Scala惯用的用分隔符拆分列表的方法是什么? [英] What's Scala's idiomatic way to split a List by separator?

问题描述

如果我有一个String类型的列表，

If I have a List of type String,

scala> val items = List("Apple","Banana","Orange","Tomato","Grapes","BREAK","Salt","Pepper","BREAK","Fish","Chicken","Beef")
items: List[java.lang.String] = List(Apple, Banana, Orange, Tomato, Grapes, BREAK, Salt, Pepper, BREAK, Fish, Chicken, Beef)

如何根据特定的字符串/模式(在本例中为"BREAK" )将其分成 n 个单独的列表.

how can I split it into n separate lists based on a certain string/pattern ("BREAK", in this case).

我已经考虑过使用 indexOf 查找"BREAK" 的位置，并以这种方式拆分列表，或者使用类似的方法使用 takeWhile(i => i！="BREAK")，但我想知道是否有更好的方法?

I've thought about finding the position of "BREAK" with indexOf, and split up the list that way, or using a similar approach with takeWhile (i => i != "BREAK") but I'm wondering if there's a better way?

如果有帮助，我知道 items 列表中将只有3套项目(因此有2个"BREAK" 标记).

If it helps, I know there will only ever be 3 sets of items in the items list (thus 2 "BREAK" markers).

推荐答案

def splitBySeparator[T](l: List[T], sep: T): List[List[T]] = {
  l.span( _ != sep ) match {
    case (hd, _ :: tl) => hd :: splitBySeparator(tl, sep)
    case (hd, _) => List(hd)
  }
}

val items = List("Apple","Banana","Orange","Tomato","Grapes","BREAK","Salt","Pepper","BREAK","Fish","Chicken","Beef")
splitBySeparator(items, "BREAK")

结果:

res1: List[List[String]] = List(List(Apple, Banana, Orange, Tomato, Grapes), List(Salt, Pepper), List(Fish, Chicken, Beef))

更新:以上版本虽然简洁有效，但存在两个问题:无法很好地处理边缘情况(如 List("BREAK")或 List("BREAK"，"Apple"，"BREAK")，并且不是尾部递归.因此，这是另一个(命令)版本，可解决此问题:

UPDATE: The above version, while concise and effective, has two problems: it does not handle well the edge cases (like List("BREAK") or List("BREAK", "Apple", "BREAK"), and is not tail recursive. So here is another (imperative) version that fixes this:

import collection.mutable.ListBuffer
def splitBySeparator[T](l: Seq[T], sep: T): Seq[Seq[T]] = {
  val b = ListBuffer(ListBuffer[T]())
  l foreach { e =>
    if ( e == sep ) {
      if  ( !b.last.isEmpty ) b += ListBuffer[T]()
    }
    else b.last += e
  }
  b.map(_.toSeq)
}

它内部使用了一个 ListBuffer ，非常类似于我在 splitBySeparator 的第一版中使用的 List.span 的实现.

It internally uses a ListBuffer, much like the implementation of List.span that I used in the first version of splitBySeparator.

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