Combining multiple Lists of arbitrary length

Question

I am looking for an approach to join multiple Lists in the following manner:

ListA a b c
ListB 1 2 3 4
ListC + # * § %
..
..
..

Resulting List: a 1 + b 2 # c 3 * 4 § %

In Words: The elements in sequential order, starting at first list combined into the resulting list. An arbitrary amount of input lists could be there varying in length.

I used multiple approaches with variants of zip, sliding iterators but none worked and especially took care of varying list lengths. There has to be an elegant way in scala ;)

Answer 1

val lists = List(ListA, ListB, ListC)

lists.flatMap(_.zipWithIndex).sortBy(_._2).map(_._1)

It's pretty self-explanatory. It just zips each value with its position on its respective list, sorts by index, then pulls the values back out.

Answer 2

Here's how I would do it:

class ListTests extends FunSuite {
  test("The three lists from his example") {
    val l1 = List("a", "b", "c")
    val l2 = List(1, 2, 3, 4)
    val l3 = List("+", "#", "*", "§", "%")

    // All lists together
    val l = List(l1, l2, l3)

    // Max length of a list (to pad the shorter ones)
    val maxLen = l.map(_.size).max

    // Wrap the elements in Option and pad with None
    val padded = l.map { list => list.map(Some(_)) ++ Stream.continually(None).take(maxLen - list.size) }

    // Transpose 
    val trans = padded.transpose

    // Flatten the lists then flatten the options
    val result = trans.flatten.flatten

    // Viola 
    assert(List("a", 1, "+", "b", 2, "#", "c", 3, "*", 4, "§", "%") === result)
  }
}