MATLAB: For loop in parfor loop

MATLABparfor

Dear all, I am trying to run the following script
a = linspace(0,10);
parfor ii=1:2
   for jj=1:length(a)
      x(ii,jj) = rand;
   end
end
which fails with an "Error: The variable x in a parfor cannot be classified." Is this a bug in Matlab? I would simply consider the vector "a" a broadcast variable, since "a" and "length(a)" are unmodified by the script.
How would you recommend to fix the script, taking into account that length(a) should be evaluated inside the parfor loop?
Thanks, Niels

Best Answer

I don't know the details (perhaps another answer can get technical), but parfor loops don't like assigning values to things like x(ii, jj) directly, so you have to use temporary variables that are accessed only in the parfor loop. Try something like this....
a = linspace(0,10);
x=NaN(2,length(a)); % pre-allocated for speed
parfor ii=1:2
      tmp_var=NaN(1,length(a)); % pre-allocate a temporary variable
      for jj=1:length(a)
          tmp_var(jj) = rand;
      end
      x(ii,:) = tmp_var; % assign it to x
end
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