Question

I am trying to program the coin change problem in Scala using recursion. The code that i have written is as follows.

def countChange(money: Int, coins: List[Int]): Int = {
  def ways(change: List[Int], size: Int, capacity: Int): Int = {
    if(capacity == 0) 1
    if((capacity < 0) || (size <= 0)) 0

    //println and readLine to check and control each recursive call.

    println("calling ways(",change, change.length-1, capacity,") + ways(",change,   change.length, capacity - change(change.length - 1),")")
    readLine()
    //

    ways(change, change.length-1, capacity) + ways(change, change.length, capacity - change(change.length - 1))
  }
  ways(coins, coins.length, money)
}

On running the code, it does not terminate and keeps on calling the first recursive call. Where am I going wrong?

Answer 1

Simply stating a value does not make Scala return it; you either need an explicit return, or it has to be the last item stated. Thus:

if (capacity == 0) return 1

or

if (capacity == 0) 1
else if (...)
else { ... }