How to use parfor in nested for loop?

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Suraj Srivastava
Suraj Srivastava on 29 May 2015
Edited: Greig on 30 May 2015
Hi, I have a nested for loop and I want to use parfor for the same, but the problem is that in my code the variable of first loop has not been used everywhere in the nested loop. And due to which it is showing error. Kindly help me out.
Thanks Suraj
  2 Comments
Suraj Srivastava
Suraj Srivastava on 29 May 2015
Edited: Walter Roberson on 29 May 2015
a = magic(3);
for i = 1:4
for j = 2:3
a(j,2) = 4;
end
end
....................then it works for for loop. BUt when I use it in following code:-
a = magic(3);
parfor i = 1:4
for j = 2:3
a(j,2) = 4;
end
end
then it gives error.
but unfortunately I want to use this parfor in same manner. Kindly suggest me for the same.

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Answers (1)

Greig
Greig on 29 May 2015
Edited: Greig on 30 May 2015
A couple of things to point out.
It is always best not to use i and j to index loops, these are both reserved for imaginary numbers.
The example you have given us is trivial and can be done without any loop at all - the loop over i isn't even used. For the point below, it would be really useful to know the details of what you are trying to implement. That way you will get the best possible answer.
What you will need to do is use a temporary variable in the j-loop. This will then assigned to your primary variable indexed in the i-loop. For example....
My_Output = NaN(20,10); % pre-allocate for speed
parfor ii = 1:10
Temporary_Var = NaN(20,1); % pre-allocate for speed
for jj = 1:20
Temporary_Var(jj) = some_function();
end
My_Output(:,ii) = Temporary_Var;
end

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