Caesar cipher

# Caesar ciphers

![centre-aligned Caesar wheel](caesarwheel1.gif)

Letter-by-letter enciphering

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layout: true

.indexlink[[Index](index.html)]

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# Enciphering and deciphering

## Arithmetic on letters

Convert .plaintext[letter] → .plaintext[number] → 
.ciphertext[number] → .ciphertext[letter]

Functions you will need

```python
ord()

chr()

%
```

* What are good test cases?

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# Using the tools

Before doing anything, create a new branch in Git

* This will keep your changes isolated

Experiment in IPython (ephemeral, for us)

Once you've got something working, copy the code into a `.py` file (permanent and reusable)

```python
from imp import reload

import test
reload(test)
from test import *
```

Re-evaluate the second cell to reload the file into the IPython notebook

When you've made progress, make a Git commit

* Commit early and often!

When you've finished, change back to `master` branch and `merge` the development branch

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# The [string module](http://docs.python.org/3.3/library/string.html) is your friend

```python
import string

string.ascii_letters
string.ascii_lowercase
string.ascii_uppercase
string.digits
string.punctuation
```

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# DRY and YAGNI

Is your code DRY?

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# Doctest

* Why document?
* Why test?

```python
def caesar_encipher_letter(letter, shift):
    """Encipher a letter, given a shift amount

>>> caesar_encipher_letter('a', 1)
    'b'
    """
    if letter in string.ascii_letters:
    .
    .
    .
```

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# The magic doctest incantation

```python
if __name__ == "__main__":
    import doctest
    doctest.testmod()
```

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# Doing the whole message

## Test-first developement

1. Write the tests.
    * They will fail. There is no code.
2. Write code until the tests pass.
3. Refactor.

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# Doing the whole message

## Abysmal

```python
ciphertext = ''
for i in range(len(plaintext)):
    ciphertext += caesar_encipher_letter(plaintext[i], key)
```

Try it in IPython

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# Doing the whole message

## Bad

```python
ciphertext = ''
for p in plaintext:
    ciphertext += caesar_encipher_letter(p, key)
```

...but easily generalisable

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# Doing the whole message

## Good (but unPythonic)

```python
ciphertext = map(lambda p: caesar_encipher_letter(p, key), plaintext)
```

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# Doing the whole message

## Best

```python
ciphertext = [caesar_encipher_letter(p, key) for p in plaintext]
```
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# Not all iterables are equal

```python
''.join()
```