How to perform transpose on List of Lists in scala?

Question

I have a scala list of following structure:

val list = List(
  List(a1,a2,a3,a4,a5),
  List(b1,b2,b3,b4,b5),
  List(c1,c2,c3,c4,c5)
)

From this list, I want to generate a list which would look like:

List(
  List(a1,b1,c1),
  List(a2,b2,c2),
  List(a3,b3,c3),
  List(a4,b4,c4),
  List(a5,b5,c5)
)

How do I achieve this in scala ?

Answer 1

If you have a fixed number of such lists, you can use zipped to zip them together (into tuples). As in your example:

scala> (List(a1,a2,a3,a4), List(b1,b2,b3,b4), List(c1,c2,c3,c4)).zipped.toList
List((a1,b1,c1),(a2,b2,c2),(a3,b3,c3),(a4,b4,c4))

If you really want the full power of what you have above, it is just the transpose of your list of lists:

scala> List(List(a1,a2,a3,a4), List(b1,b2,b3,b4), List(c1,c2,c3,c4)).transpose
List(List(a1,b1,c1),List(a2,b2,c2),List(a3,b3,c3),List(a4,b4,c4))

Answer 2

Here is a manual implementation of the library function.

object CustomZipTest extends App{
  val list1: List[String] = List("a", "1", "w")

  val list2: List[String] = List("b", "2", "x")

  val list3: List[String] = List("c", "3", "y")

  val list4: List[String] = List("d", "4", "z")

  def customZip[T](list: List[T] *): List[List[T]] = {
    def stripOneLevel(list: Seq[List[T]]): (Seq[List[T]], List[T]) = {
      list.foldRight(Seq(): Seq[List[T]], List(): List[T]){ (e, acc) =>
        if (e.size == 1) (acc._1, e.head +: acc._2)
        else (e.tail +: acc._1, e.head +: acc._2)
      }
    }

    def strip(list: Seq[List[T]], acc: List[List[T]]): List[List[T]] = {
      list match {
        case Nil => acc
        case x+:xs =>
          val stripped = stripOneLevel(list)
          strip(stripped._1, stripped._2 +: acc)
      }
    }

    strip(list, List()).reverse
  }

  println(customZip(list1, list2, list3, list4))

}