Return index vectors from unique

[ViralBShah]

In Matlab, unique can return optional index vectors, such that one can construct the unique version from the input and vice versa. This is quite handy, and would be nice to have in julia.

http://www.mathworks.com/help/matlab/ref/unique.html

We would probably need the function to be called unique_ind or something, until we have keyword args.

[johnmyleswhite]

This is closely linked to the sortperm and order discussion we had some time back. I've since come to wonder if maybe we should call these sorts of things indunique and indsort to be consistent with indmax and indmin. Kind of ugly names, but it has the benefit of being very consistent.

[ViralBShah]

I prefer the consistency but I would prefer ind to be a suffix making the functions discoverable through auto completion when you press tab.

[StefanKarpinski]

A more composable solution would be better here. Consider the way that sortperm is implemented in terms of applying sort! to an index vector and using the Perm ordering type. The same approach could be used to generically allow using the exact same code for all of these functions and getting back indices instead of the sorted data.

[ViralBShah]

+1 to following a similar approach here as well.

[ViralBShah]

What we need is uniqueperm and then implement unique using that.

[tkelman]

Would uniqueperm return the second output from the Matlab version, or the third output? Both are quite useful in different applications.

Side note, one of the most aggravating things Mathworks ever did IMO was changing the default result ordering of the index outputs from unique from "last match" to "first match." That broke so much of my code...

[StefanKarpinski]

Unique perm wouldn't actually return a permutation, would it?

[tkelman]

ah yeah it's not really the same thing, you get a many-to-few index map and/or a few-to-many index map

[StefanKarpinski]

Let's not call it "perm" then.

[ViralBShah]

Yes, it is not strictly a permutation. Perhaps a selection or something.

[johnmyleswhite]

Hi @drgar, I'm really glad you're excited about Julia. Unfortunately, posting GPL code in a public forum puts us in legal danger, so I deleted all of the code you posted without reading it. It's a frustrating limitation, but you cannot safely post code to GitHub that you don't own the copyrights to.

[johnmyleswhite]

Don't worry about it. I caught it fast enough that it's not an issue. I just wanted to make sure you understood why I edited your comment.