[WIP] constructors for Array from zeros/ones

[Sacha0]

This pull request provides constructors for Array from zeros/ones, for example Array(zeros, dims) and Vector{T}(ones, n). In other words, this pull request explores #11557 (comment). Best!

[martinholters]

Wasn't the idea to have Array(possibly_shapeless_iterator, dims) constructors and make zeros and ones iterable? Of course, this PR in a way special-cases that and might still be a reasonable optimization even if go with that general approach.

[Sacha0]

Yes, that's roughly the idea :). Establishing a beachhead to explore this direction for ones/zeros was this pull request's aim. Having since explored other approaches locally, I am not certain this approach is best. More to come after some other work. Best!

[StefanKarpinski]

I think we might want to generalize "possibly shapeless iterator" to "thing that provides a mapping from indices to values", which includes iterators as a special case where the indices are implied by counting as iteration happens. In this case the I object provides a natural mapping from pairs of indices to values, yielding ones on the diagonals and zeros off of it.

[Sacha0]

I think we might want to generalize "possibly shapeless iterator" to "thing that provides a mapping from indices to values", which includes iterators as a special case where the indices are implied by counting as iteration happens. In this case the I object provides a natural mapping from pairs of indices to values, yielding ones on the diagonals and zeros off of it.

Through working with eye/ones/zeros/junk over the last week, my thoughts have evolved in that direction as well :). Responding more completely in #11557. Best!

[JeffBezanson]

We have to be careful with that. For example, if an index-less iterator is taken too seriously to have indices 1:n, it would not initialize an OffsetArray with indices -n:-1.

[Sacha0]

We have to be careful with that. For example, if an index-less iterator is taken too seriously to have indices 1:n, it would not initialize an OffsetArray with indices -n:-1.

Ref. the last paragraph in #11557 (comment) :).

[Sacha0]

Closing for the reasons given in #24595. Thanks all!