Why am I receiving "Futures in state 'unavailable'" error when using "fetchNext" in MATLAB R2022a?

I am using a for loop to generate a list of Futures from "parfeval" as in this example:

for n = 1:10
  if (mod(n,2) == 0)
     parallelOut(n) = parfeval(@() 1, 1);
  else
     continue;
  end
end

When I try to use "fetchNext" on the "parallelOut" array it generates this error:

Error using parallel.FevalFuture/fetchNext:
fetchNext cannot operate on Futures in state 'unavailable'.

How can I avoid this error?

Answer 1

[MathWorks Support Team]

The 'unavailable' state occurs when a Future array is pre-allocated but a Future is not defined. Most operations will fail on Futures with the 'unavailable' state.

In the above example every odd element will be 'unavailable'.

To avoid 'unavailable' state Futures, only add defined Future elements to the array and do not skip defining entries. Alternatively, after creating the array, remove all Futures with the 'unavailable' state from the array before calling "fetchNext".