Selectperm bug or wrong documentation?

Wikunia · August 3, 2018, 9:01am

I think the behavior of selectperm does not fully match the documentation:
Link to documentation:

JuliaLang/julia/blob/release-0.6/base/docs/helpdb/Base.jl#L1231-L1243


      
          """
              selectperm(v, k, [alg=<algorithm>,] [by=<transform>,] [lt=<comparison>,] [rev=false])
          
          Return a partial permutation of the vector `v`, according to the order specified by
          `by`, `lt` and `rev`, so that `v[output]` returns the first `k` (or range of adjacent values
          if `k` is a range) values of a fully sorted version of `v`. If `k` is a single index
          (Integer), an array of the first `k` indices is returned; if `k` is a range, an array of
          those indices is returned. Note that the handling of integer values for `k` is different
          from [`select`](@ref) in that it returns a vector of `k` elements instead of just the `k` th
          element. Also note that this is equivalent to, but more efficient than, calling
          `sortperm(...)[k]`.
          """
          selectperm

If I have the array a = [4,3,2,1] and use selectperm(a,2) I can read this part:

If k is a single index (Integer), an array of the first k indices is returned;

then the expected behavior is: to get [4,3] as a[[4,3]] is [1,2] but regarding this:

Also note that this is equivalent to, but more efficient than, calling
sortperm(...)[k].

I would suspect to get only a single index (3) which is what selectperm actually does.

Using it without documentation I assumed to get [4,3] but I’m also happy to use selectperm(a,1:2). Anyway the documentation seems to be wrong.

StefanKarpinski · August 3, 2018, 12:46pm

Could you file a GitHub issue about this so it doesn’t get lost?

Wikunia · August 3, 2018, 12:57pm

Sure I would also do a PR if the behavior is as expected and only the docstring should be changed.

Issue:
https://github.com/JuliaLang/julia/issues/28426