Generate the combinations of K distinct objects chosen from the N elements of a list. In how many ways can a committee of 3 be chosen from a group of 12 people? We all know that there are C(12,3) = 220 possibilities (C(N,K) denotes the well-known binomial coefficient). For pure mathematicians, this result may be great. But we want to really gene…

problem26.scala

scala> combinations(3, List('a, 'b, 'c, 'd, 'e, 'f, 'g, h', i', j', k', l'))
res0: List[List[Symbol]] = List(List('a, 'b, 'c), List('a, 'b, 'd), List('a, 'b, 'e), ...

def combinations[A](size: Int, pool: List[A]): List[List[A]] = pool match {
  case Nil => Nil
  case h :: t => if(size <= 0) Nil else if (size == 1) {
    list(h) :: combinations(size, t)
  } else {
    helper(h, combinations(size-1, t)
  }
}

def helper[A](distribute: A, combs: List[List[A]]): List[List[A]] = combs match {
  case Nil => Nil
  case h :: t => (distribute :: h) :: helper (distribute, t)
}