Using parfor with a 3d array

I am trying to update a 3d array within a parfor loop. The actual code is too messy to reproduce here but the snippet of code below conveys what I am trying to do (but unable to).

a = zeros(5,5,10):
parfor t = 1:ntimes;
  temp = randn(5,5); % In my actual code, this is some meaningful quantity that depends on the loop variable 't'
  for k = 1:10;
     a(:,:,k) = a(:,:,k) + randn(5,5); 
  end
end

I get an error saying matlab is unable to classiy the variable 'a'.

Is there a way I can fix this?

Thanks!

Answer 1

[Matt J]

A variable in a parfor loop must belong to one of 5 categories. I cannot tell from your simplified example which category a is supposed to belong to (and neither can parfor), but the table here should help you decide:

https://www.mathworks.com/help/coder/ug/classification-of-variables-in-parfor-loops.html

[oceanmod]

Thanks for the reply. I did look at that link but it was not clear why 'a' in my example is not classified.

[oceanmod]

I was able to get around this error as follows:

%<same code as before>  
for k = 1:10;
     temp(:,:,k) = randn(5,5); 
end
a = a + temp;

[Matt J]

Thanks for the reply. I did look at that link but it was not clear why 'a' in my example is not classified.

If not, then which class do you think it should belong to?

[oceanmod]

I would have thought it should be a reduction variable as it is accumulating values irrespective of the parfor loop order.

I also found this statement in the matlab docs:

"MATLAB Coder does not allow reduction variables to be read anywhere in the parfor-loop except in reduction statements"

I wonder if the above also means a reduction variable cannot appear inside a nested for loop (itself inside a parfor loop).

[Matt J]

In Basic Rules for Reduction Variables,

You cannot index or subscript a reduction variable.

[oceanmod]

Ah, got it. Thanks again!