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Split a list

Three ways to split a list in half (but not necessarily in the middle). A forth version added that's very short and should be fast, as we only use List.fold. New champ found.

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module List=
    (* Execution: fast, Output: least intuitive *)
    let split s = 
        let rec loop = function
        | (_,[],a,b) -> (a,b)
        | (true,h::t,a,b) -> loop(false,t,h::a,b)
        | (false,h::t,a,b)-> loop (true,t,a,h::b)
        in loop(true,s,[],[])
    (* Execution: slowest, Output: most intuitive *)
    let split1 s =
        let len = List.length s in
        s |> List.mapi (fun i x -> (i<len/2,x))
          |> List.partition fst
          |> (fun (x,y) -> ((x |> List.map snd),(y |> List.map snd)))
    (* Execution: average, Output: fairly intuitive *)
    let split2 s =
         let len = List.length s in
         s |> List.fold (fun (n,(a,b)) x -> 
            (n+1,if n <(len/2) then (x::a,b) else (a,x::b)))
                  (0,([],[]))
           |> snd
    (* Execution: quite fast, Output: short and elegant *)
    let split3 s = List.fold (fun (xs, ys) e -> (e::ys, xs)) ([], []) s
    (* Execution: fastest, Output: fairly intuitive, but inner function needed *)
    let split4 s =
        let rec loop l (xs, ys) =
            match l with
            | x::y::rest -> loop rest (x::xs, y::ys)
            | [x] -> (x::xs, ys)
            | _ -> (xs, ys)
        loop s ([], [])
            

Timing function

let l = [1..10000000]

printfn "%d" (timeit List.split l)
printfn "%d" (timeit List.split1 l)
printfn "%d" (timeit List.split2 l)
printfn "%d" (timeit List.split3 l)
printfn "%d" (timeit List.split4 l)

(*
Results:
List.split:  1234ms
List.split1: 5242ms
List.split2: 3235ms
List.split3: 2754ms
List.split4: 1068ms
*)

(*
> List.split [1..10];;
val it : int list * int list = ([9; 7; 5; 3; 1], [10; 8; 6; 4; 2])
> List.split1 [1..10];;
val it : int list * int list = ([1; 2; 3; 4; 5], [6; 7; 8; 9; 10])
> List.split2 [1..10];;
val it : int list * int list = ([5; 4; 3; 2; 1], [10; 9; 8; 7; 6])
> List.split3 [1..10];;
val it : int list * int list = ([10; 8; 6; 4; 2], [9; 7; 5; 3; 1])
> List.split4 [1..10];;
val it : int list * int list = ([9; 7; 5; 3; 1], [10; 8; 6; 4; 2])
> 
*)
Multiple items
module List

from Microsoft.FSharp.Collections

--------------------
type List<'T> =
  | ( [] )
  | ( :: ) of Head: 'T * Tail: 'T list
  interface IEnumerable
  interface IEnumerable<'T>
  member Head : 'T
  member IsEmpty : bool
  member Item : index:int -> 'T with get
  member Length : int
  member Tail : 'T list
  static member Cons : head:'T * tail:'T list -> 'T list
  static member Empty : 'T list

Full name: Microsoft.FSharp.Collections.List<_>
val split : s:'a list -> 'a list * 'a list

Full name: Script.List.split
val s : 'a list
val loop : (bool * 'b list * 'b list * 'b list -> 'b list * 'b list)
val a : 'b list
val b : 'b list
val h : 'b
val t : 'b list
val split1 : s:'a list -> 'a list * 'a list

Full name: Script.List.split1
val len : int
val length : list:'T list -> int

Full name: Microsoft.FSharp.Collections.List.length
val mapi : mapping:(int -> 'T -> 'U) -> list:'T list -> 'U list

Full name: Microsoft.FSharp.Collections.List.mapi
val i : int
val x : 'a
val partition : predicate:('T -> bool) -> list:'T list -> 'T list * 'T list

Full name: Microsoft.FSharp.Collections.List.partition
val fst : tuple:('T1 * 'T2) -> 'T1

Full name: Microsoft.FSharp.Core.Operators.fst
val x : (bool * 'a) list
val y : (bool * 'a) list
val map : mapping:('T -> 'U) -> list:'T list -> 'U list

Full name: Microsoft.FSharp.Collections.List.map
val snd : tuple:('T1 * 'T2) -> 'T2

Full name: Microsoft.FSharp.Core.Operators.snd
val split2 : s:'a list -> 'a list * 'a list

Full name: Script.List.split2
val fold : folder:('State -> 'T -> 'State) -> state:'State -> list:'T list -> 'State

Full name: Microsoft.FSharp.Collections.List.fold
val n : int
val a : 'a list
val b : 'a list
val split3 : s:'a list -> 'a list * 'a list

Full name: Script.List.split3
val xs : 'a list
val ys : 'a list
val e : 'a
val split4 : s:'a list -> 'a list * 'a list

Full name: Script.List.split4
val loop : ('b list -> 'b list * 'b list -> 'b list * 'b list)
val l : 'b list
val xs : 'b list
val ys : 'b list
val x : 'b
val y : 'b
val rest : 'b list
let timeit func param =
    let sw = new System.Diagnostics.Stopwatch()
    sw.Start()
    let _, _ = func param
    sw.Stop()
    sw.ElapsedMilliseconds
val l : int list

Full name: Script.l
val printfn : format:Printf.TextWriterFormat<'T> -> 'T

Full name: Microsoft.FSharp.Core.ExtraTopLevelOperators.printfn
val timeit : func:('a -> 'b * 'c) -> param:'a -> int64

Full name: Script.timeit
Multiple items
module List

from Script

--------------------
module List

from Microsoft.FSharp.Collections

--------------------
type List<'T> =
  | ( [] )
  | ( :: ) of Head: 'T * Tail: 'T list
  interface IEnumerable
  interface IEnumerable<'T>
  member Head : 'T
  member IsEmpty : bool
  member Item : index:int -> 'T with get
  member Length : int
  member Tail : 'T list
  static member Cons : head:'T * tail:'T list -> 'T list
  static member Empty : 'T list

Full name: Microsoft.FSharp.Collections.List<_>

More information

Link:http://fssnip.net/3h
Posted:5 years ago
Author:Dmitri Pavlenkov
Tags: list , split , recursion , fold , partition , match