Even Fibonacci Numbers

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, …

By considering the terms in the Fibonacci sequence whose values do not exceed four million, find the sum of the even-valued terms.

Solution

The numbers are small enough to calculate it in a matter of milliseconds without any optimisations. Fibonacci implementation is done with a LazyList to make it infinite. Is this an efficient approach? Not really, because LazyList memoizes all the elements, but we need just the previous 2.

In this case, due to how easy this task is, let’s keep it as it is.

object Euler002 extends EulerApp {
  override def execute(): Int =
    fibonacci.filter(_ % 2 == 0).takeWhile(_ < 4_000_000).sum

  val fibonacci: LazyList[Int] = 1 #:: fibonacciFunction(1, 2)

  private def fibonacciFunction(a: Int, b: Int): LazyList[Int] =
    a #:: fibonacciFunction(b, a + b)
}