From 6962c914a9fae87df68ab5370e2122f143574371 Mon Sep 17 00:00:00 2001
From: "Steven G. Johnson" <stevenj@alum.mit.edu>
Date: Mon, 16 May 2022 08:06:12 -0400
Subject: [PATCH] document unique(i -> a[i], eachindex(a)) trick (#45291)

Co-authored-by: Milan Bouchet-Valat <nalimilan@club.fr>
---
 base/set.jl | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/base/set.jl b/base/set.jl
index b77d39e80ae5d..80b0138eb89f9 100644
--- a/base/set.jl
+++ b/base/set.jl
@@ -195,6 +195,28 @@ julia> unique(x -> x^2, [1, -1, 3, -3, 4])
  3
  4
 ```
+This functionality can also be used to extract the *indices* of the first
+occurrences of unique elements in an array:
+```jldoctest
+julia> a = [3.1, 4.2, 5.3, 3.1, 3.1, 3.1, 4.2, 1.7];
+
+julia> i = unique(i -> a[i], eachindex(a))
+4-element Vector{Int64}:
+ 1
+ 2
+ 3
+ 8
+
+julia> a[i]
+4-element Vector{Float64}:
+ 3.1
+ 4.2
+ 5.3
+ 1.7
+
+julia> a[i] == unique(a)
+true
+```
 """
 function unique(f, C; seen::Union{Nothing,Set}=nothing)
     out = Vector{eltype(C)}()