Sort Array by Order of another Array

Question

I have

a = ["B", "C", "A"]

and

b = [7, 10, 5]

How can I sort a by the order of the elements of b?

So, to explain, the order of the elements in b is the indexes of the sorted elements ([3,1,2]). I would like to use that to do this:

a[[3,1,2]]
["A", "B", "C"]

Answer 1

You are looking for sortperm:

sortperm(v; alg::Algorithm=DEFAULT_UNSTABLE, lt=isless, by=identity, rev::Bool=false, order::Ordering=Forward)

Return a permutation vector I that puts v[I] in sorted order. The order is specified using the same keywords as sort!. The permutation is guaranteed to be stable even if the sorting algorithm is unstable, meaning that indices of equal elements appear in ascending order.

Applied to your example:

julia> a = ["B", "C", "A"]
3-element Array{String,1}:
 "B"
 "C"
 "A"

julia> b = [7, 10, 5]
3-element Array{Int64,1}:
  7
 10
  5

julia> perm = sortperm(b)
3-element Array{Int64,1}:
 3
 1
 2

julia> a[perm]
3-element Array{String,1}:
 "A"
 "B"
 "C"