Parfor Loops

Question

Brian 2012-6-6

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/40478-parfor-loops

I have the parallel computing toolbox but I have trouble writing loops in a manner that the toolbox likes. Can someone make a suggestion as to how I can change this code so that I can run parfor on my for loops? When I try to run the loop replacing for with parfor I get the message that I can't do so because of how d is used.

Thanks a lot, Brian

d={};
  for r = 1:length(c);
      for i = 3:100
         d((r-1)*97 + i-2,1) = c(r,1);
         d((r-1)*97 + i-2,2) = c(r,2);
         d((r-1)*97 + i-2,3) = c(r,i);
         d((r-1)*97 + i-2,4) = cellstr(['F' num2str(i)-2]);
      end
      disp(r);
  end

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Answer 1

Walter Roberson 2012-6-6

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https://ww2.mathworks.cn/matlabcentral/answers/40478-parfor-loops#answer_50146

When r = 1 and i = 100, then (r-1)*97 + i-2 is 0*97 + 100 - 2 which is 98

When r = 2 and i = 3, then (r-1)*97 + i-2 is 1*97 + 3 - 2 which is 98

Your loops are overwriting locations on different iterations and that is not vectorizable.

length(3:100) is 98, not 97.

I suggest you consider pre-allocating your "d" array as cell(length(c), 98, 4), and index it at (i,r,*) and then at the end of the loop reshape it to ([], 4)

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Brian 2012-6-11

Thanks a lot Walter.

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Answer 2

Brandon Armstrong 2012-6-6

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/40478-parfor-loops#answer_50140

Nested parfor loops I believe are not allowed, but there is away around the indexing problem but putting the inner loop inside a helper function so then your code would look something like

parfor r = 1:length(c);

d = helper_function(r)

end

and helper_function the does the inner loop.