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sparsemat
Diagonal and diagonal view
.diagonal would allocate a sparse vector while .diag_view could allocate a lazy diagonal proxy, trading time for memory
This would be useful for my use-case. Any reason why this hasn't been implemented yet?
@matzhaugen I don't think we have seen enough interest to implement this yet. What would be your use case? Which APIs would you like to see on diagonals? Should we have dense and sparse diagonals?
My use case would be doing something like this
min_elem = my_sparse_matrix.diag_view().fold(1.0, |acc, elem| acc.min(*elem));
or the diag equivalent. Maybe there is a much easier way to do this that I'm not seeing. But just a sparse diagonal would be fine.
Anyway, I made a cheeky PR :p. It's more to get a feel for Rust as I'm new to the language, so feel free to let me know it I'm totally off track.
For your use case you want a diagonal iterator I guess, you'll be more efficient since you will avoid allocating. But the PR you made is already a good start!
Yes, good point, that seems like a good idea. I can change or add a PR for that, once I learn how to implement iterators in Rust :) . I'm actually not sure how a diag_view method would be implemented though since the constructor of CsVecI takes &[N]and not &[&N] for values. Not sure how to reconcile that.
But and iterator would be even better of course.
diag_view would need to return an entirely new type that would need to compute its values on demand for anything that it would be used for. Retrospectively I'm not sure it would be a good idea.
If you want to implement diag_iter in you PR, you're welcome to do so! Writing iterators is a good way to learn rust, and lifetimes in particular.
I think the signature should be something like
pub fn diag_iter(
&self,
) -> impl std::iter::DoubleEndedIterator<Item = (usize, &N)>
+ std::iter::ExactSizeIterator<Item = (usize, &N)>
+ '_
Do not hesitate to look at examples in the codebase, for instance https://github.com/vbarrielle/sprs/blob/6cc2370f685c674886f958994f1d30123c6b0c42/src/sparse/csmat.rs#L1147-L1161 is an example of an Iterator implementation. This is using the impl Trait feature of rust, which is more ergonomic than defining an iterator type by hand.
@vbarrielle Could return CsVecBase<Vec<I>, &[N]> if we allow data and indices to have separate lengths.
It could indeed, though that would not really be a view as it would be allocating. But that could definitely be useful, especially a mutable version.
Seems that approach would clash with https://docs.rs/sprs/0.9.2/sprs/struct.CsVecBase.html#method.data