Representing Characters

Characters must somehow be represented in the computer using the 1's and 0's that the hardware works with. How they are represented is arbitrary, but to make sure that characters used on one computer are understood correctly by another, there are standards defined.

The most common standard is called ASCII (American Standard Code for Information Interchange).

In ASCII, there are 128 characters to be represented, so each character is stored in one byte. Here is one representation of the ASCII character set (also see the text):

     Decimal - Character

     |  0 NUL|  1 ^A |  2 ^B |  3 ^C |  4 ^D |  5 ^E |  6 ^F |  7 ^G |

     |  8 ^H |  9 ^I | 10 ^J | 11 ^K | 12 ^L | 13 ^M | 14 ^N | 15 ^O |

     | 16 ^P | 17 ^Q | 18 ^R | 19 ^S | 20 ^T | 21 ^U | 22 ^V | 23 ^W |

     | 24 ^X | 25 ^Y | 26 ^Z | 27 ^[ | 28 ^\ | 29 ^] | 30 ^^ | 31 ^- |

     | 32 SP | 33  ! | 34  " | 35  # | 36  $ | 37  % | 38  & | 39  ' |

     | 40  ( | 41  ) | 42  * | 43  + | 44  , | 45  - | 46  . | 47  / |

     | 48  0 | 49  1 | 50  2 | 51  3 | 52  4 | 53  5 | 54  6 | 55  7 |

     | 56  8 | 57  9 | 58  : | 59  ; | 60  < | 61  = | 62  > | 63  ? |

     | 64  @ | 65  A | 66  B | 67  C | 68  D | 69  E | 70  F | 71  G |

     | 72  H | 73  I | 74  J | 75  K | 76  L | 77  M | 78  N | 79  O |

     | 80  P | 81  Q | 82  R | 83  S | 84  T | 85  U | 86  V | 87  W |

     | 88  X | 89  Y | 90  Z | 91  [ | 92  \ | 93  ] | 94  ^ | 95  _ |

     | 96  ` | 97  a | 98  b | 99  c |100  d |101  e |102  f |103  g |

     |104  h |105  i |106  j |107  k |108  l |109  m |110  n |111  o |

     |112  p |113  q |114  r |115  s |116  t |117  u |118  v |119  w |

     |120  x |121  y |122  z |123  { |124  | |125  } |126  ~ |127 DEL|

We can divide the character set into different classes:

• Alphabetic characters including:
  • lower case letters - 'a'..'z'
  • upper case letters - 'A'..'Z'
• Digit characters - '0'..'9'
• Punctuation characters - '.',';',',', etc.
• White space characters - ' ','\n','\t', etc.
• Special characters - '@',' < ', ' >', etc.
• Control characters - '^A','^B', etc.

Each character has a numeric representation. How can we find that out?

     printf("The numeric representation of %c is %d\n",'a','a');

The characters whose ASCII values are between 32 and 126 are said to be printable characters. The others have special meanings, for example, '^J' is the newline ('\n') character in Unix, '^I' is the tab ('\t') character.

The mapping of integer values to characters has some interesting and useful properties. We can use this to build a library of character utilities.