Published at Apr 05 2019
·
2 comments

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.

- 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]

- 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.

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

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

```
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 {
type Change = Option[List[Int]]
def findFewestCoins(totalPay: Int, coins: List[Int]): Change = {
@annotation.tailrec
def loop(pay: Int, resM: Map[Int, Change]): Map[Int, Change] = {
if (pay > totalPay) resM else {
loop(pay + 1, resultFor(pay, resM))
}
}
def resultFor(pay: Int, resM: Map[Int, Change]): Map[Int, Change] = {
val results = coins.filter(_ <= pay)
.foldLeft(List[Change]())((cs, coin) => resM(pay - coin).map(coin :: _) :: cs)
val result = if (results.flatten.isEmpty) None else Some(results.flatten.minBy(_.length).sorted)
resM + (pay -> result)
}
if (totalPay < 0) None else
loop(1, Map(0 -> Some(List())))(totalPay)
}
}
```

A huge amount can be learned from reading other people’s code. This is why we wanted to give exercism users the option of making their solutions public.

Here are some questions to help you reflect on this solution and learn the most from it.

- What compromises have been made?
- Are there new concepts here that you could read more about to improve your understanding?

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## Community comments

I really like your solution!!! I'm not clear why instruction

needs

`(totalPay)`

(checking method`loop`

the instruction is redundant, but without this magic compiler becomes crazy and it doesn't work. Could you please help me clarify how`(totalPay)`

is required in method call?Looks like

`totalPay`

is a map Key. So this returns the matching map Value, which is an`Option[List[Int]]`

. Without it, the statement would return the whole`Map[Int, Option[List[Int]]`

, which results in a type mismatch.