MATLAB: Problem with parfor classified variable

Question

I am trying to parallelize a for loop. Here is a minimal working example of what I am trying to do at the moment.

nx = 96;
ny = nx;
Tk_all = zeros(nx,ny);
Tkp_all = zeros(2*kmax+1,kmax+1,nx,ny);
dim2 = 31;
dim3 = 31;
kmax = min(dim2,dim3);
c = parcluster('local');
c.NumWorkers = 24;
parpool(c, c.NumWorkers);
for kxi=-kmax:kmax
  parfor kyi=0:kmax
     Tk_entry = kxi*kyi*ones(nx,ny); % should be something more meaningful
     Tk_all(nx/2+(kxi-1)+1,ny/2+(kyi-1)+1) = sum(reshape(Tk_entry,nx*ny,1));
     Tkp_all(kmax+1+(kxi-1)+1,1+(kyi-1)+1,:,:) = Tk_entry;
  end
  Tk_all(nx/2-(kxi-1)+1,ny/2-([0:kmax]-1)+1) = Tk_all(nx/2+(kxi-1)+1,ny/2+([0:kmax]-1)+1);
end
delete(gcp('nocreate'))

and I receive the error

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

I though I was satisfying the requirements from, for example here:

https://uk.mathworks.com/help/parallel-computing/sliced-variable.html

but clearly I am missing something. Any help?

I apologize if this question seems to be a duplicate of other existing questions, but I cannot seem to figure out my specific problem.

Thanks!

Answer 1

The variable Tk_alland Tkp_all are not meeting the requirements of a sliced variable. For a sliced variable, exactly one indexing expression must be of the form i, i+k, i-k, or k+i. The index i is the loop variable and k is a scalar integer constant or a simple (non-indexed) broadcast variable. Every other indexing expression is a positive integer constant, a simple (non-indexed) broadcast variable, a nested for-loop index variable, colon, or end. The following code will work in your case:

nx = 96;
ny = nx;
Tk_all = zeros(nx,ny);
dim2 = 31;
dim3 = 31;
kmax = min(dim2,dim3);
Tkp_all = zeros(2*kmax+1,kmax+1,nx,ny);
c = parcluster('local');
c.NumWorkers = 24;
parpool(c, c.NumWorkers);
for kxi=-kmax:kmax
    k1=kxi+nx/2;
    k2=ny/2;
    k3=kmax+1+kxi;
  parfor kyi=0:kmax
     Tk_entry = kxi*kyi*ones(nx,ny); % should be something more meaningful
     Tk_all(k1,k2+kyi) = 1;
     Tkp_all(k3,1+kyi,:,:) = Tk_entry;
  End
  Tk_all(nx/2-(kxi-1)+1,ny/2-((0:kmax)-1)+1) = Tk_all(nx/2+(kxi-1)+1,ny/2+((0:kmax)-1)+1);
end
delete(gcp('nocreate'));