Scala Recusion/Tail Recursion-CodePudding

I'm trying to practice recursive and tail recursive functions using Scala, I've created a tail recursive function to sum the values in two lists. I'm trying to do the same with recursion too but the only way I could think of was to modify the parameter each time the method was called like tail recursion. Can you help me out?

def callTailRecursive(list1 : List[Int], list2 : List[Int]) : List[Int] = {
  def callHelper(list1 : List[Int], list2 : List[Int], outputList : List[Int]): List[Int] ={
    if(!list1.isEmpty && !list2.isEmpty){
      callHelper(list1.tail,list2.tail,outputList: (list1.head   list2.head))
    }else if(list1.isEmpty && !list2.isEmpty){
      callHelper(list1,list2.tail,outputList: (list2.head))
    }else if(!list1.isEmpty && list2.isEmpty){
      callHelper(list1.tail,list2,outputList: (list1.head))
    }else{
      outputList
    }
  }
  callHelper(list1,list2,List())
}

def callRecursive(n : Int, list1 : List[Int], list2 : List[Int], outputList : List[Int]): List[Int]  = {

}

CodePudding user response：

Pattern matching. It's the bee's knees.

def callRecursive(list1: List[Int], list2: List[Int]): List[Int] = (list1,list2) match {
  case (hd1::tl1, hd2::tl2) => (hd1 hd2) :: callRecursive(tl1, tl2)
  case (_, Nil) => list1
  case (Nil, _) => list2
}

def notRecursive(list1: List[Int], list2: List[Int]): List[Int] =
  list1.zipAll(list2, 0, 0).map{case (a,b) => a b}