getting the index of a sorted array components at the primary array using recursive function in Juli-CodePudding

I want to get the index of sorted array components at the primary array using a recursive function.

Important note: I know this can be done using a half line of Julia code, But I'm trying to practice writing recursive functions!

Example:
input --> [5, 1, 7, 2, 3, 4]
output (Julia) --> [3, 1, 6, 5, 4, 2]
Since the argmax of the input is 3, and after that, the next argmax is 1, and so on.

And the code which I wrote for this:

julia> function sort_argmax(array)
           all(==(-Inf), array) && nothing
           any(!=(-Inf), array) && vcat(argmax(array) ,sort_argmax(vcat(array[1:argmax(array)-1], -Inf,array[argmax(array) 1:end])))
       end
sort_argmax (generic function with 1 method)

The expected output for the given [5, 1, 7, 2, 3, 4] Vector:

7-element Vector{Int64}:
 3
 1
 6
 5
 4
 2

The real output:

julia> sort_argmax([5, 1, 7, 2, 3, 4])
7-element Vector{Int64}:
 3
 1
 6
 5
 4
 2
 0

There is a surplus 0 at the end of the output, and I don't expect that! How can I achieve the expected result? And, what's the reason behind emerging the last 0?

CodePudding user response：

The fact that there is no nothing as part of the output is a clue that something may be wrong with that part of the code. When we take another look at that line:

           all(==(-Inf), array) && nothing

we see that it just evaluates to nothing, but doesn't do anything with that value. Your intention here is probably to return nothing from here - which needs an explicit return keyword unless you're at the very end of the function.

           all(==(-Inf), array) && return nothing

However, when you run that code, the output is:

julia> sort_argmax([5, 1, 7, 2, 3, 4])
7-element Vector{Union{Nothing, Int64}}:
 3
 1
 6
 5
 4
 2
  nothing

a Vector{Union{Nothing, Int64}} is not exactly what we wanted. This happens because nothing is a value on its own, and takes part in the vcat as an argument. There are a few different ways to fix this, but the easiest/laziest way here is probably to return an empty array instead:

           all(==(-Inf), array) && return Int[]

julia> sort_argmax([5, 1, 7, 2, 3, 4])
6-element Vector{Int64}:
 3
 1
 6
 5
 4
 2