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Published at Apr 19 2021
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Instructions

Test suite

Solution

The Collatz Conjecture or 3x+1 problem can be summarized as follows:

Take any positive integer n. If n is even, divide n by 2 to get n / 2. If n is odd, multiply n by 3 and add 1 to get 3n + 1. Repeat the process indefinitely. The conjecture states that no matter which number you start with, you will always reach 1 eventually.

Given a number n, return the number of steps required to reach 1.

Starting with n = 12, the steps would be as follows:

- 12
- 6
- 3
- 10
- 5
- 16
- 8
- 4
- 2
- 1

Resulting in 9 steps. So for input n = 12, the return value would be 9.

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!

An unsolved problem in mathematics named after mathematician Lothar Collatz https://en.wikipedia.org/wiki/3x_%2B_1_problem

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

```
module Tests exposing (tests)
import CollatzConjecture exposing (collatz)
import Expect
import Test exposing (..)
tests : Test
tests =
describe "CollatzConjecture tests"
[ test "zero steps for one" <|
\() ->
Expect.equal (Ok 0) (collatz 1)
-- Once you get the first test passing, remove the
-- `skip <|` (just leave the comma) on the next
-- lines to continue!
, skip <|
test "divide if even" <|
\() ->
Expect.equal (Ok 4) (collatz 16)
, skip <|
test "even and odd step" <|
\() ->
Expect.equal (Ok 9) (collatz 12)
, skip <|
test "Large number of even and odd step" <|
\() ->
Expect.equal (Ok 152) (collatz 1000000)
, skip <|
test "zero is an error" <|
\() ->
Expect.equal (Err "Only positive numbers are allowed") (collatz 0)
, skip <|
test "negative values is an error" <|
\() ->
Expect.equal (Err "Only positive numbers are allowed") (collatz -15)
]
```

```
module CollatzConjecture exposing (collatz)
-- from https://exercism.io/tracks/elm/exercises/collatz-conjecture/solutions/86afd7251cc2422092097e0588adb5db
collatz : Int -> Result String Int
collatz start =
if start <= 0 then
Err "Only positive numbers are allowed"
else
Ok (collazPos start)
collazPos n =
if n == 1 then
0
else
1
+ (if isEven n then
collazPos (n // 2)
else
collazPos (3 * n + 1)
)
isEven n =
modBy 2 n == 0
isOdd n =
not (isEven n)
```

A huge amount can be learned from reading other peopleâ€™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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