Question

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.

Clojure

#!/usr/bin/env clojure
(def fibs (lazy-cat [0 1] (map + (rest fibs) fibs)))
(println (reduce + (filter even? (take-while #(< % 4000000) fibs))))

$ time clojure fibonacci.clj
real   0m0.885s
user   0m1.720s
sys    0m0.092s

Go

package main

import "fmt"

func main() {
    sum := 0
    for a, b := 1, 2; b <= 4e6; a, b = b, a+b {
        if b%2 == 0 {
            sum += b
        }
    }
    fmt.Println(sum)
}

$ go build -o fibonacci fibonacci.go
$ time ./fibonacci
real   0m0.001s
user   0m0.000s
sys    0m0.001s

Haskell

fibs ::  [Integer]
fibs = 0 : 1 : zipWith (+) fibs (tail fibs)

main ::  IO ()
main = print $ sum $ filter even $ takeWhile (<4000000) fibs

$ ghc -O2 -o fibonacci fibonacci.hs
$ time ./fibonacci
real   0m0.002s
user   0m0.000s
sys    0m0.002s

JavaScript

let s = 0
let f = [1, 1]
while (f[0] < 4e6) {
  if (f[0] % 2 === 0) s+= f[0]
  f = [f[1], f[0] + f[1]]
}
console.log(s)

$ time node --use-strict fibonacci.js
real   0m0.052s
user   0m0.030s
sys    0m0.023s

Ruby

#!/usr/bin/env ruby
sum, a, b = 0, 1, 2
while b < 4000000
  sum += b if b.even?
  a, b = b, a + b
end
puts sum

$ time ruby fibonacci.rb
real   0m0.038s
user   0m0.038s
sys    0m0.000s

Rust

fn main() {
    let mut sum = 0;
    let mut a = 1;
    let mut b = 2;
    let mut tmp;
    while b <= 4000000 {
        if b % 2 == 0 {
            sum += b;
        }
        tmp = a + b;
        a = b;
        b = tmp;
    }
    println!("{}", sum);
}

$ rustc -C target-cpu=native -C opt-level=3 -o fibonacci fibonacci.rs
$ time ./fibonacci
real   0m0.001s
user   0m0.000s
sys    0m0.001s