parfor variable cannot be classified

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as far as i can tell, the following code should be parallelizable, however I get a 'cannot be classified' error no matter how I twist it. This is paraphrased code . It is 'theoretically parallelizable' i blv., as the result is independent of execution order.
a{1}=''
a{2}='hi'
a{3}=''
a{4}='bye'
parfor k = 1:10
j=mod(k,5)
b=a{j}
if isempty(b)
a{j} = j+3
end
end
along these lines, what can this possibly mean:
"Notably, the assignments to the variables i, t, and u do not affect variables with the same name in the context of the parfor statement. "
a more concise statement of the problem is that the following, which is in principle parallelizable for arbitrary 'index' functions f,g (ie positive integer functions in range of the array in question), cannot in fact be compiled due to matlab compiler limitations.
parfor k=1:10
index=f(k)
array(index)=g(index)
end
however as walter roberson points out below this limitation can be somewhat avoided in this case by the following:
parfor k=1:10
index=f(k)
temparray(k)=g(index)
indices(k)=index
end
for k=1:10
array(indices(k)) = temparray(k)
end
  2 Comments
Jeremy Rutman
Jeremy Rutman on 17 Nov 2015
yes, i took out a bunch of extranneous code in an attempt to make the question as clear as poss. and took out a '+1'

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Accepted Answer

Walter Roberson
Walter Roberson on 12 Nov 2015
When k = 1 or k = 6 both times j = 1 so you have multiple iterations that would be accessing the same elements of a{}. That is not permitted
The indexing of the loop variable needs to be in-line in the access, not calculated in a previous statement and then used, so that parfor can verify that there is no possibility of overlap. mod() is not one of the permitted operations in the indexing for parfor.
  6 Comments
Jeremy Rutman
Jeremy Rutman on 17 Nov 2015
ok if the function g changes with time then this would be trouble. In the meantime I adopted your solution and have run into another issue , maybe you can take a look

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