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| <div class="section"> |
| <div class="titlepage"><div><div><h4 class="title"> |
| <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 |
| more complex generator</a> |
| </h4></div></div></div> |
| <p> |
| In this section we will develop a generator for complex numbers, allowing |
| 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> |
| (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 |
| of the complex number) or as a simple <code class="computeroutput"><span class="identifier">real</span></code> |
| if the imaginary part happens to be equal to zero. This example will highlight |
| the power of <span class="emphasis"><em>Spirit.Karma</em></span> allowing to combine compile |
| time definition of formatting rules with runtime based decisions which |
| of the rules to apply. Also this time, we're using <a href="../../../../../phoenix/doc/html/index.html" target="_top">Boost.Phoenix</a> |
| to do the semantic actions. |
| </p> |
| <p> |
| Our goal is to allow for two different output formats to be applied depending |
| on whether the imaginary part of the complex number is zero or not. Let's |
| write both as a set of alternatives: |
| </p> |
| <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> |
| <span class="special">|</span> <span class="identifier">double_</span> |
| </pre> |
| <p> |
| where the first alternative should be used for numbers having a non-zero |
| imaginary part, while the second is for real numbers. Generally, alternatives |
| are tried in the sequence of their definition as long until one of the |
| expressions (as delimited by <code class="computeroutput"><span class="char">'|'</span></code>) |
| succeeds. If no generator expression succeeds the whole alternative fails. |
| </p> |
| <p> |
| If we left this formatting grammar as is our generator would always choose |
| the first alternative. We need to add some additional rules allowing to |
| make the first alternative fail. So, if the first alternative fails the |
| second one will be chosen instead. The decision about whether to choose |
| the first alternative has to be made at runtime as only then we actually |
| know the value of the imaginary part of the complex number. <span class="emphasis"><em>Spirit.Karma</em></span> |
| 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. |
| It has the property to 'succeed' generating only if its argument is true |
| (while it never generates any output on its own). |
| </p> |
| <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> |
| <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> |
| <span class="special">|</span> <span class="identifier">double_</span> |
| </pre> |
| <p> |
| If one of the generator elements of a sequence fails the whole sequence |
| will fail. This is exactly what we need, forcing the second alternative |
| to be chosen for complex numbers with imaginary parts equal to zero. |
| </p> |
| <p> |
| Now on to the full example, this time with the proper semantic actions |
| (the complete cpp file for this example can be found here: <a href="../../../../../example/karma/complex_number.cpp" target="_top">complex_number.cpp</a>). |
| </p> |
| <p> |
| 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 |
| related examples. And here you can see the full code of the generator allowing |
| to output a complex number either as a pair of numbers (if the imaginary |
| part is non-zero) or as a single number (if the complex is a real number): |
| </p> |
| <p> |
| |
| </p> |
| <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> |
| <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> |
| <span class="special">{</span> |
| <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> |
| <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> |
| <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> |
| <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> |
| |
| <span class="keyword">return</span> <span class="identifier">generate</span><span class="special">(</span><span class="identifier">sink</span><span class="special">,</span> |
| <span class="comment">// Begin grammar |
| </span> <span class="special">(</span> |
| <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> |
| <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> |
| <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="comment">// End grammar |
| </span> <span class="special">);</span> |
| <span class="special">}</span> |
| </pre> |
| <p> |
| </p> |
| <p> |
| The <code class="computeroutput"><span class="identifier">double_</span></code> generators |
| have this semantic action attached: |
| </p> |
| <pre class="programlisting"><span class="identifier">_1</span> <span class="special">=</span> <span class="identifier">n</span> |
| </pre> |
| <p> |
| which passes <code class="computeroutput"><span class="identifier">n</span></code> to the first |
| element of the s generator's attached semantic action. Remember, semantic |
| actions in <span class="emphasis"><em>Spirit.Karma</em></span> are called before the corresponding |
| generator is invoked and they are expected to provide the generator with |
| the data to be used. The semantic action above assigns the value to be |
| generated (<code class="computeroutput"><span class="identifier">n</span></code>) to the generator |
| (actually, the attribute of <code class="computeroutput"><span class="identifier">double_</span></code>). |
| <code class="computeroutput"><span class="identifier">_1</span></code> is a Phoenix placeholder |
| referring to the attribute of the semantic action's attached generator. |
| If you need more information about semantic actions, you may want to read |
| about them in this section: <a class="link" href="semantic_actions.html" title="Semantic Actions">Semantic |
| Actions</a>. |
| </p> |
| <p> |
| These semantic actions are easy to understand but have the unexpected side |
| effect of being slightly less efficient than it could be. In addition they |
| tend to make the formatting grammar less readable. We will see in one of |
| the next sections how it is possible to use other, built-in features of |
| <span class="emphasis"><em>Spirit.Karma</em></span> to get rid of the semantic actions altogether. |
| When writing your grammars in Spirit you should always try to avoid semantic |
| actions which is often possible. Semantic actions are really powerful tools |
| but grammars tend to be more efficient and readable without them. |
| </p> |
| </div> |
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| <td align="right"><div class="copyright-footer">Copyright © 2001-2010 Joel de Guzman, Hartmut Kaiser<p> |
| Distributed under the Boost Software License, Version 1.0. (See accompanying |
| 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>) |
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