1<html> 2<head> 3<title> Loops</title> 4<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1"> 5<link rel="stylesheet" href="theme/style.css" type="text/css"> 6</head> 7 8<body> 9<table width="100%" border="0" background="theme/bkd2.gif" cellspacing="2"> 10 <tr> 11 <td width="10"> 12 </td> 13 <td width="85%"> 14 <font size="6" face="Verdana, Arial, Helvetica, sans-serif"><b> Loops</b></font> 15 </td> 16 <td width="112"><a href="http://spirit.sf.net"><img src="theme/spirit.gif" width="112" height="48" align="right" border="0"></a></td> 17 </tr> 18</table> 19<br> 20<table border="0"> 21 <tr> 22 <td width="10"></td> 23 <td width="30"><a href="../index.html"><img src="theme/u_arr.gif" border="0"></a></td> 24 <td width="30"><a href="escape_char_parser.html"><img src="theme/l_arr.gif" border="0"></a></td> 25 <td width="30"><a href="character_sets.html"><img src="theme/r_arr.gif" border="0"></a></td> 26 </tr> 27</table> 28<p>So far we have introduced a couple of EBNF operators that deal with looping. 29 We have the <tt>+</tt> positive operator, which matches the preceding symbol 30 one (1) or more times, as well as the Kleene star <tt>*</tt> which matches the 31 preceding symbol zero (0) or more times.</p> 32<p>Taking this further, we may want to have a generalized loop operator. To some 33 this may seem to be a case of overkill. Yet there are grammars that are impractical 34 and cumbersome, if not impossible, for the basic EBNF iteration syntax to specify. 35 Examples:</p> 36<blockquote> 37 <p><img src="theme/bullet.gif" width="12" height="12"> A file name may have 38 a maximum of 255 characters only.<br> 39 <img src="theme/bullet.gif" width="12" height="12"> A specific bitmap file 40 format has exactly 4096 RGB color information. <br> 41 <img src="theme/bullet.gif" width="12" height="12"> A 32 bit binary string 42 (1..32 1s or 0s).</p> 43</blockquote> 44<p>Other than the Kleene star <tt>*</tt>, the Positive closure <tt>+</tt>, and 45 the optional <tt>!</tt>, a more flexible mechanism for looping is provided for 46 by the framework. <br> 47</p> 48<table width="80%" border="0" align="center"> 49 <tr> 50 <td colspan="2" class="table_title">Loop Constructs</td> 51 </tr> 52 <tr> 53 <td class="table_cells" width="26%"><b>repeat_p (n) [p]</b></td> 54 <td class="table_cells" width="74%">Repeat <b>p</b> exactly <b>n</b> times</td> 55 </tr> 56 <tr> 57 <td class="table_cells" width="26%"><b>repeat_p (n1, n2) [p]</b></td> 58 <td class="table_cells" width="74%">Repeat <b>p</b> at least <b>n1</b> times 59 and at most <b>n2</b> times</td> 60 </tr> 61 <tr> 62 <td class="table_cells" width="26%"><b>repeat_p (n, more) [p] </b></td> 63 <td class="table_cells" width="74%">Repeat <b>p</b> at least <b>n</b> times, 64 continuing until <b>p</b> fails or the input is consumed</td> 65 </tr> 66</table> 67<p>Using the <tt>repeat_p</tt> parser, we can now write our examples above:</p> 68<p>A file name with a maximum of 255 characters:<br> 69</p> 70<pre> <span class=identifier>valid_fname_chars </span><span class=special>= </span><span class=comment>/*..*/</span><span class=special>; 71 </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> 72<p>A specific bitmap file format which has exactly 4096 RGB color information:<span class=special><br> 73 </span></p> 74<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>; 75 </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> 76<p>As for the 32 bit binary string (1..32 1s or 0s), of course we could have easily 77 used the <tt>bin_p</tt> numeric parser instead. For the sake of demonstration 78 however:<span class=special><br> 79 </span></p> 80<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> 81<table width="80%" border="0" align="center"> 82 <tr> 83 <td class="note_box"><img src="theme/note.gif" width="16" height="16"> Loop 84 parsers are run-time <a href="parametric_parsers.html">parametric</a>.</td> 85 </tr> 86</table> 87<p>The Loop parsers can be dynamic. Consider the parsing of a binary file of Pascal-style 88 length prefixed string, where the first byte determines the length of the incoming 89 string. Here's a sample input: 90<blockquote> 91 <table width="363" border="0" cellspacing="0" cellpadding="0"> 92 <tr> 93 <td class="dk_grey_bkd"> 94 <table width="100%" border="0" cellspacing="2" cellpadding="2"> 95 <tr> 96 <td class="white_bkd" width=8%"> 97 <div align="center">11</div> 98 </td> 99 <td class="white_bkd" width="8%"> 100 <div align="center">h</div> 101 </td> 102 <td class="white_bkd" width="8%"> 103 <div align="center">e</div> 104 </td> 105 <td class="white_bkd" width="8%"> 106 <div align="center">l</div> 107 </td> 108 <td class="white_bkd" width="8%"> 109 <div align="center">l</div> 110 </td> 111 <td class="white_bkd" width="8%"> 112 <div align="center">o</div> 113 </td> 114 <td class="white_bkd" width="8%"> 115 <div align="center"> _</div> 116 </td> 117 <td class="white_bkd" width="8%"> 118 <div align="center">w</div> 119 </td> 120 <td class="white_bkd" width="8%"> 121 <div align="center">o</div> 122 </td> 123 <td class="white_bkd" width="8%"> 124 <div align="center">r</div> 125 </td> 126 <td class="white_bkd" width="8%"> 127 <div align="center">l</div> 128 </td> 129 <td class="white_bkd" width="8%"> 130 <div align="center">d</div> 131 </td> 132 </tr> 133 </table> 134 </td> 135 </tr> 136 </table> 137 138</blockquote> 139<p>This trivial example cannot be practically defined in traditional EBNF. Although 140 some EBNF syntax allow more powerful repetition constructs other than the Kleene 141 star, we are still limited to parsing fixed strings. The nature of EBNF forces 142 the repetition factor to be a constant. On the other hand, Spirit allows the 143 repetition factor to be variable at run time. We could write a grammar that 144 accepts the input string above:</p> 145<pre><span class=identifier> </span><span class=keyword>int </span><span class=identifier>c</span><span class=special>; 146 </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>>> </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> 147<p>The expression</p> 148<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> 149<p>extracts the first character from the input and puts it in <tt>c</tt>. What 150 is interesting is that in addition to constants, we can also use variables as 151 parameters to <tt>repeat_p</tt>, as demonstrated in </p> 152<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> 153<p>Notice that <tt>boost::ref</tt> is used to reference the integer <tt>c</tt>. 154 This usage of <tt>repeat_p</tt> makes the parser defer the evaluation of the 155 repetition factor until it is actually needed. Continuing our example, since 156 the value 11 is already extracted from the input, <tt>repeat_p</tt> is is now 157 expected to loop exactly 11 times.</p> 158<table border="0"> 159 <tr> 160 <td width="10"></td> 161 <td width="30"><a href="../index.html"><img src="theme/u_arr.gif" border="0"></a></td> 162 <td width="30"><a href="escape_char_parser.html"><img src="theme/l_arr.gif" border="0"></a></td> 163 <td width="30"><a href="character_sets.html"><img src="theme/r_arr.gif" border="0"></a></td> 164 </tr> 165</table> 166<br> 167<hr size="1"> 168<p class="copyright">Copyright © 1998-2003 Joel de Guzman<br> 169 <br> 170 <font size="2">Use, modification and distribution is subject to the Boost Software 171 License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at 172 http://www.boost.org/LICENSE_1_0.txt) </font> </p> 173</body> 174</html> 175
