MATLAB: Parfor variable cannot be classified

Parallel Computing Toolboxparfor

I'm completely stumped by my inability to use parfor for a simple problem. I've boiled it down to a ridiculous case. Can someone please explain?
function b=wtf(a)
sz = size(a);
b=a;
parfor x=1:sz(1)
    for y=1:sz(2)
        b(x,y) = 5;
    end
end
Calling this results in an error:
>> wtf(zeros(3))
Error: File: wtf.m Line: 8 Column: 9
The variable b in a parfor cannot be classified.
See Parallel for Loops in MATLAB, "Overview".
Is it actually not possible to use parfor when the size of the input is not absolutely fixed ahead of time?

Best Answer

You've tripped over one of the limitations of for loops nested within parfor. This is described in the doc. The requirement is:
You must define the range of a for-loop nested in a parfor-loop by constant numbers or broadcast variables.
Unfortunately, even indexing a broadcast variable for the range of the for loop is not permitted, so what you need to do is pull out the elements of the size vector outside the loop like this:
function b = fixed(a)
[m,n] = size(a);
b=a;
parfor x=1:m
    for y=1:n
        b(x,y) = 5;
    end
end
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