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   9 <title>section 2.8: Increment and Decrement Operators</title>
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  15 <H2>section 2.8: Increment and Decrement Operators</H2>
  16
  17 <p>The distinction between the prefix and postfix forms of
  18 <TT>++</TT> and <TT>--</TT>
  19 will probably
  20 seem strained at first,
  21 but it will make more sense once we begin using these operators
  22 in more realistic situations.
  23 </p><p>The authors point out that an expression like <TT>(i+j)++</TT> is illegal,
  24 and it's
  25 worth thinking for a moment about <em>why</em>.
  26 The <TT>++</TT> operator doesn't just mean ``add one'';
  27 it means ``add one <em>to a variable</em>''
  28 or ``make a variable's value one more than it was before.''
  29 But <TT>(i+j)</TT> is not a variable,
  30 it's an expression;
  31 so there's no place for <TT>++</TT> to store the incremented result.
  32 If you were bound and determined to use <TT>++</TT> here,
  33 you'd have to introduce another variable:
  34 <pre>   int k = i + j;
  35         k++;
  36 </pre>But really,
  37 when you want to add one to an expression, just use
  38 <pre>   i + j + 1
  39 </pre></p><p>Another unfortunate
  40 (and utterly meaningless)
  41 example is
  42 <pre>   i = i++;
  43 </pre>If you want to increment <TT>i</TT>
  44 (that is,
  45 add one to it,
  46 and store the result back in <TT>i</TT>),
  47 either use
  48 <pre>   i = i + 1;
  49 </pre>or
  50 <pre>   i++;
  51 </pre>Don't try to combine the two.
  52 </p><p>page 47
  53 </p><p>Deep sentence:
  54 <blockquote>In a context where no value is wanted,
  55 just the incrementing effect,
  56 as in
  57 <pre>   if(c == '\n')
  58                 nl++;
  59 </pre>prefix and postfix are the same.
  60 </blockquote>In other words,
  61 when you're just incrementing some variable,
  62 you can use either the <TT>nl++</TT> or <TT>++nl</TT> form.
  63 But when you're immediately using the result,
  64 as in the examples
  65 we'll look at later,
  66 using one or the other makes a big difference.
  67 </p><p>In that light,
  68 study
  69 one of the examples on this page--<TT>squeeze</TT>,
  70 the modified <TT>getline</TT>,
  71 or <TT>strcat</TT>--and convince yourself that it would
  72 <em>not</em> work if the wrong form of increment
  73 (<TT>++i</TT> or <TT>++j</TT>)
  74 were used.
  75 </p><p>You may note that all three examples
  76 on pages 47-48
  77 use the postfix form.
  78 Postfix increment is probably more common,
  79 though prefix definitely has its uses, too.
  80 </p><p>You may notice the keyword <TT>void</TT> popping up in a few code examples.
  81 <TT>void</TT> is a type we haven't met yet;
  82 it's a type with no values and no operations.
  83 When a function is declared as ``returning'' <TT>void</TT>,
  84 as in the <TT>squeeze</TT> and <TT>strcat</TT> examples
  85 on pages 47 and 48,
  86 it means that the function does not return a value.
  87 (This was briefly mentioned on page 30 in chapter 1.)
  88
  89 </p><hr>
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  98 This page by <a href="http://www.eskimo.com/~scs/">Steve Summit</a>
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