Exercism v3 launches on Sept 1st 2021. Learn more! ๐๐๐

Published at Jul 13 2018
·
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Instructions

Test suite

Solution

Given a number, find the sum of all the unique multiples of particular numbers up to but not including that number.

If we list all the natural numbers below 20 that are multiples of 3 or 5, we get 3, 5, 6, 9, 10, 12, 15, and 18.

The sum of these multiples is 78.

Refer to the Exercism help page for Elm installation and learning resources.

The first time you start an exercise, you'll need to ensure you have the appropriate dependencies installed.

```
$ elm-package install --yes
```

Execute the tests with:

```
$ elm-test
```

Automatically run tests again when you save changes:

```
$ elm-test --watch
```

As you work your way through the test suite, be sure to remove the `skip <|`

calls from each test until you get them all passing!

A variation on Problem 1 at Project Euler http://projecteuler.net/problem=1

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

```
module Tests exposing (..)
import Expect
import SumOfMultiples exposing (sumOfMultiples)
import Test exposing (..)
tests : Test
tests =
describe "Sum Of Multiples"
[ test "[3, 5] 15" <|
\() -> Expect.equal 45 (sumOfMultiples [ 3, 5 ] 15)
, skip <|
test "[7, 13, 17] 20" <|
\() -> Expect.equal 51 (sumOfMultiples [ 7, 13, 17 ] 20)
, skip <|
test "[4, 6] 15" <|
\() -> Expect.equal 30 (sumOfMultiples [ 4, 6 ] 15)
, skip <|
test "[5, 6, 8] 150" <|
\() -> Expect.equal 4419 (sumOfMultiples [ 5, 6, 8 ] 150)
, skip <|
test "[43, 47] 10000" <|
\() -> Expect.equal 2203160 (sumOfMultiples [ 43, 47 ] 10000)
, skip <|
test "[5, 25] 51" <|
\() -> Expect.equal 275 (sumOfMultiples [ 5, 25 ] 51)
]
```

```
{
"version": "3.0.0",
"summary": "Exercism problems in Elm.",
"repository": "https://github.com/exercism/elm.git",
"license": "BSD3",
"source-directories": [
".",
".."
],
"exposed-modules": [],
"dependencies": {
"elm-lang/core": "5.0.0 <= v < 6.0.0",
"elm-community/elm-test": "4.0.0 <= v < 5.0.0"
},
"elm-version": "0.18.0 <= v < 0.19.0"
}
```

```
module SumOfMultiples exposing (..)
import Set
sumOfMultiples: List Int -> Int -> Int
sumOfMultiples numbers maxMultiple =
numbers
|> List.concatMap (\number -> multiplesOf number maxMultiple [])
|> removeDuplicates
|> List.sum
multiplesOf: Int -> Int -> List Int -> List Int
multiplesOf baseNumber maxMultiple multiplesSoFar =
let nextMultiple = calculateNextMultiple baseNumber (List.head multiplesSoFar) in
if nextMultiple < maxMultiple then
multiplesOf baseNumber maxMultiple (nextMultiple :: multiplesSoFar)
else
multiplesSoFar
calculateNextMultiple: Int -> Maybe Int -> Int
calculateNextMultiple baseNumber previousMultiple =
baseNumber + Maybe.withDefault 0 previousMultiple
removeDuplicates: List Int -> List Int
removeDuplicates numbers =
numbers
|> Set.fromList
|> Set.toList
```

A huge amount can be learned from reading other peopleโs 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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