Get the permutation vector from sortrows and sortcols

[ViralBShah]

Getting the permutation vector from sortrows and sortcols is often required, but we don't have a way to do it. For now, one can concatenate [1:n] to the end of the array and calling sortrows, which will become the permutation vector when sortrows completes.

Discussion here:
https://groups.google.com/forum/?hl=en#!topic/julia-users/zgWwkgcHpFY

cc: @kmsquire

[StefanKarpinski]

The code that implements sortrows and sortcols is pretty simple:

function sortrows(A::AbstractMatrix; kws...)
    c = 1:size(A,2)
    rows = [ sub(A,i,c) for i=1:size(A,1) ]
    p = sortperm(rows; kws..., order=Lexicographic)
    A[p,:]
end

function sortcols(A::AbstractMatrix; kws...)
    r = 1:size(A,1)
    cols = [ sub(A,r,i) for i=1:size(A,2) ]
    p = sortperm(cols; kws..., order=Lexicographic)
    A[:,p]
end

As you can see, the permutation is right there. The main issue is just the API for getting at it. In some sense, since permuting the matrix is easy, we should maybe just be returning the permutation instead of doing that for the user. That's a major API change though and would definitely break code.

[ViralBShah]

Yes, the api question was really what was on my mind too.

One possibility would be to make this capability part of sortperm.

[simonster]

The API question seems related to #1845

[tkelman]

Did not make it into 0.4, removing from milestone.

[LilithHafner]

Now that eachslice is infers well, sortperm(eachslice(x; dims)) solves this issue.