How can I take last N items from a list in F#?

Question

I would do something like

let last n xs = xs |> List.rev |> Seq.take n |> List.ofSeq |> List.rev

I am not sure about turning a list to a sequence and back though. Is this how you do it F#?

Answer 1

Seq + Skip

Taking the last N items is equivalent to skipping the first (length - N) items, so for a Sequence as input (and output), you could do something like:

let last n xs = Seq.skip ((Seq.length xs) - n) xs

(or, with piping, let last n xs = xs |> Seq.skip (Seq.length xs - n)

and for a List as input (and output) you could do:

let last n xs = List.toSeq xs |> Seq.skip (xs.Length - n) |> Seq.toList

or by defining both, just pipe it to the sequence one:

let lastList n xs = List.toSeq xs |> last n |> Seq.toList

Tail + Recursion

Alternatively, this can be achieved by (tail) recursively applying Tail as so:

let rec last n xs =
  if List.length xs <= n then xs
  else last n xs.Tail