70 lines
2.2 KiB
Markdown
70 lines
2.2 KiB
Markdown
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# Collatz Conjecture
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Welcome to Collatz Conjecture on Exercism's Python Track.
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If you need help running the tests or submitting your code, check out `HELP.md`.
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## Instructions
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The Collatz Conjecture or 3x+1 problem can be summarized as follows:
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Take any positive integer n.
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If n is even, divide n by 2 to get n / 2.
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If n is odd, multiply n by 3 and add 1 to get 3n + 1.
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Repeat the process indefinitely.
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The conjecture states that no matter which number you start with, you will always reach 1 eventually.
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Given a number n, return the number of steps required to reach 1.
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## Examples
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Starting with n = 12, the steps would be as follows:
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0. 12
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1. 6
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2. 3
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3. 10
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4. 5
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5. 16
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6. 8
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7. 4
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8. 2
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9. 1
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Resulting in 9 steps.
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So for input n = 12, the return value would be 9.
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## Exception messages
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Sometimes it is necessary to [raise an exception](https://docs.python.org/3/tutorial/errors.html#raising-exceptions). When you do this, you should always include a **meaningful error message** to indicate what the source of the error is. This makes your code more readable and helps significantly with debugging. For situations where you know that the error source will be a certain type, you can choose to raise one of the [built in error types](https://docs.python.org/3/library/exceptions.html#base-classes), but should still include a meaningful message.
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The Collatz Conjecture is only concerned with **strictly positive integers**, so this exercise expects you to use the [raise statement](https://docs.python.org/3/reference/simple_stmts.html#the-raise-statement) and "throw" a `ValueError` in your solution if the given value is zero or a negative integer. The tests will only pass if you both `raise` the `exception` and include a message with it.
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To raise a `ValueError` with a message, write the message as an argument to the `exception` type:
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```python
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# example when argument is zero or a negative integer
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raise ValueError("Only positive integers are allowed")
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```
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## Source
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### Created by
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- @zwaltman
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### Contributed to by
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- @BethanyG
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- @cmccandless
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- @Dog
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- @ikhadykin
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- @K4cePhoenix
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- @N-Parsons
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- @smt923
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- @Stigjb
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- @thomasjpfan
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- @tqa236
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### Based on
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An unsolved problem in mathematics named after mathematician Lothar Collatz - https://en.wikipedia.org/wiki/3x_%2B_1_problem
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