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   9 <title>18.2.5: The Conditional Operator</title>
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  15 <H3>18.2.5: The Conditional Operator</H3>
  16
  17 <p>[This section corresponds to K&amp;R Sec. 2.11]
  18 </p><p>C has one last operator which we haven't seen yet.
  19 It's called the
  20 conditional or ``ternary'' or <TT>?:</TT> operator,
  21 and in action it looks something like this:
  22 <pre>
  23         average = (n &gt; 0) ? sum / n : 0
  24 </pre>
  25 </p><p>The syntax of the conditional operator is
  26 <pre>
  27         <I>e1</I> ? <I>e2</I> : <I>e3</I>
  28 </pre>
  29 and what happens is that <I>e1</I> is evaluated,
  30 and if it's true then <I>e2</I> is evaluated
  31 and becomes the result of the expression,
  32 otherwise
  33 <I>e3</I> is evaluated and becomes the result of the expression.
  34 In other words,
  35 the conditional expression is
  36 sort of an <TT>if</TT>/<TT>else</TT> statement
  37 buried inside of an expression.
  38 The above computation of <TT>average</TT> could be written out
  39 in a longer form using an <TT>if</TT> statement:
  40 <pre>
  41         if(n &gt; 0)
  42                 average = sum / n;
  43         else    average = 0;
  44 </pre>
  45 </p><p>The conditional operator,
  46 however,
  47 forms an expression
  48 and can therefore be used wherever an expression can be used.
  49 This makes it more convenient to use
  50 when an <TT>if</TT> statement
  51 would otherwise cause other
  52 sections
  53 of code to be needlessly repeated.
  54 For example,
  55 suppose we were trying to write a complicated function call
  56 <pre>
  57         func(a, b + 1, c + d, <I>xx</I>, (g + h + i) / 2);
  58 </pre>
  59 where <I>xx</I> was supposed to be <TT>f</TT> if <TT>e</TT> was true
  60 and 0 if it was not.
  61 Using an <TT>if</TT> statement, we'd have to write:
  62 <pre>
  63         if(e)
  64                 func(a, b + 1, c + d, f, (g + h + i) / 2);
  65         else    func(a, b + 1, c + d, 0, (g + h + i) / 2);
  66 </pre>
  67 We could write this more compactly, more readably, and more safely
  68 (it's easier both to see and to guarantee
  69 that the other arguments are always the same)
  70 by writing
  71 <pre>
  72         func(a, b + 1, c + d, e ? f : 0, (g + h + i) / 2);
  73 </pre>
  74 </p><p>(The obscure name ``ternary,'' by the way,
  75 comes from the fact that the conditional operator
  76 is neither unary nor binary;
  77 it takes <em>three</em> operands.)
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