Functional Programming: A PragPub Anthology

Counting Coins

To explore the expressive power of pattern matching, let’s write a function that will count the number of nickels, dimes, and quarters in a given collection:

val coins = List(25, 10, 5, 10, 5, 25, 10, 5, 5, 25)

Using the traditional if-else construct, the code would look like this:

def countCoins(coins : Seq[Int], nickels : Int = 0,
  dimes : Int = 0, quarters : Int = 0)  : Map[String, Int] = {

  if (coins.isEmpty)
    Map("nickels" -> nickels, "dimes" -> dimes,
      "quarters" -> quarters)
  else {
    if (5 == coins.head) {
      countCoins(coins.tail, nickels + 1, dimes, quarters)
    }
    else {
      if (10 == coins.head) {
        countCoins(coins.tail, nickels, dimes + 1, quarters)
      } else {
        if (25 == coins.head) {
 countCoins(coins.tail, ...