Create an implementation of the rotational cipher, also sometimes called the Caesar cipher.
The Caesar cipher is a simple shift cipher that relies on
transposing all the letters in the alphabet using an integer key
between 0
and 26
. Using a key of 0
or 26
will always yield
the same output due to modular arithmetic. The letter is shifted
for as many values as the value of the key.
The general notation for rotational ciphers is ROT + <key>
.
The most commonly used rotational cipher is ROT13
.
A ROT13
on the Latin alphabet would be as follows:
Plain: abcdefghijklmnopqrstuvwxyz
Cipher: nopqrstuvwxyzabcdefghijklm
It is stronger than the Atbash cipher because it has 27 possible keys, and 25 usable keys.
Ciphertext is written out in the same formatting as the input including spaces and punctuation.
omg
gives trl
c
gives c
Cool
gives Cool
The quick brown fox jumps over the lazy dog.
gives Gur dhvpx oebja sbk whzcf bire gur ynml qbt.
Gur dhvpx oebja sbk whzcf bire gur ynml qbt.
gives The quick brown fox jumps over the lazy dog.
Sometimes it is necessary to raise an exception. When you do this, you should 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. Not every exercise will require you to raise an exception, but for those that do, the tests will only pass if you include a message.
To raise a message with an exception, just write it as an argument to the exception type. For example, instead of
raise Exception
, you should write:
raise Exception("Meaningful message indicating the source of the error")
To run the tests, run the appropriate command below (why they are different):
py.test rotational_cipher_test.py
pytest rotational_cipher_test.py
Alternatively, you can tell Python to run the pytest module (allowing the same command to be used regardless of Python version):
python -m pytest rotational_cipher_test.py
pytest
options-v
: enable verbose output-x
: stop running tests on first failure--ff
: run failures from previous test before running other test casesFor other options, see python -m pytest -h
Note that, when trying to submit an exercise, make sure the solution is in the $EXERCISM_WORKSPACE/python/rotational-cipher
directory.
You can find your Exercism workspace by running exercism debug
and looking for the line that starts with Workspace
.
For more detailed information about running tests, code style and linting, please see the help page.
Wikipedia https://en.wikipedia.org/wiki/Caesar_cipher
It's possible to submit an incomplete solution so you can see how others have completed the exercise.
import unittest
import rotational_cipher
# Tests adapted from `problem-specifications//canonical-data.json` @ v1.2.0
class RotationalCipherTest(unittest.TestCase):
def test_rotate_a_by_0(self):
self.assertEqual(rotational_cipher.rotate('a', 0), 'a')
def test_rotate_a_by_1(self):
self.assertEqual(rotational_cipher.rotate('a', 1), 'b')
def test_rotate_a_by_26(self):
self.assertEqual(rotational_cipher.rotate('a', 26), 'a')
def test_rotate_m_by_13(self):
self.assertEqual(rotational_cipher.rotate('m', 13), 'z')
def test_rotate_n_by_13_with_wrap_around_alphabet(self):
self.assertEqual(rotational_cipher.rotate('n', 13), 'a')
def test_rotate_capital_letters(self):
self.assertEqual(rotational_cipher.rotate('OMG', 5), 'TRL')
def test_rotate_spaces(self):
self.assertEqual(rotational_cipher.rotate('O M G', 5), 'T R L')
def test_rotate_numbers(self):
self.assertEqual(
rotational_cipher.rotate('Testing 1 2 3 testing', 4),
'Xiwxmrk 1 2 3 xiwxmrk')
def test_rotate_punctuation(self):
self.assertEqual(
rotational_cipher.rotate("Let's eat, Grandma!", 21),
"Gzo'n zvo, Bmviyhv!")
def test_rotate_all_letters(self):
self.assertEqual(
rotational_cipher.rotate("The quick brown fox jumps"
" over the lazy dog.", 13),
"Gur dhvpx oebja sbk whzcf bire gur ynml qbt.")
if __name__ == '__main__':
unittest.main()
def rotate(text, key):
rotated = ""
lower_set = ""
# populate a the cipher loop
for i in range(ord('a'), ord('z') + 1):
lower_set += chr(i)
# rotate each key in string
for textchar in text:
newchar = ""
for index, setchar in enumerate(lower_set):
if textchar == setchar:
newchar = lower_set[(index + key) % len(lower_set)]
break
elif textchar == setchar.upper():
newchar = lower_set[(index + key) % len(lower_set)].upper()
break
if newchar:
rotated += newchar
else:
rotated += textchar
return rotated
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