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daewon's solution

to Change in the Scala Track

Published at Jul 13 2018 · 1 comment
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 findFewestCoins(_amount: Int, _totalCoins: List[Int]): Option[List[Int]] = {
    def recur(remain: Int, coins: List[Int], used: List[Int]): List[Int] = {
      if (remain == 0) used
      else if (coins.isEmpty || remain < 0) Nil
      else {
        val coin :: remains = coins

        val coinUsed = recur(remain - coin, coins, coin :: used)
        val coinNotUsed = recur(remain, remains, used)

        (coinUsed, coinNotUsed) match {
          case (Nil, b) => b
          case (a, Nil) => a
          case (a, b) => if (a.length > b.length) b else a
        }
      }
    }

    if (_amount == 0) Some(Nil)
    else recur(_amount, _totalCoins, Nil) match {
      case Nil => None
      case coins => Some(coins.sorted)
    }
  }
}

Community comments

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Avatar of daewon
daewon
Solution Author
commented about 3 years ago

Naive solution. It can't pass largest TC, I will fix it later.

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