The variable x in a parfor cannot be classified

When I run this code, I get an error "The variable x in a parfor cannot be classified" that points to x(j,:) in the line sum = sum + A(i,j)*x(j,:);. Can you please suggest what should I do?

parfor i = 2:n
     while (j < i) 
         if  (A(i,j) ~= 0)
                sum = sum + A(i,j)*x(j,:);     
         end
           j = j+1;
     end
end

[KSSV]

YOu need not to use a parfor for this. YOu can achieve it stright away by vectorization which would be very fast.

Answer 1

[Edric Ellis]

Hm, I didn't see that precise error. I modified your example just a little so that it was actually executable, like so:

n = 4;
A = rand(n);
x = rand(n);
sum = 0; % note 1
parfor i = 2:n
    j = 1; % note 2
    while (j < i)
        if  (A(i,j) ~= 0)
            sum = sum + A(i,j)*x(j,:);
        end
        j = j+1;
    end
end

A couple of changes:

1. I set an initial value for sum
2. You must reset j each time around the parfor loop so that the parfor machinery can tell that you intend j to be a temporary variable (and that you aren't doing anything order-dependent).

[Priyabrata Senapati]

Hi, I implemented the changes that you mentioned and I get the same error. The error says that the way I have implemeted x in the sum variable is wrong for parfor. Can you please suggest?