| <html> |
| <head> |
| <meta http-equiv="Content-Type" content="text/html; charset=US-ASCII"> |
| <title>Tutorial</title> |
| <link rel="stylesheet" href="../../../doc/src/boostbook.css" type="text/css"> |
| <meta name="generator" content="DocBook XSL Stylesheets V1.75.2"> |
| <link rel="home" href="../index.html" title="The Boost C++ Libraries BoostBook Documentation Subset"> |
| <link rel="up" href="../boost_random.html" title="Chapter 16. Boost.Random"> |
| <link rel="prev" href="../boost_random.html" title="Chapter 16. Boost.Random"> |
| <link rel="next" href="reference.html" title="Reference"> |
| </head> |
| <body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> |
| <table cellpadding="2" width="100%"><tr> |
| <td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../boost.png"></td> |
| <td align="center"><a href="../../../index.html">Home</a></td> |
| <td align="center"><a href="../../../libs/libraries.htm">Libraries</a></td> |
| <td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> |
| <td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> |
| <td align="center"><a href="../../../more/index.htm">More</a></td> |
| </tr></table> |
| <hr> |
| <div class="spirit-nav"> |
| <a accesskey="p" href="../boost_random.html"><img src="../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../boost_random.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="reference.html"><img src="../../../doc/src/images/next.png" alt="Next"></a> |
| </div> |
| <div class="section"> |
| <div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
| <a name="boost_random.tutorial"></a><a class="link" href="tutorial.html" title="Tutorial">Tutorial</a> |
| </h2></div></div></div> |
| <div class="toc"><dl> |
| <dt><span class="section"><a href="tutorial.html#boost_random.tutorial.generating_integers_in_a_range">Generating |
| integers in a range</a></span></dt> |
| <dt><span class="section"><a href="tutorial.html#boost_random.tutorial.generating_integers_with_different_probabilities">Generating |
| integers with different probabilities</a></span></dt> |
| </dl></div> |
| <div class="section"> |
| <div class="titlepage"><div><div><h3 class="title"> |
| <a name="boost_random.tutorial.generating_integers_in_a_range"></a><a class="link" href="tutorial.html#boost_random.tutorial.generating_integers_in_a_range" title="Generating integers in a range">Generating |
| integers in a range</a> |
| </h3></div></div></div> |
| <p> |
| For the source of this example see <a href="../../..//libs/random/example/die.cpp" target="_top">die.cpp</a>. |
| First we include the headers we need for <code class="computeroutput"><a class="link" href="../boost/mt19937.html" title="Type definition mt19937">mt19937</a></code> |
| and <code class="computeroutput"><a class="link" href="../boost/uniform_int.html" title="Class template uniform_int">uniform_int</a></code>. |
| </p> |
| <p> |
| |
| </p> |
| <pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">random</span><span class="special">/</span><span class="identifier">mersenne_twister</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> |
| <span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">random</span><span class="special">/</span><span class="identifier">uniform_int</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> |
| <span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">random</span><span class="special">/</span><span class="identifier">variate_generator</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span></pre> |
| <p> |
| </p> |
| <p> |
| We use <code class="computeroutput"><a class="link" href="../boost/mt19937.html" title="Type definition mt19937">mt19937</a></code> with the default |
| seed as a source of randomness. The numbers produced will be the same every |
| time the program is run. One common method to change this is to seed with |
| the current time (<code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">time</span><span class="special">(</span><span class="number">0</span><span class="special">)</span></code> defined in ctime). |
| </p> |
| <p> |
| |
| </p> |
| <pre class="programlisting"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">mt19937</span> <span class="identifier">gen</span><span class="special">;</span></pre> |
| <p> |
| </p> |
| <div class="note"><table border="0" summary="Note"> |
| <tr> |
| <td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td> |
| <th align="left">Note</th> |
| </tr> |
| <tr><td align="left" valign="top"><p> |
| We are using a <span class="emphasis"><em>global</em></span> generator object here. This |
| is important because we don't want to create a new <a class="link" href="reference.html#boost_random.reference.concepts.pseudo_random_number_generator" title="Pseudo-Random Number Generator">pseudo-random |
| number generator</a> at every call |
| </p></td></tr> |
| </table></div> |
| <p> |
| Now we can define a function that simulates an ordinary six-sided die. |
| </p> |
| <p> |
| |
| </p> |
| <pre class="programlisting"><span class="keyword">int</span> <span class="identifier">roll_die</span><span class="special">()</span> <span class="special">{</span> |
| <a class="co" name="boost_random0co" href="tutorial.html#boost_random0"><img src="../../../doc/src/images/callouts/1.png" alt="1" border="0"></a><span class="identifier">boost</span><span class="special">::</span><span class="identifier">uniform_int</span><span class="special"><></span> <span class="identifier">dist</span><span class="special">(</span><span class="number">1</span><span class="special">,</span> <span class="number">6</span><span class="special">);</span> |
| <a class="co" name="boost_random1co" href="tutorial.html#boost_random1"><img src="../../../doc/src/images/callouts/2.png" alt="2" border="0"></a><span class="identifier">boost</span><span class="special">::</span><span class="identifier">variate_generator</span><span class="special"><</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">mt19937</span><span class="special">&,</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">uniform_int</span><span class="special"><></span> <span class="special">></span> <span class="identifier">die</span><span class="special">(</span><span class="identifier">gen</span><span class="special">,</span> <span class="identifier">dist</span><span class="special">);</span> |
| <a class="co" name="boost_random2co" href="tutorial.html#boost_random2"><img src="../../../doc/src/images/callouts/3.png" alt="3" border="0"></a><span class="keyword">return</span> <span class="identifier">die</span><span class="special">();</span> |
| <span class="special">}</span> |
| </pre> |
| <p> |
| </p> |
| <div class="calloutlist"><table border="0" summary="Callout list"> |
| <tr> |
| <td width="5%" valign="top" align="left"><p><a name="boost_random0"></a><a href="#boost_random0co"><img src="../../../doc/src/images/callouts/1.png" alt="1" border="0"></a> </p></td> |
| <td valign="top" align="left"> |
| <p> |
| <code class="computeroutput"><a class="link" href="../boost/mt19937.html" title="Type definition mt19937">mt19937</a></code> produces integers |
| in the range [0, 2<sup>32</sup>-1]. However, we want numbers in the range [1, 6]. |
| The distribution <code class="computeroutput"><a class="link" href="../boost/uniform_int.html" title="Class template uniform_int">uniform_int</a></code> |
| performs this transformation. |
| </p> |
| <div class="warning"><table border="0" summary="Warning"> |
| <tr> |
| <td rowspan="2" align="center" valign="top" width="25"><img alt="[Warning]" src="../../../doc/src/images/warning.png"></td> |
| <th align="left">Warning</th> |
| </tr> |
| <tr><td align="left" valign="top"><p> |
| Contrary to common C++ usage <code class="computeroutput"><a class="link" href="../boost/uniform_int.html" title="Class template uniform_int">uniform_int</a></code> |
| does not take a <span class="emphasis"><em>half-open range</em></span>. Instead it takes |
| a <span class="emphasis"><em>closed range</em></span>. Given the parameters 1 and 6, |
| <code class="computeroutput"><a class="link" href="../boost/uniform_int.html" title="Class template uniform_int">uniform_int</a></code> can can |
| produce any of the values 1, 2, 3, 4, 5, or 6. |
| </p></td></tr> |
| </table></div> |
| </td> |
| </tr> |
| <tr> |
| <td width="5%" valign="top" align="left"><p><a name="boost_random1"></a><a href="#boost_random1co"><img src="../../../doc/src/images/callouts/2.png" alt="2" border="0"></a> </p></td> |
| <td valign="top" align="left"> |
| <p> |
| <code class="computeroutput"><a class="link" href="../boost/variate_generator.html" title="Class template variate_generator">variate_generator</a></code> |
| combines a generator with a distribution. |
| </p> |
| <div class="important"><table border="0" summary="Important"> |
| <tr> |
| <td rowspan="2" align="center" valign="top" width="25"><img alt="[Important]" src="../../../doc/src/images/important.png"></td> |
| <th align="left">Important</th> |
| </tr> |
| <tr><td align="left" valign="top"><p> |
| We pass <code class="computeroutput"><a class="link" href="../boost/mt19937.html" title="Type definition mt19937">boost::mt19937&</a></code> |
| to <code class="computeroutput"><a class="link" href="../boost/variate_generator.html" title="Class template variate_generator">variate_generator</a></code> |
| instead of just <code class="computeroutput"><a class="link" href="../boost/mt19937.html" title="Type definition mt19937">boost::mt19937</a></code> |
| (note the reference). Without the reference, <code class="computeroutput"><a class="link" href="../boost/variate_generator.html" title="Class template variate_generator">variate_generator</a></code> |
| would make a copy of the generator and would leave the global <code class="computeroutput"><span class="identifier">gen</span></code> unchanged. Consequently, <code class="computeroutput"><span class="identifier">roll_die</span></code> would produce <span class="bold"><strong>the same value</strong></span> every time it was called. |
| </p></td></tr> |
| </table></div> |
| </td> |
| </tr> |
| <tr> |
| <td width="5%" valign="top" align="left"><p><a name="boost_random2"></a><a href="#boost_random2co"><img src="../../../doc/src/images/callouts/3.png" alt="3" border="0"></a> </p></td> |
| <td valign="top" align="left"><p> |
| A <code class="computeroutput"><a class="link" href="../boost/variate_generator.html" title="Class template variate_generator">variate_generator</a></code> |
| is a function object. |
| </p></td> |
| </tr> |
| </table></div> |
| </div> |
| <div class="section"> |
| <div class="titlepage"><div><div><h3 class="title"> |
| <a name="boost_random.tutorial.generating_integers_with_different_probabilities"></a><a class="link" href="tutorial.html#boost_random.tutorial.generating_integers_with_different_probabilities" title="Generating integers with different probabilities">Generating |
| integers with different probabilities</a> |
| </h3></div></div></div> |
| <p> |
| For the source of this example see <a href="../../..//libs/random/example/weighted_die.cpp" target="_top">weighted_die.cpp</a>. |
| </p> |
| <p> |
| |
| </p> |
| <pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">random</span><span class="special">/</span><span class="identifier">mersenne_twister</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> |
| <span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">random</span><span class="special">/</span><span class="identifier">uniform_real</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> |
| <span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">random</span><span class="special">/</span><span class="identifier">variate_generator</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> |
| <span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">vector</span><span class="special">></span> |
| <span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">algorithm</span><span class="special">></span> |
| <span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">numeric</span><span class="special">></span> |
| |
| <span class="identifier">boost</span><span class="special">::</span><span class="identifier">mt19937</span> <span class="identifier">gen</span><span class="special">;</span></pre> |
| <p> |
| </p> |
| <p> |
| This time, instead of a fair die, the probability of rolling a 1 is 50% (!). |
| The other five faces are all equally likely. |
| </p> |
| <p> |
| |
| </p> |
| <pre class="programlisting"><span class="keyword">static</span> <span class="keyword">const</span> <span class="keyword">double</span> <span class="identifier">probabilities</span><span class="special">[]</span> <span class="special">=</span> <span class="special">{</span> |
| <span class="number">0.5</span><span class="special">,</span> <span class="number">0.1</span><span class="special">,</span> <span class="number">0.1</span><span class="special">,</span> <span class="number">0.1</span><span class="special">,</span> <span class="number">0.1</span><span class="special">,</span> <span class="number">0.1</span> |
| <span class="special">};</span></pre> |
| <p> |
| </p> |
| <p> |
| Now define a function that simulates rolling this die. Note that the C++0x |
| library contains a <code class="computeroutput"><span class="identifier">discrete_distribution</span></code> |
| class which would be a better way to do this. |
| </p> |
| <p> |
| |
| </p> |
| <pre class="programlisting"><span class="keyword">int</span> <span class="identifier">roll_weighted_die</span><span class="special">()</span> <span class="special">{</span> |
| <span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special"><</span><span class="keyword">double</span><span class="special">></span> <span class="identifier">cumulative</span><span class="special">;</span> |
| <span class="identifier">std</span><span class="special">::</span><span class="identifier">partial_sum</span><span class="special">(&</span><span class="identifier">probabilities</span><span class="special">[</span><span class="number">0</span><span class="special">],</span> <span class="special">&</span><span class="identifier">probabilities</span><span class="special">[</span><span class="number">0</span><span class="special">]</span> <span class="special">+</span> <span class="number">6</span><span class="special">,</span> |
| <span class="identifier">std</span><span class="special">::</span><span class="identifier">back_inserter</span><span class="special">(</span><span class="identifier">cumulative</span><span class="special">));</span> |
| <span class="identifier">boost</span><span class="special">::</span><span class="identifier">uniform_real</span><span class="special"><></span> <span class="identifier">dist</span><span class="special">(</span><span class="number">0</span><span class="special">,</span> <span class="identifier">cumulative</span><span class="special">.</span><span class="identifier">back</span><span class="special">());</span> |
| <span class="identifier">boost</span><span class="special">::</span><span class="identifier">variate_generator</span><span class="special"><</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">mt19937</span><span class="special">&,</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">uniform_real</span><span class="special"><></span> <span class="special">></span> <span class="identifier">die</span><span class="special">(</span><span class="identifier">gen</span><span class="special">,</span> <span class="identifier">dist</span><span class="special">);</span> |
| <a class="co" name="boost_random3co" href="tutorial.html#boost_random3"><img src="../../../doc/src/images/callouts/1.png" alt="1" border="0"></a><span class="keyword">return</span> <span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">lower_bound</span><span class="special">(</span><span class="identifier">cumulative</span><span class="special">.</span><span class="identifier">begin</span><span class="special">(),</span> <span class="identifier">cumulative</span><span class="special">.</span><span class="identifier">end</span><span class="special">(),</span> <span class="identifier">die</span><span class="special">())</span> <span class="special">-</span> <span class="identifier">cumulative</span><span class="special">.</span><span class="identifier">begin</span><span class="special">())</span> <span class="special">+</span> <span class="number">1</span><span class="special">;</span> |
| <span class="special">}</span> |
| |
| </pre> |
| <p> |
| </p> |
| <div class="calloutlist"><table border="0" summary="Callout list"><tr> |
| <td width="5%" valign="top" align="left"><p><a name="boost_random3"></a><a href="#boost_random3co"><img src="../../../doc/src/images/callouts/1.png" alt="1" border="0"></a> </p></td> |
| <td valign="top" align="left"><p> |
| Find the position within the sequence and add 1 (to make sure that the |
| result is in the range [1,6] instead of [0,5]) |
| </p></td> |
| </tr></table></div> |
| </div> |
| </div> |
| <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> |
| <td align="left"></td> |
| <td align="right"><div class="copyright-footer">Copyright © 2000 -2005 Jens Maurer, 2009 Steven Watanabe<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>) |
| </p> |
| </div></td> |
| </tr></table> |
| <hr> |
| <div class="spirit-nav"> |
| <a accesskey="p" href="../boost_random.html"><img src="../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../boost_random.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="reference.html"><img src="../../../doc/src/images/next.png" alt="Next"></a> |
| </div> |
| </body> |
| </html> |
