Merge two collections by interleaving values

Question

How can I merge two lists / Seqs so it takes 1 element from list 1, then 1 element from list 2, and so on, instead of just appending list 2 at the end of list 1?

E.g [1,2] + [3,4] = [1,3,2,4]

and not [1,2,3,4]

Any ideas? Most concat methods I've looked at seem to do to the latter and not the former.

What if the lists are not the same size? What behavior do you expect? — Pedro Correia Luís, Aug 01 '19 at 17:01
@PedroCorreiaLuís Append the final elements of the longer list at the end — Ali, Aug 02 '19 at 08:17
So solutions with zip or transpose can't do it, only jwvh or Mario Galic (updated solution) would satisfy your desired behavior — Pedro Correia Luís, Aug 02 '19 at 10:02

Krzysztof Atłasik · Answer 1 · 2019-08-01T17:01:46.197

7

Another way:

List(List(1,2), List(3,4)).transpose.flatten

edited Aug 01 '19 at 17:01

answered Aug 01 '19 at 16:05

Krzysztof Atłasik

21,985
6
54
76

jwvh · Accepted Answer · 2019-08-01T18:07:16.423

6

So maybe your collections aren't always the same size. Using zip in that situation would create data loss.

def interleave[A](a :Seq[A], b :Seq[A]) :Seq[A] =
  if (a.isEmpty) b else if (b.isEmpty) a
  else a.head +: b.head +: interleave(a.tail, b.tail)

interleave(List(1, 2, 17, 27)
          ,Vector(3, 4))  //res0: Seq[Int] = List(1, 3, 2, 4, 17, 27)

edited Aug 01 '19 at 18:07

answered Aug 01 '19 at 17:04

jwvh

50,871
7
38
64

That is what I was thinking, you can´t use zip. Your solution is great – Pedro Correia Luís Aug 01 '19 at 17:06

Mario Galic · Answer 3 · 2019-08-01T21:27:58.480

Try

List(1,2)
  .zip(List(3,4))
  .flatMap(v => List(v._1, v._2))

which outputs

res0: List[Int] = List(1, 3, 2, 4)

Also consider the following implicit class

implicit class ListIntercalate[T](lhs: List[T]) {
  def intercalate(rhs: List[T]): List[T] = lhs match {
    case head :: tail => head :: (rhs.intercalate(tail))
    case _ => rhs
  }
}

List(1,2) intercalate List(3,4)
List(1,2,5,6,6,7,8,0) intercalate List(3,4)

which outputs

res2: List[Int] = List(1, 3, 2, 4)
res3: List[Int] = List(1, 3, 2, 4, 5, 6, 6, 7, 8, 0)

score 5 · Answer 4 · answered Aug 01 '19 at 16:00

5

You can do:

val l1 = List(1, 2)
val l2 = List(3, 4)
l1.zip(l2).flatMap { case (a, b) => List(a, b) }

answered Aug 01 '19 at 16:00

Jean Logeart

52,687
11
83
118

Merge two collections by interleaving values

4 Answers4