"unable to classify the variable in the body of the parfor-loop"

I tried

X=ones(b*n,5)

parfor (a=1:n, 12)
        Y=X((a-1)*b+1:1:a*b,3:4);
        sorted=sort(Y(:));
        Z=sorted(end-2); 
        X((a-1)*b+1:1:a*b,5)=Z;
end

and obtained

Error: Unable to classify the variable 'bidder_samples' in the body of the parfor-loop. For more information, see Parallel for Loops in MATLAB, "Solve Variable Classification Issues in parfor-Loops".

What would be the next step?

Answer 1

[Edric Ellis]

The doc page you mention describes the valid ways of getting results out of a parfor loop. In your case, you appear to be computing distinct chunks of column 5 of X based on chunks of columns 3:4 of X. To fit in with the parfor rules though, you must re-write your code a little to make the references into X be compatible with "slicing". One way to do that would be to make X be n-by-b-by-5, and then do something like this (completely untested):

X34 = ones(n, b, 2); % corresponds to X(:,3:4)
Xout = ones(n, b); % corresponds to X(:,5)

parfor a = 1:n
    Y = X34(a,:,:);
    % compute Z
    Xout(a,:) = Z;
end

[alpedhuez]

Essentially, is it that parfor can only deal with a single row operation?

[Edric Ellis]

The restrictions of parfor sliced indexing are that you slice an array in a single dimension, and that the index in that dimension must be the loop variable (possibly with a constant offset, but that's not relevant here).

[alpedhuez]

More or less.