scala - 哪个功能最快，如何？

Question

哪一个在性能方面更快。在这种情况下，视图如何使其更快？

def f(arr: List[Int]): List[Int] =
  arr.zipWithIndex.filter(_._2 % 2 == 1).map(_._1)

def f(arr: List[Int]): List[Int] =
  arr.view.zipWithIndex.filter { _._2 % 2 != 0 }.map(_._1).toList

def f(arr: List[Int]): List[Int] =
  arr.view.zipWithIndex.collect { case (a, b) if (b % 2 == 0) => a }.toList

标签： scala

Answer 1

def f(arr: List[Int]): List[Int] =
  arr.grouped(2).flatMap(_.drop(1)).toList

:)

---

更严重的是：

def f(arr: List[Int]): List[Int] = {
  @annotation.tailrec
  def loop(rem: List[Int], res: List[Int]): List[Int] =
    rem match {
      case _::b::tail => loop(tail, b +: res)
      case _ => res.reverse
    }

  loop(arr, Nil)
}