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      <font size="6" face="Verdana, Arial, Helvetica, sans-serif"><b> Loops</b></font>
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<p>So far we have introduced a couple of EBNF operators that deal with looping. 
  We have the <tt>+</tt> positive operator, which matches the preceding symbol 
  one (1) or more times, as well as the Kleene star <tt>*</tt> which matches the 
  preceding symbol zero (0) or more times.</p>
<p>Taking this further, we may want to have a generalized loop operator. To some 
  this may seem to be a case of overkill. Yet there are grammars that are impractical 
  and cumbersome, if not impossible, for the basic EBNF iteration syntax to specify. 
  Examples:</p>
<blockquote>
  <p><img src="theme/bullet.gif" width="12" height="12"> A file name may have 
    a maximum of 255 characters only.<br>
    <img src="theme/bullet.gif" width="12" height="12"> A specific bitmap file 
    format has exactly 4096 RGB color information. <br>
    <img src="theme/bullet.gif" width="12" height="12"> A 32 bit binary string 
    (1..32 1s or 0s).</p>
</blockquote>
<p>Other than the Kleene star <tt>*</tt>, the Positive closure <tt>+</tt>, and 
  the optional <tt>!</tt>, a more flexible mechanism for looping is provided for 
  by the framework. <br>
</p>
<table width="80%" border="0" align="center">
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    <td colspan="2" class="table_title">Loop Constructs</td>
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    <td class="table_cells" width="26%"><b>repeat_p (n) [p]</b></td>
    <td class="table_cells" width="74%">Repeat <b>p</b> exactly <b>n</b> times</td>
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    <td class="table_cells" width="26%"><b>repeat_p (n1, n2) [p]</b></td>
    <td class="table_cells" width="74%">Repeat <b>p</b> at least <b>n1</b> times 
      and at most <b>n2</b> times</td>
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    <td class="table_cells" width="26%"><b>repeat_p (n, more) [p] </b></td>
    <td class="table_cells" width="74%">Repeat <b>p</b> at least <b>n</b> times, 
      continuing until <b>p</b> fails or the input is consumed</td>
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<p>Using the <tt>repeat_p</tt> parser, we can now write our examples above:</p>
<p>A file name with a maximum of 255 characters:<br>
</p>
<pre>    <span class=identifier>valid_fname_chars </span><span class=special>= </span><span class=comment>/*..*/</span><span class=special>;
    </span><span class=identifier>filename </span><span class=special>= </span><span class=identifier>repeat_p</span><span class=special>(</span><span class=number>1</span><span class=special>, </span><span class=number>255</span><span class=special>)[</span><span class=identifier>valid_fname_chars</span><span class=special>];</span></pre>
<p>A specific bitmap file format which has exactly 4096 RGB color information:<span class=special><br>
  </span></p>
<pre>    <span class=identifier>uint_parser</span><span class=special>&lt;</span><span class=keyword>unsigned</span><span class=special>, </span><span class=number>16</span><span class=special>, </span><span class=number>6</span><span class=special>, </span><span class=number>6</span><span class=special>&gt; </span><span class=identifier>rgb_p</span><span class=special>;
    </span><span class=identifier>bitmap </span><span class=special>= </span><span class=identifier>repeat_p</span><span class=special>(</span><span class=number>4096</span><span class=special>)[</span><span class=identifier>rgb_p</span><span class=special>];</span></pre>
<p>As for the 32 bit binary string (1..32 1s or 0s), of course we could have easily 
  used the <tt>bin_p</tt> numeric parser instead. For the sake of demonstration 
  however:<span class=special><br>
  </span></p>
<pre>    <span class=identifier>bin</span><span class=number>32</span> <span class=special>= </span><span class=identifier>lexeme_d</span><span class=special>[</span><span class=identifier>repeat_p</span><span class=special>(</span>1, <span class=number>32</span><span class=special>)[</span><span class=identifier>ch_p</span><span class=special>(</span><span class=literal>'1'</span><span class=special>) </span><span class=special>| </span><span class=literal>'0'</span><span class=special>]];</span></pre>
<table width="80%" border="0" align="center">
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    <td class="note_box"><img src="theme/note.gif" width="16" height="16"> Loop 
      parsers are run-time <a href="parametric_parsers.html">parametric</a>.</td>
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</table>
<p>The Loop parsers can be dynamic. Consider the parsing of a binary file of Pascal-style 
  length prefixed string, where the first byte determines the length of the incoming 
  string. Here's a sample input: 
<blockquote> 
  <table width="363" border="0" cellspacing="0" cellpadding="0">
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            <td class="white_bkd" width=8%"> 
              <div align="center">11</div>
            </td>
            <td class="white_bkd" width="8%"> 
              <div align="center">h</div>
            </td>
            <td class="white_bkd" width="8%"> 
              <div align="center">e</div>
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            <td class="white_bkd" width="8%"> 
              <div align="center">l</div>
            </td>
            <td class="white_bkd" width="8%"> 
              <div align="center">l</div>
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            <td class="white_bkd" width="8%"> 
              <div align="center">o</div>
            </td>
            <td class="white_bkd" width="8%"> 
              <div align="center"> _</div>
            </td>
            <td class="white_bkd" width="8%"> 
              <div align="center">w</div>
            </td>
            <td class="white_bkd" width="8%"> 
              <div align="center">o</div>
            </td>
            <td class="white_bkd" width="8%"> 
              <div align="center">r</div>
            </td>
            <td class="white_bkd" width="8%"> 
              <div align="center">l</div>
            </td>
            <td class="white_bkd" width="8%"> 
              <div align="center">d</div>
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</blockquote>
<p>This trivial example cannot be practically defined in traditional EBNF. Although 
  some EBNF syntax allow more powerful repetition constructs other than the Kleene 
  star, we are still limited to parsing fixed strings. The nature of EBNF forces 
  the repetition factor to be a constant. On the other hand, Spirit allows the 
  repetition factor to be variable at run time. We could write a grammar that 
  accepts the input string above:</p>
<pre><span class=identifier>    </span><span class=keyword>int </span><span class=identifier>c</span><span class=special>;
    </span><span class=identifier>r </span><span class=special>= </span><span class=identifier>anychar_p</span><span class=special>[</span><span class=identifier>assign_a</span><span class=special>(</span><span class=identifier>c</span><span class=special>)] </span><span class=special>&gt;&gt; </span><span class=identifier>repeat_p</span><span class=special>(</span><span class=identifier>boost</span><span class=special>::</span><span class=identifier>ref</span><span class=special>(</span><span class=identifier>c</span><span class=special>))[</span><span class=identifier>anychar_p</span><span class=special>];</span></pre>
<p>The expression</p>
<pre>    <span class=identifier>anychar_p</span><span class=special>[</span><span class=identifier>assign_a</span><span class=special>(</span><span class=identifier>c</span><span class=special>)]</span></pre>
<p>extracts the first character from the input and puts it in <tt>c</tt>. What 
  is interesting is that in addition to constants, we can also use variables as 
  parameters to <tt>repeat_p</tt>, as demonstrated in </p>
<pre>    <span class=identifier>repeat_p</span><span class=special>(</span><span class=identifier>boost</span><span class=special>::</span><span class=identifier>ref</span><span class=special>(</span><span class=identifier>c</span><span class=special>)</span><span class=special>)</span><span class=special>[</span><span class=identifier>anychar_p</span><span class=special>]</span></pre>
<p>Notice that <tt>boost::ref</tt> is used to reference the integer <tt>c</tt>. 
  This usage of <tt>repeat_p</tt> makes the parser defer the evaluation of the 
  repetition factor until it is actually needed. Continuing our example, since 
  the value 11 is already extracted from the input, <tt>repeat_p</tt> is is now 
  expected to loop exactly 11 times.</p>
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<p class="copyright">Copyright &copy; 1998-2003 Joel de Guzman<br>
  <br>
  <font size="2">Use, modification and distribution is subject to the Boost Software
    License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at
    http://www.boost.org/LICENSE_1_0.txt) </font> </p>
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