How to use parfor in nested for loop?

Question

Suraj Srivastava 2015-5-29

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/220928-how-to-use-parfor-in-nested-for-loop

编辑： Greig 2015-5-30

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

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Adam 2015-5-29

You have to give more information than that, preferably including a relevant code snippet or create some test code that contains only those elements relevant to replicating the problem.

Suraj Srivastava 2015-5-29

编辑：Walter Roberson 2015-5-29

在 MATLAB Online 中打开

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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Answer 1

Greig 2015-5-29

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https://ww2.mathworks.cn/matlabcentral/answers/220928-how-to-use-parfor-in-nested-for-loop#answer_180946

编辑：Greig 2015-5-30

在 MATLAB Online 中打开

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