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Published at Jun 10 2020
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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 Installing Elm page for information about installing elm.

The code you have to write is located inside the `src/`

directory of the exercise.
Elm automatically installs packages dependencies the first time you run the tests
so we can start by running the tests from the exercise directory with:

```
$ elm-test
```

To automatically run tests again when you save changes:

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

As you work your way through the tests suite in the file `tests/Tests.elm`

,
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 is possible to submit an incomplete solution so you can see how others have completed the exercise.

```
module Tests exposing (tests)
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)
]
```

```
module SumOfMultiples exposing (sumOfMultiples)
import Set
sumOfMultiples : List Int -> Int -> Int
sumOfMultiples divisors limit =
let
multiples : List (List Int)
multiples =
List.map (getMultiples limit) divisors
digits =
List.map2 evaluateValues divisors multiples
in
-- use Set to remove dups from lists
Set.foldr (+) 0 <|
Set.fromList <|
List.concat digits
getMultiples : Int -> Int -> List Int
getMultiples limit num =
let
max =
limit // num
range =
List.range 1
in
-- we need to check if the value is evenly divisble by the limit
-- if so, the set of mupltiples is not inclusive of the final number
if remainderBy num limit == 0 then
range <| decrement max
else
range max
evaluateValues : Int -> List Int -> List Int
evaluateValues divisor list =
List.map ((*) divisor) list
decrement val =
val - 1
```

small refactors, fun with currying and math

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