"the variable in a parfor cannot be classified." error in Matlab

Question

I am trying to implement a very simple program with parfor but I get some errors. I saw nearly all of the SO questions for a possible duplication but non of them was similar to my question situation. The error I get is :

Error: The variable log_likelihood_II_with_entropy in a parfor cannot be classified.

My code is written below:

em_iterations=10;

users=5;
log_likelihood_II_with_entropy=zeros(users,em_iterations);

parfor u = 1:1:users
      for current_iter=1:1:em_iterations
           log_likelihood_II_with_entropy(u,current_iter)=rand();               
      end
end

Answer 1

Since log_likelihood_II_with_entropy relies on both the parfor index (u) and an "inside index" (current_iter) it cannot be classified. Every parfor iteration is independent from the others and they are not executed in order (that is, u will not necessarily go from 1 to users in order 1,2,3,4,...,users).

My suggestion is to let the single parfor iteration (worker) build an entire row of log_likelihood_II_with_entropy.

parfor u=1:users
    single_row=zeros(1,em_iterations);
    for current_iter=1:1:em_iterations
        single_row(current_iter)=rand();
    end
    log_likelihood_II_with_entropy(u,:)=single_row;
end

In this manner every parfor task (the parfor body itself) will preallocate and evaluate a single row, no matter what the u value is. And then it will replace/concatenate such value in the log_likelihood_II_with_entropy matrix.