scala - 有没有办法在 Scala 中突破 @tailrec ？

Question

我有一个递归的方法。scala中是否有办法根据缓冲区的大小进行突破（如下所示）？例如，当 elementList.size > 5 时爆发的情况？

val elementList = ListBuffer.empty[Book]
@tailrec
def getBooks(elements: List[Element]) {
  elements match {
    case Nil => info("Reached end of elements list.")
    case element :: rest if element.getElementType == ElementType.BOOK => {
      elementList.append(element.getBook) 
      getLooks(rest)
    }
    case _ => getBooks(elements.tail)
  }
}

Answer 1

I guess the most simple way would be to just wrap an if statement around your match statement like this:

val elementList = ListBuffer.empty[Book]
@tailrec
def getBooks(elements: List[Element]) {
  if (elementList.size <= 5){
    elements match {
      case Nil => info("Reached end of elements list.")
      case element :: rest if element.getElementType == ElementType.BOOK => {
        elementList.append(element.getBook) 
        getLooks(rest)
      }
      case _ => getBooks(elements.tail)
    }
  }
}

Answer 2

一般来说，您可以尝试在递归中传递剩余元素的数量。

例如：

  def get(list: List[Int], max: Int): List[Int] = {

    @tailrec
    def get(list: List[Int], acc: List[Int], remaining: Int): List[Int] = {
      list match {

        case h :: tail if remaining > 0 =>
          get(tail, h :: acc, remaining - 1)

        case _ =>
          acc
      }
    }

    // Requires reverse() !
    get(list, Nil, max).reverse

至于累加器：您可以改用缓冲区，以防止reverse()最后出现。