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81 lines
3.1 KiB
Markdown
81 lines
3.1 KiB
Markdown
# Rotational Cipher
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Create an implementation of the rotational cipher, also sometimes called the Caesar cipher.
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The Caesar cipher is a simple shift cipher that relies on
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transposing all the letters in the alphabet using an integer key
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between `0` and `26`. Using a key of `0` or `26` will always yield
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the same output due to modular arithmetic. The letter is shifted
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for as many values as the value of the key.
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The general notation for rotational ciphers is `ROT + <key>`.
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The most commonly used rotational cipher is `ROT13`.
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A `ROT13` on the Latin alphabet would be as follows:
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```text
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Plain: abcdefghijklmnopqrstuvwxyz
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Cipher: nopqrstuvwxyzabcdefghijklm
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```
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It is stronger than the Atbash cipher because it has 27 possible keys, and 25 usable keys.
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Ciphertext is written out in the same formatting as the input including spaces and punctuation.
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## Examples
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- ROT5 `omg` gives `trl`
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- ROT0 `c` gives `c`
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- ROT26 `Cool` gives `Cool`
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- ROT13 `The quick brown fox jumps over the lazy dog.` gives `Gur dhvpx oebja sbk whzcf bire gur ynml qbt.`
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- ROT13 `Gur dhvpx oebja sbk whzcf bire gur ynml qbt.` gives `The quick brown fox jumps over the lazy dog.`
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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 rotational_cipher_test.py`
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- Python 3.4+: `pytest rotational_cipher_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 rotational_cipher_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/rotational-cipher` 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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Wikipedia [https://en.wikipedia.org/wiki/Caesar_cipher](https://en.wikipedia.org/wiki/Caesar_cipher)
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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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