zipWith in Scala using map

Question

Define zipWith. It should zip two lists, but instead of zipping elements into a tuple, it should use a function to combine two elements.

Example: zipWith(List(1, 2, 3), List(10, 11, 12), (x: Int, y: Int) => x+y) Should return: List(11,13,15)

use map and zip.

def zipWith[A,B,C](xs: List[A], ys: List[B], f: (A, B) => C): List[C] = {
        val zs = xs.zip(ys)
        //I don't know how to do this because if myMap(zs, f) 

        //myMap takes a functin f:(a)=>b instead of f: (A, B) => C
    }
}

Answer 1

It sounds like you looking for something like this:

def zipWith[A,B,C](xs: List[A], ys: List[B], f: (A, B) => C): List[C] = {
  (xs, ys) match {
    case (Nil, _) => Nil
    case (_, Nil) => Nil
    case (x :: xs, y :: ys) => f(x, y) :: zipWith(xs, ys, f)
  }
}

Hope that helps.

Update

Here is the same function but being tail-recursive:

    def zipWith[A, B, C](xs: List[A], ys: List[B], f: (A, B) => C): List[C] = {
      @tailrec
      def zipAccumulatingResult(xs: List[A], ys: List[B], f: (A, B) => C, acc: List[C]): List[C] = {
        (xs, ys) match {
          case (Nil, _) => acc
          case (_, Nil) => acc
          case (x :: xs, y :: ys) => zipAccumulatingResult(xs, ys, f, acc :+ f(x, y))
        }
      }

      zipAccumulatingResult(xs, ys, f, Nil)
    }