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   9 <title>2.2 Constants</title>
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  15 <H2>2.2 Constants</H2>
  16
  17 <p>[This section corresponds to K&amp;R Sec. 2.3]
  18 </p><p></p><p>A <dfn>constant</dfn> is just an immediate, absolute value
  19 found in an expression.
  20 The simplest constants are decimal integers,
  21 e.g. <TT>0</TT>, <TT>1</TT>, <TT>2</TT>, <TT>123</TT>
  22 .
  23 Occasionally it is useful to specify constants in base 8 or base 16
  24 (octal or hexadecimal);
  25 this is done by prefixing an extra <TT>0</TT> (zero) for octal,
  26 or <TT>0x</TT> for hexadecimal:
  27 the constants <TT>100</TT>, <TT>0144</TT>, and <TT>0x64</TT>
  28 all represent the same number.
  29 (If you're not using these non-decimal constants,
  30 just remember not to use any leading zeroes.
  31 If you accidentally write <TT>0123</TT>
  32 intending to get one hundred and twenty three,
  33 you'll get 83 instead, which is
  34 123 base 8.)
  35 </p><p>We write constants in decimal, octal, or hexadecimal
  36 for our convenience,
  37 not the compiler's.
  38 The compiler doesn't care;
  39 it always converts everything into binary internally, anyway.
  40 (There is, however,
  41 no good way to specify constants in source code in binary.)
  42 </p><p>A constant can be forced to be of type <TT>long int</TT>
  43 by suffixing it with the letter <TT>L</TT>
  44 (in upper or lower case, although upper case is strongly
  45 recommended, because a lower case <TT>l</TT> looks too much like
  46 the digit <TT>1</TT>).
  47 </p><p>A constant that contains a decimal point or the letter
  48 <TT>e</TT> (or both) is a floating-point constant:
  49 <TT>3.14</TT>, <TT>10.</TT>, <TT>.01</TT>,
  50 <TT>123e4</TT>, <TT>123.456e7</TT>
  51 .
  52 The <TT>e</TT> indicates multiplication by a power of 10;
  53 <TT>123.456e7</TT>
  54 is
  55 123.456
  56 times 10 to the 7th,
  57 or
  58 1,234,560,000.
  59 (Floating-point constants are of type <TT>double</TT> by default.)
  60 </p><p>We also have constants for specifying characters and strings.
  61 (Make sure you understand the difference between a character
  62 and a string:
  63 a character is exactly one character;
  64 a string is a set of zero or more characters;
  65 a string containing one character is distinct from a lone character.)
  66 A character constant is simply a single character between
  67 single quotes: <TT>'A'</TT>, <TT>'.'</TT>, <TT>'%'</TT>.
  68 The numeric value of a character constant is,
  69 naturally enough,
  70 that character's value in the machine's character set.
  71 (In ASCII, for example, <TT>'A'</TT> has the value 65.)
  72 </p><p>A <dfn>string</dfn> is represented in C as a
  73 sequence or
  74 array of characters.
  75 (We'll have more to say about arrays in general,
  76 and strings in particular, later.)
  77 A string constant is a sequence of zero or more characters
  78 enclosed in double quotes:
  79 <TT>"apple"</TT>,
  80 <TT>"hello, world"</TT>,
  81 <TT>"this is a test"</TT>.
  82 </p><p>Within character and string constants,
  83 the backslash character <TT>\</TT> is special,
  84 and is used to represent characters
  85 not easily typed on the keyboard
  86 or for various reasons not easily typed in constants.
  87 The most common of these ``character escapes'' are:
  88 <br>
  89 <br>
  90 <pre>
  91         <TT>\n</TT>     a ``newline'' character
  92         <TT>\b</TT>     a backspace
  93         <TT>\r</TT>     a carriage return (without a line feed)
  94         <TT>\'</TT>     a single quote (e.g. in a character constant)
  95         <TT>\"</TT>     a double quote (e.g. in a string constant)
  96         <TT>\\</TT>     a single backslash
  97 </pre>
  98 </p><p>For example,
  99 <TT>"he said \"hi\""</TT> is a string constant
 100 which contains two double quotes,
 101 and
 102 <TT>'\''</TT> is a character constant consisting of a
 103 (single)
 104 single quote.
 105 Notice once again that the character constant <TT>'A'</TT>
 106 is very different from the string constant <TT>"A"</TT>.
 107
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