slides/caesar-encipher.html

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  28       .plaintext {
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  34       .ciphertext {
  35         background: #272822;
  36         color: #ff6666;
  37         text-shadow: 0 0 20px #333;
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  49
  50 # Caesar ciphers
  51
  52 ![centre-aligned Caesar wheel](caesarwheel1.gif)
  53
  54 Letter-by-letter enciphering
  55
  56 ---
  57
  58 layout: true
  59
  60 .indexlink[[Index](index.html)]
  61
  62 ---
  63
  64 # Enciphering and deciphering
  65
  66 ## Arithmetic on letters
  67
  68 Convert .plaintext[letter] → .plaintext[number] →
  69 .ciphertext[number] → .ciphertext[letter]
  70
  71 Functions you will need
  72
  73 ```python
  74 ord()
  75
  76 chr()
  77
  78 %
  79 ```
  80
  81 * What are good test cases?
  82
  83 ---
  84
  85 # Using the tools
  86
  87 Before doing anything, create a new branch in Git
  88
  89 * This will keep your changes isolated
  90
  91 Experiment in IPython (ephemeral, for us)
  92
  93 Once you've got something working, export the code into a `.py` file (permanent and reusable)
  94
  95 ```python
  96 from imp import reload
  97
  98 import test
  99 reload(test)
 100 from test import *
 101 ```
 102
 103 Re-evaluate the second cell to reload the file into the IPython notebook
 104
 105 When you've made progress, make a Git commit
 106
 107 * Commit early and often!
 108
 109 When you've finished, change back to `master` branch and `merge` the development branch
 110
 111 ---
 112
 113 # The [string module](http://docs.python.org/3.3/library/string.html) is your friend
 114
 115 ```python
 116 import string
 117
 118 string.ascii_letters
 119 string.ascii_lowercase
 120 string.ascii_uppercase
 121 string.digits
 122 string.punctuation
 123 ```
 124
 125 ---
 126
 127 # DRY and YAGNI
 128
 129 Is your code DRY?
 130
 131 ---
 132
 133 # Doctest
 134
 135 * Why document?
 136 * Why test?
 137
 138 ```python
 139 def caesar_encipher_letter(letter, shift):
 140     """Encipher a letter, given a shift amount
 141
 142     >>> caesar_encipher_letter('a', 1)
 143     'b'
 144     """
 145     if letter in string.ascii_letters:
 146     .
 147     .
 148     .
 149 ```
 150
 151 ---
 152
 153 # The magic doctest incantation
 154
 155 ```python
 156 if __name__ == "__main__":
 157     import doctest
 158     doctest.testmod()
 159 ```
 160
 161 ---
 162
 163 # Doing the whole message
 164
 165 ## Test-first developement
 166
 167 1. Write the tests.
 168     * They will fail. There is no code.
 169 2. Write code until the tests pass.
 170 3. Refactor.
 171
 172 ---
 173
 174 # Doing the whole message
 175
 176 ## Abysmal
 177
 178 ```python
 179 ciphertext = ''
 180 for i in range(len(plaintext)):
 181     ciphertext += caesar_encipher_letter(plaintext[i], key)
 182 ```
 183
 184 Try it in IPython
 185
 186 ---
 187
 188 # Doing the whole message
 189
 190 ## Bad
 191
 192 ```python
 193 ciphertext = ''
 194 for p in plaintext:
 195     ciphertext += caesar_encipher_letter(p, key)
 196 ```
 197
 198 ...but easily generalisable
 199
 200 ---
 201
 202 # Doing the whole message
 203
 204 ## Good (but unPythonic)
 205
 206 ```python
 207 ciphertext = map(lambda p: caesar_encipher_letter(p, key), plaintext)
 208 ```
 209
 210 ---
 211
 212 # Doing the whole message
 213
 214 ## Best
 215
 216 ```python
 217 ciphertext = [caesar_encipher_letter(p, key) for p in plaintext]
 218 ```
 219 ---
 220
 221 # Not all iterables are equal
 222
 223 ```python
 224 ''.join()
 225 ```
 226
 227 You'll be doing this a lot, so define a couple of utility functions:
 228
 229 ```python
 230 cat = ''.join
 231 wcat = ' '.join
 232 ```
 233
 234 `cat` after the Unix command (_concatenate_ files), `wcat` for _word concatenate_.
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