samskivert: Euler 021

18 January 2008

Problem 021:

object Euler21 extends Application {
  def sumdiv (n :Int) = List.range(1, n/2+1).filter(div => n % div == 0).foldLeft(0)(_+_)
  println(List.range(1, 10000).filter(n => {
    val sdn = sumdiv(n); (n == sumdiv(sdn) && n != sdn && sdn < 10000)
  }).foldLeft(0)(_+_));
}

This one is almost readable — if you've been staring at a lot of Scala recently. It also uses a less efficient method for computing divisors because it fits on one line.

The above runs in 1176ms but the version using:

def sumdiv (x :Int) = (1 :: List.flatten(for {
    divis < - List.range(2, Math.sqrt(x)+1)
    if x % divis == 0
  } yield List(divis, x / divis).removeDuplicates)).foldLeft(0)(_+_)

runs in 157ms. Oh the sacrifices we make for brevity.

©1999–2015 Michael Bayne