Appending elements to a list in parallel table

Question

Is there any way to store values in a list that is being computed in parallel? For example, if ParallelTable is computing some {x,y} values, how would I store those computed values to a list? The order does not matter. I can always use Sort later.

For example, the following calculation can be done using Table:

list = {};
Table[Block[{x = x2, y},
   y = 2 x^2;
   AppendTo[list, {x, y}];
   ];,
 {x2, 1, 2}]

This simply returns {{1,2},{2,8}}. But if I want to do this computation in parallel the calculation can't be stored in a list. This is obviously a simplified version of my algorithm which is very lengthy and I would like it to store in parallel.

Take same exmaple:

list = {};
ParallelTable[Block[{x = x2, y},
   y = 2 x^2;
   AppendTo[list, {x, y}];
   ];,
 {x2, 1, 2}]

This returns an empty list. It makes sense that appending to a list is not a operation that can be done it parallel since the kernal needs to keep track of the order in the list, therefore multiple kernals can't manipulate it simultaneously. But I feel like there is some way around that by avoiding the ordered property of the list.

Answer 1

If you want to use AppendTo, you could just make list a shared variable and change your ParallelTable to a ParallelDo:

list = {};
SetSharedVariable[list];
ParallelDo[Block[{x = x2, y}, y = 2 x^2;
   AppendTo[list, {x, y}];];, {x2, 1, 4}]