samskivert: Euler 55

01 January 2010

Problem 055:

object Euler55 extends EulerApp {
  def ispal (n :String) = n.take(n.length/2) == n.takeRight(n.length/2).reverse
  def islychrel (n :BigInt, iter :Int = 0) :Int =
    if (iter > 0 && ispal(n.toString)) 0
    else if (iter == 50) 1
    else islychrel(n + BigInt(n.toString.reverse), iter+1)
  println((1 to 9999).map(n => islychrel(n)).sum)

Nothing much to see here, other than that I took advantage of a couple of nice features of Scala 2.8 while cleaning this up for posting.

The first is the use of a default argument to islychrel so that when I call it non-recursively, I don't have to manually specify 0 for the 0th iteration. I do still have to explicitly write n => islychrel(n) because I'm taking advantage of the implicit conversion to BigInt. In theory I could do map(BigInt.apply).map(islychrel) but, not surprisingly, that causes Scala's implicit argument support to do le freak out.

The other nice 2.8ism I'm taking advantage of is that RichString.take and RichString.takeRight both return String, so where I used to be calling mkString to get the faster string comparison, I no longer need to do so. Yay!

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