slides/caesar-encipher.html

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  28       .plaintext {
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  34       .ciphertext {
  35         background: #272822;
  36         color: #ff6666;
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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 # The [string module](http://docs.python.org/3.3/library/string.html) is your friend
  86
  87 ```python
  88 import string
  89
  90 string.ascii_letters
  91 string.ascii_lowercase
  92 string.ascii_uppercase
  93 string.digits
  94 string.punctuation
  95 ```
  96
  97 ---
  98 # DRY and YAGNI
  99
 100 Is your code DRY?
 101
 102 ---
 103
 104 # Doctest
 105
 106 * Why document?
 107 * Why test?
 108
 109 ```python
 110 def caesar_encipher_letter(letter, shift):
 111     """Encipher a letter, given a shift amount
 112
 113     >>> caesar_encipher_letter('a', 1)
 114     'b'
 115     """
 116     if letter in string.ascii_letters:
 117     .
 118     .
 119     .
 120 ```
 121
 122 ---
 123
 124 # The magic doctest incantation
 125
 126 ```python
 127 if __name__ == "__main__":
 128     import doctest
 129     doctest.testmod()
 130 ```
 131
 132 ---
 133
 134 # Doing all the letters
 135
 136 ## Test-first developement
 137
 138 1. Write the tests.
 139     * They will fail. There is no code.
 140 2. Write code until the tests pass.
 141 3. Refactor.
 142
 143 ---
 144
 145 # Doing all the letters
 146
 147 ## Abysmal
 148
 149 ```python
 150 ciphertext = ''
 151 for i in range(len(plaintext)):
 152     ciphertext += caesar_encipher_letter(plaintext[i], key)
 153 ```
 154
 155 ---
 156
 157 # Doing all the letters
 158
 159 ## Bad
 160
 161 ```python
 162 ciphertext = ''
 163 for p in plaintext:
 164     ciphertext += caesar_encipher_letter(p, key)
 165 ```
 166
 167 ...but easily generalisable
 168
 169 ---
 170
 171 # Doing all the letters
 172
 173 ## Good (but unPythonic)
 174
 175 ```python
 176 ciphertext = map(lambda p: caesar_encipher_letter(p, key), plaintext)
 177 ```
 178
 179 ---
 180
 181 # Doing all the letters
 182
 183 ## Best
 184
 185 ```python
 186 ciphertext = [caesar_encipher_letter(p, key) for p in plaintext]
 187 ```
 188 ---
 189
 190 # Not all iterables are equal
 191
 192 ```python
 193 ''.join()
 194 ```
 195
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