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   9 <title>section 3.6: Loops -- Do-while</title>
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  15 <H2>section 3.6: Loops -- Do-while</H2>
  16
  17 page 63
  18 <p>Note the semicolon following the parenthesized expression
  19 in the <TT>do</TT>-<TT>while</TT> loop;
  20 it's a required part of the syntax.
  21 </p><p>Make sure you understand the difference between a
  22 <TT>while</TT> loop and a <TT>do</TT>-<TT>while</TT> loop.
  23 A <TT>while</TT> loop executes strictly according to its
  24 conditional expression:
  25 if the expression is never true,
  26 the loop executes zero times.
  27 The <TT>do</TT>-<TT>while</TT> loop,
  28 on the other hand,
  29 makes an initial ``no peek'' foray through the loop body no
  30 matter what.
  31 </p><p>To see the difference,
  32 let's imagine three different ways of writing the loop in the
  33 <TT>itoa</TT> function on page 64.
  34 Suppose we somehow forgot to use a termination condition at all,
  35 and wrote something like
  36 <pre>   for(;;) {
  37                 s[i++] = n % 10 + '0';
  38                 n /= 10;
  39         }
  40 </pre>Eventually, <TT>n</TT> becomes zero, but we keep going around the loop,
  41 and we convert a number like 123 into a string like
  42 <TT>"0000000000123"</TT>, except with an infinite number of
  43 leading zeroes.
  44 (Mathematically, this is correct, but it's not what we want here,
  45 especially if we want our program to use a finite amount of
  46 time and space.)
  47 </p><p>Our next attempt might be
  48 <pre>   while(n &gt; 0) {
  49                 s[i++] = n % 10 + '0';
  50                 n /= 10;
  51         }
  52 </pre>so that we stop creating digits when <TT>n</TT> reaches 0.
  53 This works fine for positive numbers,
  54 but for 0, it stops too soon:
  55 it would convert the number 0 to the empty string <TT>""</TT>.
  56 That's why the <TT>do</TT>-<TT>while</TT> loop is appropriate here;
  57 the fact that it always makes at least one pass through the loop
  58 makes sure that we always generate at least one digit,
  59 even it it's 0.
  60 </p><p>(It's also useful to look at the invariants in this loop:
  61 during each trip through the loop,
  62 <TT>n</TT> contains the rest of the number we have to convert,
  63 <TT>s[]</TT> contains the digits we've already converted,
  64 and <TT>i</TT> points at the next cell in <TT>s[]</TT> which is to receive a digit.
  65 Each trip through the loop converts one digit,
  66 increments <TT>i</TT>,
  67 and divides <TT>n</TT> by 10.)
  68 </p><hr>
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  77 This page by <a href="http://www.eskimo.com/~scs/">Steve Summit</a>
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