slides/caesar-encipher.html

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  34       .ciphertext {
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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
 111 ---
 112
 113 # The magic doctest incantation
 114
 115 ```python
 116 if __name__ == "__main__":
 117     import doctest
 118     doctest.testmod()
 119 ```
 120
 121 ---
 122
 123 # Doing all the letters
 124
 125 ## Test-first developement
 126
 127 1. Write the tests.
 128     * They will fail. There is no code.
 129 2. Write code until the tests pass.
 130 3. Refactor.
 131
 132 ---
 133
 134 # Doing all the letters
 135
 136 ## Abysmal
 137
 138 ```python
 139 ciphertext = ''
 140 for i in range(len(plaintext)):
 141     ciphertext += caesar_encipher_letter(plaintext[i], key)
 142 ```
 143
 144 ---
 145
 146 # Doing all the letters
 147
 148 ## Bad
 149
 150 ```python
 151 ciphertext = ''
 152 for p in plaintext:
 153     ciphertext += caesar_encipher_letter(p, key)
 154 ```
 155
 156 ...but easily generalisable
 157
 158 ---
 159
 160 # Doing all the letters
 161
 162 ## Good (but unPythonic)
 163
 164 ```python
 165 ciphertext = map(lambda p: caesar_encipher_letter(p, key), plaintext)
 166 ```
 167
 168 ---
 169
 170 # Doing all the letters
 171
 172 ## Best
 173
 174 ```python
 175 ciphertext = [caesar_encipher_letter(p, key) for p in plaintext]
 176 ```
 177 ---
 178
 179 # Not all iterables are equal
 180
 181 ```python
 182 ''.join()
 183 ```
 184
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