📂Julia

Application for sortperm in julia

Code

sortperm returns an array of indices that would sort the given array¹. It might sound complicated when you hear it, but looking at an example makes it immediately clear.

julia> foo = ['다', '나', '라', '가']
4-element Vector{Char}:
 '다': Unicode U+B2E4 (category Lo: Letter, other)
 '나': Unicode U+B098 (category Lo: Letter, other)
 '라': Unicode U+B77C (category Lo: Letter, other)
 '가': Unicode U+AC00 (category Lo: Letter, other)

julia> sortperm(foo)
4-element Vector{Int64}:
 4
 2
 1
 3

julia> foo[sortperm(foo)]
4-element Vector{Char}:
 '가': Unicode U+AC00 (category Lo: Letter, other)
 '나': Unicode U+B098 (category Lo: Letter, other)
 '다': Unicode U+B2E4 (category Lo: Letter, other)
 '라': Unicode U+B77C (category Lo: Letter, other)

Tricks

While not a built-in function, here’s a common trick using sortperm.

ranking

ranking(x) = sortperm(sortperm(x, rev = true))

This returns the ranking of an array. There is a ranking function in the statistical package StatBase.jl as well². However, since it can be easily implemented with sortperm, this method is often used when only ranking is needed without loading the package.

julia> a = randn(5)
5-element Vector{Float64}:
  0.7795283609374186
  0.6230493124556412
  1.7323736283590032
 -2.0790567409341945
 -0.05984808360056012

julia> ranking(a)
5-element Vector{Int64}:
 2
 3
 1
 5
 4

argnth

argnth(n, A) = sortperm(A, rev = true)[n]

This returns the index of the n-th largest element. It’s tricky to implement without sortperm.

julia> b = randn(5)
5-element Vector{Float64}:
 -0.45523175238444824
  0.2671034422090365
 -1.0703159524443844
 -0.4218688982647255
 -0.27806227968448743

julia> argnth(3, b)
4

Environment

• OS: Windows
• julia: v1.10.0