Scala function to get all sorted subsets of size k

Question

I am given a set of size L and want to generate every sorted subset of size k. Would be great if your solution is in scala but maybe I am able to translate by myself.

An example run for L = 6 and k = 3 should yield.

1, 2, 3
1, 2, 4
1, 2, 5
1, 2, 6
1, 3, 4
1, 3, 5
1, 3, 6
1, 4, 5
1, 4, 6
1, 5, 6
2, 3, 4
2, 3, 5
2, 3, 6
2, 4, 5
2, 4, 6
2, 5, 6
3, 4, 5
3, 4, 6
3, 5, 6
4, 5, 6

My scala attempt was something like:

object Util {
  def main(args : Array[String]) : Unit = {
    starts(6,3,1)
  }

  def starts(L: Int, num: Int, level: Int) : List[List[Int]] = {
    if( num == 0 ) {
      return List()
    }else{
      (level to (L-num+1)).map( o => o :: starts(L,num-1,level+1))
    }
  }
}

I hope you can help me.

Answer 1

All you need is

def subsets(L: Int, k: Int) =  
  1 to L combinations k

Results:

scala> subsets(6, 3) foreach println
Vector(1, 2, 3)
Vector(1, 2, 4)
Vector(1, 2, 5)
Vector(1, 2, 6)
Vector(1, 3, 4)
Vector(1, 3, 5)
Vector(1, 3, 6)
Vector(1, 4, 5)
Vector(1, 4, 6)
Vector(1, 5, 6)
Vector(2, 3, 4)
Vector(2, 3, 5)
Vector(2, 3, 6)
Vector(2, 4, 5)
Vector(2, 4, 6)
Vector(2, 5, 6)
Vector(3, 4, 5)
Vector(3, 4, 6)
Vector(3, 5, 6)
Vector(4, 5, 6)

as required.

Answer 2

You could start from that

def subsets(start: Int, end: Int, count: Int) :Seq[Seq[Int]] = (
  if (count == 0) 
    List(Nil)
  else 
    for(head <- start to end; tail <- subsets(head + 1, end, count -1)) 
    yield head +: tail
)