How to generate the power set of a set in Scala

Question

I have a Set of items of some type and want to generate its power set.

I searched the web and couldn\'t find any Scala code that adresses this specific task.

Answer 1

Here's a simple, recursive solution using a helper function:

def concatElemToList[A](a: A, list: List[A]): List[Any] = (a,list) match {
    case (x, Nil)                 => List(List(x))
    case (x, ((h:List[_]) :: t))  => (x :: h) :: concatElemToList(x, t)
    case (x, (h::t))              => List(x, h) :: concatElemToList(x, t)
}

def powerSetRec[A] (a: List[A]): List[Any] = a match {
    case Nil    => List()
    case (h::t) => powerSetRec(t) ++ concatElemToList(h, powerSetRec (t))
}

so the call of

powerSetRec(List("a", "b", "c"))

will give the result

List(List(c), List(b, c), List(b), List(a, c), List(a, b, c), List(a, b), List(a))

Answer 2

Looks like no-one knew about it back in July, but there's a built-in method: subsets.

scala> Set(1,2,3).subsets foreach println
Set()
Set(1)
Set(2)
Set(3)
Set(1, 2)
Set(1, 3)
Set(2, 3)
Set(1, 2, 3)