S04

S-99: Ninety-Nine Scala Problems
を試してみる。

//P04 (*) Find the number of elements of a list.
//Example:
//scala> length(List(1, 1, 2, 3, 5, 8))
//res0: Int = 6

def lengthBuiltIn[T](xs: List[T]): Int = xs.length

// 末尾再帰版
def lengthTailRecursive[T](xs: List[T]): Int = {
  def recursive(l: List[T], n: Int): Int = l match {
    case Nil => 0
    case _ :: Nil => n + 1
    case _ :: tail => recursive(tail, n + 1)
  }
  recursive(xs, 0)
}

def length[T](xs: List[T]): Int = {
  def recursive(l: List[T]): Int = l match {
    case Nil => 0
    case _ :: tail => 1 + recursive(tail)
  }
  recursive(xs)
}

解答例