F# split sequence into sub lists on every nth element

Question

Say I have a sequence of 100 elements. Every 10th element I want a new list of the previous 10 elements. In this case I will end up with a list of 10 sublists.

Seq.t

Answer 1

Perhaps this simple pure implementation might be useful:

let splitAt n xs =  (Seq.truncate n xs, if Seq.length xs < n then Seq.empty else Seq.skip n xs)

let rec chunk n xs =
    if Seq.isEmpty xs then Seq.empty
    else
        let (ys,zs) = splitAt n xs
        Seq.append (Seq.singleton ys) (chunk n zs)

For example:

> chunk 10 [1..100];;
val it : seq<seq<int>> =
  seq
    [seq [1; 2; 3; 4; ...]; seq [11; 12; 13; 14; ...]; 
     seq [21; 22; 23; 24; ...]; seq [31; 32; 33; 34; ...]; ...]

> chunk 5 [1..12];;
val it : seq<seq<int>> =
  seq [seq [1; 2; 3; 4; ...]; seq [6; 7; 8; 9; ...]; seq [11; 12]]

Answer 2

I found this to be easily the fastest:

let windowChunk n xs =
    let range = [0 .. Seq.length xs]
    Seq.windowed n xs |> Seq.zip range 
                      |> Seq.filter (fun d -> (fst d) % n = 0)
                      |> Seq.map(fun x -> (snd x))

i.e. window the list, zip with a list of integers, remove all the overlapping elements, and then drop the integer portion of the tuple.