Scala-如何以功能方式在列表周围循环

Question

我需要遍历一个列表，并将列表中的上一个元素与当前元素进行比较。从传统的编程背景开始，我使用带变量的循环来保存先前的值，但是对于Scala，我认为应该有一种更好的方法。

for (item <- dataList)
{
   // Code to compare previous list element
   prevElement = item
}

上面的代码可以正常工作并给出正确的结果，但是我想探索编写相同代码的功能方法。

Answer 1

有许多不同的解决方案。也许最通用的是使用辅助函数：

// This example finds the minimum value (the hard way) in a list of integers. It's
// not looking at the previous element, as such, but you can use the same approach.
// I wanted to make the example something meaningful.
def findLowest(xs: List[Int]): Option[Int] = {

  @tailrec // Guarantee tail-recursive implementation for safety & performance.
  def helper(lowest: Int, rem: List[Int]): Int = {

    // If there are no elements left, return lowest found.
    if(rem.isEmpty) lowest

    // Otherwise, determine new lowest value for next iteration. (If you need the
    // previous value, you could put that here.)
    else {
      val newLow = Math.min(lowest, rem.head)
      helper(newLow, rem.tail)
    }
  }

  // Start off looking at the first member, guarding against an empty list.
  xs match {
    case x :: rem => Some(helper(x, rem))
    case _ => None
  }
}

在这种情况下，可以用代替fold。

def findLowest(xs: List[Int]): Option[Int] = xs match {
  case h :: _ => Some(xs.fold(h)((b, m) => Math.min(b, m)))
  case _ => None
}

可以进一步简化为：

def findLowest(xs: List[Int]): Option[Int] = xs match {
  case h :: _ => Some(xs.foldLeft(h)(Math.min))
  case _ => None
}

在这种情况下，我们使用操作的zero参数fold存储最小值。

如果这看起来太抽象了，您可以在循环中发布更多有关您想做的事情的详细信息，我可以在回答中解决这个问题吗？

更新：好的，因此，看到您对日期的评论后，这里是一个更具体的版本。DateRange在此示例中，我使用了（您将需要用实际类型替换它）。你还必须确定什么overlap和merge需要做的。但是，这是您的解决方案：

// Determine whether two date ranges overlap. Return true if so; false otherwise.
def overlap(a: DateRange, b: DateRange): Boolean = { ... }

// Merge overlapping a & b date ranges, return new range.
def merge(a: DateRange, b: DateRange): DateRange = { ... }

// Convert list of date ranges into another list, in which all consecutive,
// overlapping ranges are merged into a single range.
def mergeDateRanges(dr: List[DateRange]): List[DateRange] = {

  // Helper function. Builds a list of merged date ranges.
  def helper(prev: DateRange, ret: List[DateRange], rem: List[DateRange]):
  List[DateRange] = {

    // If there are no more date ranges to process, prepend the previous value
    // to the list that we'll return.
    if(rem.isEmpty) prev :: ret

    // Otherwise, determine if the previous value overlaps with the current
    // head value. If it does, create a new previous value that is the merger
    // of the two and leave the returned list alone; if not, prepend the
    // previous value to the returned list and make the previous value the
    // head value.
    else {
      val (newPrev, newRet) = if(overlap(prev, rem.head)) {
        (merge(prev, rem.head), ret)
      }
      else (rem.head, prev :: ret)

      // Next iteration of the helper (I missed this off, originally, sorry!)
      helper(newPrev, newRet, rem.tail)
    }
  }

  // Start things off, trapping empty list case. Because the list is generated by pre-pending elements, we have to reverse it to preserve the order.
  dr match {
    case h :: rem => helper(h, Nil, rem).reverse // <- Fixed bug here too...
    case _ => Nil
  }
}