Parfor Indexing -- Basic question

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Matlab2010
Matlab2010 on 25 Oct 2013
Answered: Matt J on 25 Oct 2013
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?

Answers (3)

Edric Ellis
Edric Ellis on 25 Oct 2013
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));
  1 Comment
Matt J
Matt J on 25 Oct 2013
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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Matlab2010
Matlab2010 on 25 Oct 2013
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()
  1 Comment
Matt J
Matt J on 25 Oct 2013
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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Matt J
Matt J on 25 Oct 2013
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{:})

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