Scala Saturday – The List.++ Method

Occasionally, you need to combine two lists (or arrays or vectors or sequences) into one. List.++ to the rescue! (Along with Array.++, and Vector.++, and Seq.++.)

A widow Carol has three daughters: Marcia, Jan, and Cindy.

val ladies = List("Carol", "Marcia", "Jan", "Cindy")

A widower Mike has three sons: Greg, Peter, and Bobby.

val fellas = List("Mike", "Greg", "Peter", "Bobby")

That lovely lady meets that fellow, and they know it is much more than a hunch. They marry and form a family:

val bunch = ladies ++ fellas
// bunch: List[String] = 
//   List(Carol, Marcia, Jan, Cindy,
//        Mike, Greg, Peter, Bobby)

Of course, as you probably have guessed, order matters. Let’s reverse the arguments:

val bunch2 = fellas ++ ladies
// bunch2: List[String] = 
//   List(Mike, Greg, Peter, Bobby,
//        Carol, Marcia, Jan, Cindy)

You can also use ++ to chain a series of lists together:

val hobbits = List("Frodo", "Sam", "Pippin", "Merry")
val men = List("Aragorn", "Boromir")
val dwarves = List("Gimli")
val elves = List("Legolas")
val maiar = List("Gandalf")
val fellowship = hobbits ++ men ++ dwarves ++
                 elves ++ maiar
// fellowship: List[String] = 
//   List(Frodo, Sam, Pippin, Merry, Aragorn, 
//        Boromir, Gimli, Legolas, Gandalf)

And there’s nothing special about ++ in this regard. Because Scala allows infix notation, you can similarly chain other operations together:

val fellowslip = 
  hobbits ++ men ++ dwarves ++
  elves ++ maiar filter {
    !_.startsWith("G")
  } map {
    _.toUpperCase
  }
// fellowslip: List[String] =
//   List(FRODO, SAM, PIPPIN, MERRY,
//        ARAGORN, BOROMIR, LEGOLAS)