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Filtering lists

Two functions showing how to filter functional lists using the specified predicate. First version uses naive recursion and the second one is tail-recursive using the accumulator parameter.

Naive recursive implementation

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  /// Create a list containing all elements 
  /// of 'list' that match the predicate 'f'
  let rec filter f list = 
    match list with 
    | x::xs when f x -> x::(filter f xs)
    | _::xs -> filter f xs
    | [] -> []

Tail-recursive implementation

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  /// Create a list containing all elements 
  /// of 'list' that match the predicate 'f'
  let filter f list = 
    /// Inner recursive function that uses
    /// accumulator argument to construct list
    let rec filterAux acc list = 
      match list with 
      | x::xs when f x -> filterAux (x::acc) xs
      | _::xs -> filterAux acc xs
      | [] -> acc |> List.rev
    filterAux [] list

val filter : f:('a -> bool) -> list:'a list -> 'a list

Full name: Script.L1.filter

Create a list containing all elements
of 'list' that match the predicate 'f'

val f : ('a -> bool)

Multiple items
val list : 'a list

--------------------
type 'T list = List<'T>

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

val x : 'a

val xs : 'a list

val filter : f:('a -> bool) -> list:'a list -> 'a list

Full name: Script.L2.filter

Create a list containing all elements
of 'list' that match the predicate 'f'

val filterAux : ('a list -> 'a list -> 'a list)

Inner recursive function that uses
accumulator argument to construct list

val acc : 'a list

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 rev : list:'T list -> 'T list

Full name: Microsoft.FSharp.Collections.List.rev

Raw view Test code New version

More information

Link:	http://fssnip.net/m
Posted:	13 years ago
Author:	Tomas Petricek
Tags:	list , filter , recursion , pattern matching