Parfor Indexing -- Basic question

Question

Matlab2010 2013-10-25

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回答： Matt J 2013-10-25

How do you index into a variable if you dont know the size of the loop?

In the first example below parfor works fine.

 A = 1:10;
  parfor i = 1:length(A)
      tmp = rand(A(i));
     B(i) = tmp(1);
  end

In this example, parfor doesnt work."cannot be run due to the way variable B is used". Such a situation might occur if the case was more complex where B was only assigned an output under certain conditions (IE you dont know the final size of B at the start).

A = 1:10;
B = [];
parfor i = 1:length(A)
    tmp = rand(A(i));
   B(end+1) = tmp(1);
end

also doesnt work:

    A = 1:10;
    B = [];
    cnt = 1;
    parfor i = 1:length(A)
        tmp = rand(A(i));
       B(cnt) = tmp(1);
       cnt = cnt + 1;
     end

Hence my question is, how do you index into B in such a case?

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Answer 1

Edric Ellis 2013-10-25

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Here are two ways you could address this. Firstly, using concatenation:

B1 = [];
parfor idx = 1:1000
    x = rand();
    if x > 0.5
        B1 = [B1, x];
    end
end

Or, build B with invalid values and strip them later

B2 = NaN(1, 1000);
parfor idx = 1:1000
    x = rand();
    if x > 0.5
        B2(idx) = x;
    end
end
B2 = B2(~isnan(B2));

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Matt J 2013-10-25

编辑：Matt J 2013-10-25

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Similarly, you could use a cell array and then post-concatenate

   parfor i = 1:1000
      x = rand();
      if x > 0.5
          B{i} = x;
      end
   end
    B=cell2mat(B),

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Answer 2

Matlab2010 2013-10-25

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ok. perhaps the original example wasnt the best!

In my specific case B is a struct. with a very large number of fields. Hence generating each one is time consuming and thus why I want to use parfor.

How to use parfor when the output is a struct and you dont know the final number. Assignment is to a cell eg B{cnt} = struct()

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Matt J 2013-10-25

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Yes, you could do that,

   parfor i = 1:1000
      x = rand();
      if x > 0.5
          B{i}.field1=...;
          B{i}.field2=...;
      end
   end
   B=[B{:}];

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Answer 3

Matt J 2013-10-25

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Or, perhaps you meant something like this

   fields = {'f1','f2','f3','f4'};
   N=length(fields);
   vals=cell(1,N);
   parfor i = 1:N
          switch fields{i}
              case 'f1'   
              vals{i}=1;
              otherwise
                 vals{i}=0;
          end
   end
        args=[fields;vals];
        B=struct(args{:})