SparseMatrix Interface

[quinnj]

Add rowvals(A) and nzrange(A, col) to make the sparse matrix interface a little friendlier.

cc @ViralBShah, @tanmaykm

[mlubin]

This description would be pretty confusing to someone not familiar with CSC format. Maybe a bit more context?

[tkelman]

No reason not to have convenience functions like these. What I'd really like to see is dense and sparse matrices supporting similar "iterate by columns through nonzero elements" (or by rows for transpose types?) API's, so various operators and reductions can extend efficiently to sparse while reusing generic implementations.

[ViralBShah]

The iterator would be nice but getting the row index and passing it was slow when I first tried this, and I don't think it would have changed.

[johnmyleswhite]

I was thinking about this recently and though that adding an enumerate method to both dense and sparse arrays would be great.

[ViralBShah]

How about calling it rowval so that it matches the internal structure, and also having nzval(A)?

[quinnj]

Updated with suggestions.

[mlubin]

Given the history of heated debates on naming (#6769), what's the motivation for renaming nonzeros to nzval? Seems like unnecessary churn, and the whole point of these accessors is that they don't need to match the names of the internal fields.

[ViralBShah]

Oops, I forgot about nonzeros. I was just mechanically thinking about accessors for the various SparseMatrixCSC fields. We don't need nzval then. Sorry for the noise.

[quinnj]

Ok, reverted the nzval(A) chnage and kept the documentation updates as suggested by @mlubin

[mlubin]

thanks, LGTM