indexing within parfor loop

Dear All,

I am having troubles with setting up the following parfor loop.

Basically, I have two variables T and tau and I want to loop over them in parallel.

parfor i = 1:length(tau)*length(T)
        k = ceil(i/length(T)); %tau
        j = i-(k-1)*length(T); %T

          p.T = T(j);
          p.tau = tau(k);
          fun(p);   
  end

I get the error message that the problem is with the use of variable p.

Could someone help me how to fix this?

Thank you

[Guillaume]

What is the error message?

I would recommend you use numel instead of length. With numel your code will work whether tau and T are matrices or vectors. With length it will break in interesting ways if any of them are matrices.

Answer 1

[Rik]

parfor loops don't like temporary variables, because it will not be able to guarantee which assignment will be the last, so it can't tell which version of p to keep available. You can fix this by either creating a double nested loop (which you can make parfor if you like), or indexing p.

parfor i = 1:numel(tau)*numel(T)
    k = ceil(i/numel(T)); %tau
    j = i-(k-1)*numel(T); %T
    p(i).T = T(j);
    p(i).tau = tau(k);
    fun(p(i));
end

or

for i_tau = 1:numel(tau)
    for i_T=1:numel(T)
        p.T = T(i_T);
        p.tau = tau(i_tau);
        fun(p);
    end
end

Answer 2

[Edric Ellis]

The problem here is that parfor can't tell whether you're updating all fields of the variable p, and therefore it thinks there might be order-dependent stuff going on. The simplest way to fix this is to ensure you completely overwrite p on each iteration of your loop, like this:

parfor i = 1:length(tau)*length(T)
    k = ceil(i/length(T)); %tau
    j = i-(k-1)*length(T); %T
    p = struct('T', T(j), 'tau', tau(k));
    fun(p);
end