Question

I’m trying to write a scala function which will recursively sum the values in a list. Here is what I have so far :

  def sum(xs: List[Int]): Int = {
    val num = List(xs.head)   
    if(!xs.isEmpty) {
      sum(xs.tail)
    }
    0
   }

I dont know how to sum the individual Int values as part of the function. I am considering defining a new function within the function sum and have using a local variable which sums values as List is beuing iterated upon. But this seems like an imperative approach. Is there an alternative method ?

Answer 1

Here’s the the “standard” recursive approach:

def sum(xs: List[Int]): Int = {
  xs match {
    case x :: tail => x + sum(tail) // if there is an element, add it to the sum of the tail
    case Nil => 0 // if there are no elements, then the sum is 0
  }
}

And, here’s a tail-recursive function. It will be more efficient than a non-tail-recursive function because the compiler turns it into a while loop that doesn’t require pushing a new frame on the stack for every recursive call:

def sum(xs: List[Int]): Int = {
  @tailrec
  def inner(xs: List[Int], accum: Int): Int = {
    xs match {
      case x :: tail => inner(tail, accum + x)
      case Nil => accum
    }
  }
  inner(xs, 0)
}