Chapel: Creating a dense array out of a sparse one

[e-kayrakli]

Currently there is nothing in the sparse array interface that allows creating a dense array from a sparse one. Some method that can do that can help with linear algebra routines and some others.

[e-kayrakli]

Scipy has toarray method for something like that. I am not quite fond of the name as sparse arrays are still arrays in Chapel. I am thinking toDense but it sort of implies that it converts the existing sparse array to dense, which is not what that method is going to do. I am open to suggestions in terms of the method name here. I will look around a little bit myself, as well.

Possibly interested are @wongalvis14 and @ben-albrecht

[benharsh]

Should this method simply return the internal parentDom field? If so, this method could apply more generally to any subdomain.

[ben-albrecht]

We should also consider how this might generalize to other layouts, e.g. should we have similar methods for converting CSR <-> CSC <-> COO <-> Dense?

[cassella]

parentDom might not be dense, if it's a sparse subdomain itself..

[bradcray]

@benharsh: As I understand it, the goal is to get an array, not a domain, but otherwise I think basing the array on the parentDom has potential merit. Something like:

var DenserArr: [parentDom] eltType;
forall ind in parentDom do
  DenserArr = dsiAccess[ind];

ought to do it? As Paul points, out this doesn't guarantee a dense array, so we might also want a "denseParentDom" method that gets the first dense domain in the parent domain links and does the similar thing.

[bradcray]

(for that matter, doing this outside of the array implementation would be simple if we had a "denseParentDom()" query. The user should just be able to write:

var DenseArr: [SparseArr.denseParentDom()] SparseArr.eltType = SparseArr;

[cassella]

)

I'd tried yesterday with var DA: [DD] SA.eltType = SA;, but hit

sparse2dense.chpl:12: error: Sparse iterators can't yet be zippered with others

[bradcray]

Oh shoot, we should really fix that. You ought to be able to assign a sparse array to a dense.

I think the workaround would be:

var DA: [DD] SA.eltType = [i in DD] SA[i];

but this is going to be much more expensive than just implementing a follower iterator on sparse arrays that knows how to yield IRVs until it hits the next nonzero.

[bradcray]

I added issue #13452 to capture this longstanding desire to assign a sparse array to a dense one.

[e-kayrakli]

I am currently thinking that #13452 is a higher-priority issue. It is more generic and can potentially allow more idioms. If there is a roadblock resolving that issue then this can be an alternative stopgap measure.

Or we might consider adding a toArray one-liner only for convenience.