<html> <head> <title> Loops</title> <meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1"> <link rel="stylesheet" href="theme/style.css" type="text/css"> </head> <body> <table width="100%" border="0" background="theme/bkd2.gif" cellspacing="2"> <tr> <td width="10"> </td> <td width="85%"> <font size="6" face="Verdana, Arial, Helvetica, sans-serif"><b> Loops</b></font> </td> <td width="112"><a href="http://spirit.sf.net"><img src="theme/spirit.gif" width="112" height="48" align="right" border="0"></a></td> </tr> </table> <br> <table border="0"> <tr> <td width="10"></td> <td width="30"><a href="../index.html"><img src="theme/u_arr.gif" border="0"></a></td> <td width="30"><a href="escape_char_parser.html"><img src="theme/l_arr.gif" border="0"></a></td> <td width="20"><a href="character_sets.html"><img src="theme/r_arr.gif" border="0"></a></td> </tr> </table> <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"> <tr> <td colspan="2" class="table_title">Loop Constructs</td> </tr> <tr> <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> </tr> <tr> <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> </tr> <tr> <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> </tr> </table> <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><</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>> </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"> <tr> <td class="note_box"><img src="theme/note.gif" width="16" height="16"> Loop parsers are run-time <a href="doc/parametric_parsers.html">parametric</a>.</td> </tr> </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"> <tr> <td class="dk_grey_bkd"> <table width="100%" border="0" cellspacing="2" cellpadding="2"> <tr> <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> </td> <td class="white_bkd" width="8%"> <div align="center">l</div> </td> <td class="white_bkd" width="8%"> <div align="center">l</div> </td> <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> </td> </tr> </table> </td> </tr> </table> </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</span><span class=special>(</span><span class=identifier>c</span><span class=special>)] </span><span class=special>>> </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</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> <table border="0"> <tr> <td width="10"></td> <td width="30"><a href="../index.html"><img src="theme/u_arr.gif" border="0"></a></td> <td width="30"><a href="escape_char_parser.html"><img src="theme/l_arr.gif" border="0"></a></td> <td width="20"><a href="character_sets.html"><img src="theme/r_arr.gif" border="0"></a></td> </tr> </table> <br> <hr size="1"> <p class="copyright">Copyright © 1998-2002 Joel de Guzman<br> <br> <font size="2">Permission to copy, use, modify, sell and distribute this document is granted provided this copyright notice appears in all copies. This document is provided "as is" without express or implied warranty, and with no claim as to its suitability for any purpose. </font> </p> </body> </html>