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Project Euler #2

Solution to Project Euler problem 2: Find sum of even terms in fibonacci sequence which do not exceed four million. Comments about the first version: 1. It does't use memoize becouse of recursive call to unmemoized function "fibs". So it take over 20 sec to get answer. A minor change ("fibs" to "fibs' ") reduces this time to 140ms.

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// The first version fixed to do use memoize
let memoize (f: 'a -> 'b) =
    let t = new System.Collections.Generic.Dictionary<'a,'b>()
    fun n ->
        let ok, res = t.TryGetValue(n)
        if ok then res
        else let res = f n
             t.Add(n,res)
             res

let fibs n = 
    let rec fibs' = fun n -> match n with 0 -> 1 | 1 -> 1 | _ -> (fibs' (n - 1)) + (fibs' (n - 2))
    memoize(fibs') n


let rec fibSeq n = 
    if fibs n > 4000000 then [] else fibs n :: fibSeq  (n + 1)

printf "sum: %d\n" (fibSeq 1 |>List.filter (fun x -> x % 2 = 0) |> List.sum)
val memoize : f:('a -> 'b) -> ('a -> 'b) (requires equality)

Full name: Script.memoize
val f : ('a -> 'b) (requires equality)
val t : System.Collections.Generic.Dictionary<'a,'b> (requires equality)
namespace System
namespace System.Collections
namespace System.Collections.Generic
Multiple items
type Dictionary<'TKey,'TValue> =
  new : unit -> Dictionary<'TKey, 'TValue> + 5 overloads
  member Add : key:'TKey * value:'TValue -> unit
  member Clear : unit -> unit
  member Comparer : IEqualityComparer<'TKey>
  member ContainsKey : key:'TKey -> bool
  member ContainsValue : value:'TValue -> bool
  member Count : int
  member GetEnumerator : unit -> Enumerator<'TKey, 'TValue>
  member GetObjectData : info:SerializationInfo * context:StreamingContext -> unit
  member Item : 'TKey -> 'TValue with get, set
  ...
  nested type Enumerator
  nested type KeyCollection
  nested type ValueCollection

Full name: System.Collections.Generic.Dictionary<_,_>

--------------------
System.Collections.Generic.Dictionary() : unit
System.Collections.Generic.Dictionary(capacity: int) : unit
System.Collections.Generic.Dictionary(comparer: System.Collections.Generic.IEqualityComparer<'TKey>) : unit
System.Collections.Generic.Dictionary(dictionary: System.Collections.Generic.IDictionary<'TKey,'TValue>) : unit
System.Collections.Generic.Dictionary(capacity: int, comparer: System.Collections.Generic.IEqualityComparer<'TKey>) : unit
System.Collections.Generic.Dictionary(dictionary: System.Collections.Generic.IDictionary<'TKey,'TValue>, comparer: System.Collections.Generic.IEqualityComparer<'TKey>) : unit
val n : 'a (requires equality)
val ok : bool
val res : 'b
System.Collections.Generic.Dictionary.TryGetValue(key: 'a, value: byref<'b>) : bool
System.Collections.Generic.Dictionary.Add(key: 'a, value: 'b) : unit
val fibs : n:int -> int

Full name: Script.fibs
val n : int
val fibs' : (int -> int)
val fibSeq : n:int -> int list

Full name: Script.fibSeq
val printf : format:Printf.TextWriterFormat<'T> -> 'T

Full name: Microsoft.FSharp.Core.ExtraTopLevelOperators.printf
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 filter : predicate:('T -> bool) -> list:'T list -> 'T list

Full name: Microsoft.FSharp.Collections.List.filter
val x : int
val sum : list:'T list -> 'T (requires member ( + ) and member get_Zero)

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

More information

Link:http://fssnip.net/1p
Posted:8 years ago
Author:D
Tags: euler problem