ErikSchierboom's solution to Change on the Scala track

Instructions

Test suite

Solution

Correctly determine the fewest number of coins to be given to a customer such that the sum of the coins' value would equal the correct amount of change.

For example

An input of 15 with [1, 5, 10, 25, 100] should return one nickel (5) and one dime (10) or [0, 1, 1, 0, 0]
An input of 40 with [1, 5, 10, 25, 100] should return one nickel (5) and one dime (10) and one quarter (25) or [0, 1, 1, 1, 0]

Edge cases

Does your algorithm work for any given set of coins?
Can you ask for negative change?
Can you ask for a change value smaller than the smallest coin value?

The Scala exercises assume an SBT project scheme. The exercise solution source should be placed within the exercise directory/src/main/scala. The exercise unit tests can be found within the exercise directory/src/test/scala.

To run the tests simply run the command sbt test in the exercise directory.

For more detailed info about the Scala track see the help page.

Source

Software Craftsmanship - Coin Change Kata https://web.archive.org/web/20130115115225/http://craftsmanship.sv.cmu.edu:80/exercises/coin-change-kata

Submitting Incomplete Solutions

It's possible to submit an incomplete solution so you can see how others have completed the exercise.

ChangeTest.scala

import org.scalatest.{Matchers, FunSuite}

/** @version 1.2.0 */
class ChangeTest extends FunSuite with Matchers {
  
  test("single coin change") {
    Change.findFewestCoins(25, List(1, 5, 10, 25, 100)) should be (Some(List(25)))
  }

  test("multiple coin change") {
    pending
    Change.findFewestCoins(15, List(1, 5, 10, 25, 100)) should be (Some(List(5, 10)))
  }

  test("change with Lilliputian Coins") {
    pending
    Change.findFewestCoins(23, List(1, 4, 15, 20, 50)) should be (Some(List(4, 4, 15)))
  }

  test("change with Lower Elbonia Coins") {
    pending
    Change.findFewestCoins(63, List(1, 5, 10, 21, 25)) should be (Some(List(21, 21, 21)))
  }

  test("large target values") {
    pending
    Change.findFewestCoins(999, List(1, 2, 5, 10, 20, 50, 100)) should be (Some(List(2, 2, 5, 20, 20, 50, 100, 100, 100, 100, 100, 100, 100, 100, 100)))
  }

  test("possible change without unit coins available") {
    pending
    Change.findFewestCoins(21, List(2, 5, 10, 20, 50)) should be (Some(List(2, 2, 2, 5, 10)))
  }

  test("another possible change without unit coins available") {
    pending
    Change.findFewestCoins(27, List(4, 5)) should be (Some(List(4, 4, 4, 5, 5, 5)))
  }

  test("no coins make 0 change") {
    pending
    Change.findFewestCoins(0, List(1, 5, 10, 21, 25)) should be (Some(List()))
  }

  test("error testing for change smaller than the smallest of coins") {
    pending
    Change.findFewestCoins(3, List(5, 10)) should be (None)
  }

  test("error if no combination can add up to target") {
    pending
    Change.findFewestCoins(94, List(5, 10)) should be (None)
  }

  test("cannot find negative change values") {
    pending
    Change.findFewestCoins(-5, List(1, 2, 5)) should be (None)
  }
}

object Change {
  def minimalCoins(coins: List[Int], minimalCoinsMap: Map[Int, List[Int]], target: Int): Option[List[Int]] =
    coins
      .filter { _ <= target}
      .map { coin => coin::minimalCoinsMap(target - coin) }
      .sortBy { _.length }
      .headOption

  def updateMinimalCoinsMap(coins: List[Int], minimalCoinsMap: Map[Int, List[Int]], target: Int): Map[Int, List[Int]] =
    minimalCoins(coins, minimalCoinsMap, target)
      .map { minimalCoinsMap.updated(target, _) }
      .getOrElse(minimalCoinsMap)

  def findFewestCoins(target: Int, coins: List[Int]): Option[List[Int]] =
    (1 to target)
    .foldLeft(Map((0, List[Int]()))) { (minimalCoinsMap, change) => updateMinimalCoinsMap(coins, minimalCoinsMap, change)}
    .get(target)
}

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ErikSchierboom's solution

to Change in the Scala Track