Problem Two is defined as

 

Each new term in the Fibonacci sequence is generated by adding the previous two terms. By starting with 1 and 2, the first 10 terms will be:

1, 2, 3, 5, 8, 13, 21, 34, 55, 89, ...

Find the sum of all the even-valued terms in the sequence which do not exceed four million.

 F# Code: 

let fib = Seq.unfold (fun (n0,n1) -> Some(n0,(n1, n0 + n1)))(1I,1I)

let fiblessthan n = {

  let e = fib.GetEnumerator()

  while e.MoveNext() && e.Current < n do

  yield e.Current }

let even = fiblessthan 4000000I |> Seq.filter (fun x -> x % 2I = 0I) 

let p2 = even |> Seq.fold1 (fun sum i -> sum + i)

print_any p2

After finishing my version of this problem in F#, I went online and look at some other solutions.  One of the best, I found was by Dustin Campbell on his blog "Did it with .NET"

C# 3.0 (using extension method) 

static public class Numbers

{

    public static IEnumerable<Int64> Fibonacci

    {

        get

        {

            int a = 1;

            int b = 2;

            int t = 0;

            yield return a;

            yield return b;

            while (true)

            {

                yield return t = a + b; a = b; b = t;

            }

        }

    }

 

}

 

Numbers.Fibonacci.TakeWhile(x => x <= 4000000).Where(x => x % 2 == 0).Sum()
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