My first attempt at this problem using functional programming with F#. I would've liked to use some kind of take-while function to keep generating Fibonacci numbers until they fulfilled certain conditions, but alas I could not find such a thing in F#.

open System

let rec fib n = match n with
                | 0 | 1 | 2 -> n
                | _ -> fib (n - 1) + fib (n - 2)

let rec nOfFibTermsUpUntil x y = if fib (y + 1) < x then nOfFibTermsUpUntil x (y + 1) else y
                     
[for n in 0..nOfFibTermsUpUntil 4000000 0 -> fib n]
|> List.filter (fun x -> x % 2 = 0)
|> List.sum
|> printf "The sum of Fibonacci terms up until 4,000,000 is: %d"
Console.ReadKey () |> ignore

Project Euler Problem 2 asks for the sum of all even Fibonacci numbers below 4 million.

My first attempt at this problem using functional programming with F#. I would've liked to use some kind of take-while function to keep generating Fibonacci numbers until they fulfilled certain conditions, but alas I could not find such a thing in F#.

open System

let rec fib n = match n with
                | 0 | 1 | 2 -> n
                | _ -> fib (n - 1) + fib (n - 2)

let rec nOfFibTermsUpUntil x y = if fib (y + 1) < x then nOfFibTermsUpUntil x (y + 1) else y
                     
[for n in 0..nOfFibTermsUpUntil 4000000 0 -> fib n]
|> List.filter (fun x -> x % 2 = 0)
|> List.sum
|> printf "The sum of Fibonacci terms up until 4,000,000 is: %d"
Console.ReadKey () |> ignore