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82 lines
2.8 KiB
Markdown
82 lines
2.8 KiB
Markdown
# Collatz Conjecture
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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. If n is even, divide n by 2 to get n / 2. If n is
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odd, multiply n by 3 and add 1 to get 3n + 1. Repeat the process indefinitely.
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The conjecture states that no matter which number you start with, you will
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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. So for input n = 12, the return value would be 9.
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## Notes
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The Collatz Conjecture is only concerned with strictly positive integers, so your solution should raise a `ValueError` with a meaningful message if given 0 or a negative integer.
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## Exception messages
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Sometimes it is necessary to raise an exception. When you do this, you should include a meaningful error message to
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indicate what the source of the error is. This makes your code more readable and helps significantly with debugging. Not
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every exercise will require you to raise an exception, but for those that do, the tests will only pass if you include
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a message.
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To raise a message with an exception, just write it as an argument to the exception type. For example, instead of
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`raise Exception`, you should write:
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```python
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raise Exception("Meaningful message indicating the source of the error")
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```
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## Running the tests
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To run the tests, run the appropriate command below ([why they are different](https://github.com/pytest-dev/pytest/issues/1629#issue-161422224)):
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- Python 2.7: `py.test collatz_conjecture_test.py`
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- Python 3.4+: `pytest collatz_conjecture_test.py`
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Alternatively, you can tell Python to run the pytest module (allowing the same command to be used regardless of Python version):
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`python -m pytest collatz_conjecture_test.py`
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### Common `pytest` options
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- `-v` : enable verbose output
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- `-x` : stop running tests on first failure
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- `--ff` : run failures from previous test before running other test cases
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For other options, see `python -m pytest -h`
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## Submitting Exercises
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Note that, when trying to submit an exercise, make sure the solution is in the `$EXERCISM_WORKSPACE/python/collatz-conjecture` directory.
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You can find your Exercism workspace by running `exercism debug` and looking for the line that starts with `Workspace`.
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For more detailed information about running tests, code style and linting,
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please see [Running the Tests](http://exercism.io/tracks/python/tests).
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## Source
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An unsolved problem in mathematics named after mathematician Lothar Collatz [https://en.wikipedia.org/wiki/3x_%2B_1_problem](https://en.wikipedia.org/wiki/3x_%2B_1_problem)
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## Submitting Incomplete Solutions
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It's possible to submit an incomplete solution so you can see how others have completed the exercise.
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