1<html> 2<head> 3<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 4<title>Complex - A first more complex generator</title> 5<link rel="stylesheet" href="../../../../../../../doc/src/boostbook.css" type="text/css"> 6<meta name="generator" content="DocBook XSL Stylesheets V1.79.1"> 7<link rel="home" href="../../../index.html" title="Spirit 2.5.8"> 8<link rel="up" href="../tutorials.html" title="Tutorials"> 9<link rel="prev" href="semantic_actions.html" title="Generator Semantic Actions"> 10<link rel="next" href="karma_easier_complex.html" title="Complex - Made easier"> 11</head> 12<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> 13<table cellpadding="2" width="100%"><tr> 14<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../../../boost.png"></td> 15<td align="center"><a href="../../../../../../../index.html">Home</a></td> 16<td align="center"><a href="../../../../../../../libs/libraries.htm">Libraries</a></td> 17<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> 18<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> 19<td align="center"><a href="../../../../../../../more/index.htm">More</a></td> 20</tr></table> 21<hr> 22<div class="spirit-nav"> 23<a accesskey="p" href="semantic_actions.html"><img src="../../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../tutorials.html"><img src="../../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../../index.html"><img src="../../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="karma_easier_complex.html"><img src="../../../../../../../doc/src/images/next.png" alt="Next"></a> 24</div> 25<div class="section"> 26<div class="titlepage"><div><div><h4 class="title"> 27<a name="spirit.karma.tutorials.karma_complex"></a><a class="link" href="karma_complex.html" title="Complex - A first more complex generator">Complex - A first 28 more complex generator</a> 29</h4></div></div></div> 30<p> 31 In this section we will develop a generator for complex numbers, allowing 32 to represent a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">complex</span></code> either as <code class="computeroutput"><span class="special">(</span><span class="identifier">real</span><span class="special">,</span> <span class="identifier">imag</span><span class="special">)</span></code> 33 (where <code class="computeroutput"><span class="identifier">real</span></code> and <code class="computeroutput"><span class="identifier">imag</span></code> are the real and imaginary parts 34 of the complex number) or as a simple <code class="computeroutput"><span class="identifier">real</span></code> 35 if the imaginary part happens to be equal to zero. This example will highlight 36 the power of <span class="emphasis"><em>Spirit.Karma</em></span> allowing to combine compile 37 time definition of formatting rules with runtime based decisions which 38 of the rules to apply. Also this time, we're using <a href="../../../../../../../libs/phoenix/doc/html/index.html" target="_top">Boost.Phoenix</a> 39 to do the semantic actions. 40 </p> 41<p> 42 Our goal is to allow for two different output formats to be applied depending 43 on whether the imaginary part of the complex number is zero or not. Let's 44 write both as a set of alternatives: 45 </p> 46<pre class="programlisting"> <span class="char">'('</span> <span class="special"><<</span> <span class="identifier">double_</span> <span class="special"><<</span> <span class="string">", "</span> <span class="special"><<</span> <span class="identifier">double_</span> <span class="special"><<</span> <span class="char">')'</span> 47<span class="special">|</span> <span class="identifier">double_</span> 48</pre> 49<p> 50 where the first alternative should be used for numbers having a non-zero 51 imaginary part, while the second is for real numbers. Generally, alternatives 52 are tried in the sequence of their definition as long until one of the 53 expressions (as delimited by <code class="computeroutput"><span class="char">'|'</span></code>) 54 succeeds. If no generator expression succeeds the whole alternative fails. 55 </p> 56<p> 57 If we left this formatting grammar as is our generator would always choose 58 the first alternative. We need to add some additional rules allowing to 59 make the first alternative fail. So, if the first alternative fails the 60 second one will be chosen instead. The decision about whether to choose 61 the first alternative has to be made at runtime as only then we actually 62 know the value of the imaginary part of the complex number. <span class="emphasis"><em>Spirit.Karma</em></span> 63 provides us with with a primitive generator <code class="computeroutput"><span class="identifier">eps</span><span class="special">()</span></code>, which is usable as a semantic predicate. 64 It has the property to 'succeed' generating only if its argument is true 65 (while it never generates any output on its own). 66 </p> 67<pre class="programlisting"><span class="keyword">double</span> <span class="identifier">imag</span> <span class="special">=</span> <span class="special">...;</span> <span class="comment">// imaginary part</span> 68 69 <span class="identifier">eps</span><span class="special">(</span><span class="identifier">imag</span> <span class="special">!=</span> <span class="number">0</span><span class="special">)</span> <span class="special"><<</span> <span class="char">'('</span> <span class="special"><<</span> <span class="identifier">double_</span> <span class="special"><<</span> <span class="string">", "</span> <span class="special"><<</span> <span class="identifier">double_</span> <span class="special"><<</span> <span class="char">')'</span> 70<span class="special">|</span> <span class="identifier">double_</span> 71</pre> 72<p> 73 If one of the generator elements of a sequence fails the whole sequence 74 will fail. This is exactly what we need, forcing the second alternative 75 to be chosen for complex numbers with imaginary parts equal to zero. 76 </p> 77<p> 78 Now on to the full example, this time with the proper semantic actions 79 (the complete cpp file for this example can be found here: <a href="../../../../../example/karma/complex_number.cpp" target="_top">complex_number.cpp</a>). 80 </p> 81<p> 82 We will use the <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">complex</span></code> type for this and all subsequent 83 related examples. And here you can see the full code of the generator allowing 84 to output a complex number either as a pair of numbers (if the imaginary 85 part is non-zero) or as a single number (if the complex is a real number): 86 </p> 87<p> 88</p> 89<pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">OutputIterator</span><span class="special">></span> 90<span class="keyword">bool</span> <span class="identifier">generate_complex</span><span class="special">(</span><span class="identifier">OutputIterator</span> <span class="identifier">sink</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">complex</span><span class="special"><</span><span class="keyword">double</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">c</span><span class="special">)</span> 91<span class="special">{</span> 92 <span class="keyword">using</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">spirit</span><span class="special">::</span><span class="identifier">karma</span><span class="special">::</span><span class="identifier">eps</span><span class="special">;</span> 93 <span class="keyword">using</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">spirit</span><span class="special">::</span><span class="identifier">karma</span><span class="special">::</span><span class="identifier">double_</span><span class="special">;</span> 94 <span class="keyword">using</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">spirit</span><span class="special">::</span><span class="identifier">karma</span><span class="special">::</span><span class="identifier">_1</span><span class="special">;</span> 95 <span class="keyword">using</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">spirit</span><span class="special">::</span><span class="identifier">karma</span><span class="special">::</span><span class="identifier">generate</span><span class="special">;</span> 96 97 <span class="keyword">return</span> <span class="identifier">generate</span><span class="special">(</span><span class="identifier">sink</span><span class="special">,</span> 98 <span class="comment">// Begin grammar</span> 99 <span class="special">(</span> 100 <span class="identifier">eps</span><span class="special">(</span><span class="identifier">c</span><span class="special">.</span><span class="identifier">imag</span><span class="special">()</span> <span class="special">!=</span> <span class="number">0</span><span class="special">)</span> <span class="special"><<</span> 101 <span class="char">'('</span> <span class="special"><<</span> <span class="identifier">double_</span><span class="special">[</span><span class="identifier">_1</span> <span class="special">=</span> <span class="identifier">c</span><span class="special">.</span><span class="identifier">real</span><span class="special">()]</span> <span class="special"><<</span> <span class="string">", "</span> <span class="special"><<</span> <span class="identifier">double_</span><span class="special">[</span><span class="identifier">_1</span> <span class="special">=</span> <span class="identifier">c</span><span class="special">.</span><span class="identifier">imag</span><span class="special">()]</span> <span class="special"><<</span> <span class="char">')'</span> 102 <span class="special">|</span> <span class="identifier">double_</span><span class="special">[</span><span class="identifier">_1</span> <span class="special">=</span> <span class="identifier">c</span><span class="special">.</span><span class="identifier">real</span><span class="special">()]</span> 103 <span class="special">)</span> 104 <span class="comment">// End grammar</span> 105 <span class="special">);</span> 106<span class="special">}</span> 107</pre> 108<p> 109 </p> 110<p> 111 The <code class="computeroutput"><span class="identifier">double_</span></code> generators 112 have this semantic action attached: 113 </p> 114<pre class="programlisting"><span class="identifier">_1</span> <span class="special">=</span> <span class="identifier">n</span> 115</pre> 116<p> 117 which passes <code class="computeroutput"><span class="identifier">n</span></code> to the first 118 element of the s generator's attached semantic action. Remember, semantic 119 actions in <span class="emphasis"><em>Spirit.Karma</em></span> are called before the corresponding 120 generator is invoked and they are expected to provide the generator with 121 the data to be used. The semantic action above assigns the value to be 122 generated (<code class="computeroutput"><span class="identifier">n</span></code>) to the generator 123 (actually, the attribute of <code class="computeroutput"><span class="identifier">double_</span></code>). 124 <code class="computeroutput"><span class="identifier">_1</span></code> is a Phoenix placeholder 125 referring to the attribute of the semantic action's attached generator. 126 If you need more information about semantic actions, you may want to read 127 about them in this section: <a class="link" href="semantic_actions.html" title="Generator Semantic Actions">Semantic 128 Actions</a>. 129 </p> 130<p> 131 These semantic actions are easy to understand but have the unexpected side 132 effect of being slightly less efficient than it could be. In addition they 133 tend to make the formatting grammar less readable. We will see in one of 134 the next sections how it is possible to use other, built-in features of 135 <span class="emphasis"><em>Spirit.Karma</em></span> to get rid of the semantic actions altogether. 136 When writing your grammars in Spirit you should always try to avoid semantic 137 actions which is often possible. Semantic actions are really powerful tools 138 but grammars tend to be more efficient and readable without them. 139 </p> 140</div> 141<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 142<td align="left"></td> 143<td align="right"><div class="copyright-footer">Copyright © 2001-2011 Joel de Guzman, Hartmut Kaiser<p> 144 Distributed under the Boost Software License, Version 1.0. (See accompanying 145 file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>) 146 </p> 147</div></td> 148</tr></table> 149<hr> 150<div class="spirit-nav"> 151<a accesskey="p" href="semantic_actions.html"><img src="../../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../tutorials.html"><img src="../../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../../index.html"><img src="../../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="karma_easier_complex.html"><img src="../../../../../../../doc/src/images/next.png" alt="Next"></a> 152</div> 153</body> 154</html> 155
