozubik on 21 Mar 2018
Commented: Rik on 15 Nov 2022
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
Matt J on 7 Nov 2021
Edited: Matt J on 7 Nov 2021
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.

Namnendra on 19 Jun 2022
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 on 15 Nov 2022
Comment posted as flag by @ahmad eldeeb:
Could you give example?