MATLAB: Array indexing in parfor loops, why does this simple code not work

Question

Hi everybody,

I have recently been looking into parfor loops, mostly out of curiosity.

I have lots of loops which follow this syntax:

A = {};
B = [1 3 6];
parfor i = 1:3
  A{B(i)} = i;
end
A

I don't understand why this code produces an error, I know that the order in which the calculations may be carried out are random but both A and B are defined outside the loop, the index into both is given by the loop number 'i' and the results can be entered into A randomly as it's an array. So why does this code fail?

Any help would be greatly appreciated,

Rod.

EDIT

My problem is basically that I have input matrices or vectors and for loops which I want to swap to parfor, but often I only want to access certain parts of each input matrix or vector. parfor won't accept a non continuous index like parfor i = [1 3 6], but you also can't use [1 3 6] as an index inside the loop because values might be duplicates.

The solution I've found is the inclusion of an if statement:

A = {};
parfor i = 1:6
  if i == 1||i == 3||i == 6
    A{i} = i;
  end
end
A

Serious coders here will probably laugh at me, but it does what I want.

Answer 1

You're getting the cell index for A out of vector B, however B could be defined with multiple elements having the same value (e.g. B=[1 1 6];), if this would occur multiple parallel instances would be attempting to access the same cell of A at the same time