slides/caesar-encipher.html

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  44
  45 # Caesar ciphers
  46
  47 ![centre-aligned Caesar wheel](caesarwheel1.gif)
  48
  49 Letter-by-letter enciphering
  50
  51 ---
  52
  53 # Enciphering and deciphering
  54
  55 ## Arithmetic on letters
  56
  57 Convert .plaintext[letter] → .plaintext[number] →
  58 .ciphertext[number] → .ciphertext[letter]
  59
  60 Functions you will need
  61
  62 ```python
  63 ord()
  64
  65 chr()
  66
  67 %
  68 ```
  69
  70 * What are good test cases?
  71
  72 ---
  73
  74 # The [string module](http://docs.python.org/3.3/library/string.html) is your friend
  75
  76 ```python
  77 import string
  78
  79 string.ascii_letters
  80 string.ascii_lowercase
  81 string.ascii_uppercase
  82 string.digits
  83 string.punctuation
  84 ```
  85
  86 ---
  87 # DRY and YAGNI
  88
  89 Is your code DRY?
  90
  91 ---
  92
  93 # Doctest
  94
  95 * Why document?
  96 * Why test?
  97
  98 ```python
  99 def caesar_encipher_letter(letter, shift):
 100     """Encipher a letter, given a shift amount
 101
 102     >>> caesar_encipher_letter('a', 1)
 103     'b'
 104     """
 105     if letter in string.ascii_letters:
 106 ```
 107
 108 ---
 109
 110 # My tests
 111
 112 ```python
 113 def caesar_encipher_letter(letter, shift):
 114     """Encipher a letter, given a shift amount
 115
 116     >>> caesar_encipher_letter('a', 1)
 117     'b'
 118     >>> caesar_encipher_letter('a', 2)
 119     'c'
 120     >>> caesar_encipher_letter('b', 2)
 121     'd'
 122     >>> caesar_encipher_letter('x', 2)
 123     'z'
 124     >>> caesar_encipher_letter('y', 2)
 125     'a'
 126     >>> caesar_encipher_letter('z', 2)
 127     'b'
 128     >>> caesar_encipher_letter('z', -1)
 129     'y'
 130     >>> caesar_encipher_letter('a', -1)
 131     'z'
 132     """
 133     if letter in string.ascii_letters:
 134 ```
 135 ---
 136
 137 # The magic doctest incantation
 138
 139 ```python
 140 if __name__ == "__main__":
 141     import doctest
 142     doctest.testmod()
 143 ```
 144
 145 ---
 146
 147 # Doing all the letters
 148
 149 ## Test-first developement
 150
 151 1. Write the tests.
 152     * They will fail. There is no code.
 153 2. Write code until the tests pass.
 154 3. Refactor.
 155
 156 ---
 157
 158 # Doing all the letters
 159
 160 ## Abysmal
 161
 162 ```python
 163 ciphertext = ''
 164 for i in range(len(plaintext)):
 165     ciphertext += caesar_encipher_letter(plaintext[i], key)
 166 ```
 167
 168 ---
 169
 170 # Doing all the letters
 171
 172 ## Bad
 173
 174 ```python
 175 ciphertext = ''
 176 for p in plaintext:
 177     ciphertext += caesar_encipher_letter(p, key)
 178 ```
 179
 180 ...but easily generalisable
 181
 182 ---
 183
 184 # Doing all the letters
 185
 186 ## Good (but unPythonic)
 187
 188 ```python
 189 ciphertext = map(lambda p: caesar_encipher_letter(p, key), plaintext)
 190 ```
 191
 192 ---
 193
 194 # Doing all the letters
 195
 196 ## Best
 197
 198 ```python
 199 ciphertext = [caesar_encipher_letter(p, key) for p in plaintext]
 200 ```
 201 ---
 202
 203 # Not all iterables are equal
 204
 205 ```python
 206 ''.join()
 207 ```
 208
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