How to share numbered variables between kernels

Question

Supose I have two variables v[1] and v[2]. I used:

SetSharedVariable[{v[1], v[2]}]

Then, I used:

$SharedVariables
(*{}*)

It seems that in this way I can't share these two variables among the kernels. However, when I use:

SetSharedVariable[{a, b}]

and then:

$SharedVariables
{Hold[a], Hold[b]}

it's possible to share a and b.

Why can't I share v[1] and v[2] but it's possible to do it for a and b?

Answer 1

I believe that v[1] is a DownValue of a function v, not a variable. Thus, it should be shared among Kernels by means of SetSharedFunction

SetSharedFunction[v]

See SetSharedFunction and DownValues for examples of usage.

Answer 2

The reason this doesn't work is that you have defined v as a function, not a variable. If you want v to be a variable, then to access its elements one uses v[[1]] and v[[2]].

With your current definition, you can still share the values, but you need to use SetSharedFunction on v.

SetSharedFunction[v]
v[1] = 1;
v[2] = 2;

Parallel evaluations will now work with the function v. For example:

In[6]:= ParallelEvaluate[v[$KernelID]]
Out[6]= {1, 2}