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<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 V1.78.1"> <link rel="home" href="../index.html" title="The Boost C++ Libraries BoostBook Documentation Subset"> <link rel="up" href="../boost_random.html" title="Chapter 24. Boost.Random"> <link rel="prev" href="../boost_random.html" title="Chapter 24. 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 class="toc"> <dt><a href="tutorial.html#boost_random.tutorial.generating_integers_in_a_range">Generating integers in a range</a></dt> <dt><a href="tutorial.html#boost_random.tutorial.generating_integers_with_different_probabilities">Generating integers with different probabilities</a></dt> <dt><a href="tutorial.html#boost_random.tutorial.generating_a_random_password">Generating a random password</a></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> 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/random/mt19937.html" title="Type definition mt19937">mt19937</a></code> and <code class="computeroutput"><a class="link" href="../boost/random/uniform_int_distribution.html" title="Class template uniform_int_distribution">uniform_int_distribution</a></code>. <pre class="programlisting">#include <boost/random/mersenne_twister.hpp> #include <boost/random/uniform_int_distribution.hpp> </pre> We use <code class="computeroutput"><a class="link" href="../boost/random/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">std::time(0)</code> defined in ctime). <pre class="programlisting">boost::random::mt19937 gen; </pre> <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"> We are using a global 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 </td></tr> </table></div> Now we can define a function that simulates an ordinary six-sided die. <pre class="programlisting">int roll_die() { <a class="co" name="boost_random.tutorial.generating_integers_in_a_range.c0" href="tutorial.html#boost_random.tutorial.generating_integers_in_a_range.c1"><img src="../../../doc/src/images/callouts/1.png" alt="1" border="0"></a>boost::random::uniform_int_distribution<> dist(1, 6); <a class="co" name="boost_random.tutorial.generating_integers_in_a_range.c2" href="tutorial.html#boost_random.tutorial.generating_integers_in_a_range.c3"><img src="../../../doc/src/images/callouts/2.png" alt="2" border="0"></a>return dist(gen); } </pre> <div class="calloutlist"><table border="0" summary="Callout list"> <tr> <td width="5%" valign="top" align="left"><a name="boost_random.tutorial.generating_integers_in_a_range.c1"></a><a href="#boost_random.tutorial.generating_integers_in_a_range.c0"><img src="../../../doc/src/images/callouts/1.png" alt="1" border="0"></a> </td> <td valign="top" align="left"> <code class="computeroutput"><a class="link" href="../boost/random/mt19937.html" title="Type definition mt19937">mt19937</a></code> produces integers in the range [0, 232-1]. However, we want numbers in the range [1, 6]. The distribution <code class="computeroutput"><a class="link" href="../boost/random/uniform_int_distribution.html" title="Class template uniform_int_distribution">uniform_int_distribution</a></code> performs this transformation. <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"> Contrary to common C++ usage <code class="computeroutput"><a class="link" href="../boost/random/uniform_int_distribution.html" title="Class template uniform_int_distribution">uniform_int_distribution</a></code> does not take a half-open range. Instead it takes a closed range. Given the parameters 1 and 6, <code class="computeroutput"><a class="link" href="../boost/random/uniform_int_distribution.html" title="Class template uniform_int_distribution">uniform_int_distribution</a></code> can produce any of the values 1, 2, 3, 4, 5, or 6. </td></tr> </table></div> </td> </tr> <tr> <td width="5%" valign="top" align="left"><a name="boost_random.tutorial.generating_integers_in_a_range.c3"></a><a href="#boost_random.tutorial.generating_integers_in_a_range.c2"><img src="../../../doc/src/images/callouts/2.png" alt="2" border="0"></a> </td> <td valign="top" align="left"> A distribution is a function object. We generate a random number by calling <code class="computeroutput">dist</code> with the generator. </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> For the source of this example see <a href="../../..//libs/random/example/weighted_die.cpp" target="_top">weighted_die.cpp</a>. <pre class="programlisting">#include <boost/random/mersenne_twister.hpp> #include <boost/random/discrete_distribution.hpp> boost::mt19937 gen; </pre> This time, instead of a fair die, the probability of rolling a 1 is 50% (!). The other five faces are all equally likely. <code class="computeroutput"><a class="link" href="../boost/random/discrete_distribution.html" title="Class template discrete_distribution">discrete_distribution</a></code> works nicely here by allowing us to assign weights to each of the possible outcomes. <div class="tip"><table border="0" summary="Tip"> <tr> <td rowspan="2" align="center" valign="top" width="25"><img alt="[Tip]" src="../../../doc/src/images/tip.png"></td> <th align="left">Tip</th> </tr> <tr><td align="left" valign="top"> If your compiler supports <code class="computeroutput">std::initializer_list</code>, you can initialize <code class="computeroutput"><a class="link" href="../boost/random/discrete_distribution.html" title="Class template discrete_distribution">discrete_distribution</a></code> directly with the weights. </td></tr> </table></div> <pre class="programlisting">double probabilities[] = { 0.5, 0.1, 0.1, 0.1, 0.1, 0.1 }; boost::random::discrete_distribution<> dist(probabilities); </pre> Now define a function that simulates rolling this die. <pre class="programlisting">int roll_weighted_die() { <a class="co" name="boost_random.tutorial.generating_integers_with_different_probabilities.c0" href="tutorial.html#boost_random.tutorial.generating_integers_with_different_probabilities.c1"><img src="../../../doc/src/images/callouts/1.png" alt="1" border="0"></a>return dist(gen) + 1; } </pre> <div class="calloutlist"><table border="0" summary="Callout list"><tr> <td width="5%" valign="top" align="left"><a name="boost_random.tutorial.generating_integers_with_different_probabilities.c1"></a><a href="#boost_random.tutorial.generating_integers_with_different_probabilities.c0"><img src="../../../doc/src/images/callouts/1.png" alt="1" border="0"></a> </td> <td valign="top" align="left"> Add 1 to make sure that the result is in the range [1,6] instead of [0,5]. </td> </tr></table></div> </div> <div class="section"> <div class="titlepage"><div><div><h3 class="title"> <a name="boost_random.tutorial.generating_a_random_password"></a><a class="link" href="tutorial.html#boost_random.tutorial.generating_a_random_password" title="Generating a random password">Generating a random password</a> </h3></div></div></div> For the source of this example see <a href="../../..//libs/random/example/password.cpp" target="_top">password.cpp</a>. This example demonstrates generating a random 8 character password. <pre class="programlisting">#include <boost/random/random_device.hpp> #include <boost/random/uniform_int_distribution.hpp> int main() { <a class="co" name="boost_random.tutorial.generating_a_random_password.c0" href="tutorial.html#boost_random.tutorial.generating_a_random_password.c1"><img src="../../../doc/src/images/callouts/1.png" alt="1" border="0"></a>std::string chars( "abcdefghijklmnopqrstuvwxyz" "ABCDEFGHIJKLMNOPQRSTUVWXYZ" "1234567890" "!@#$%^&*()" "`~-_=+[{]}\\|;:'\",<.>/? "); <a class="co" name="boost_random.tutorial.generating_a_random_password.c2" href="tutorial.html#boost_random.tutorial.generating_a_random_password.c3"><img src="../../../doc/src/images/callouts/2.png" alt="2" border="0"></a>boost::random::random_device rng; <a class="co" name="boost_random.tutorial.generating_a_random_password.c4" href="tutorial.html#boost_random.tutorial.generating_a_random_password.c5"><img src="../../../doc/src/images/callouts/3.png" alt="3" border="0"></a>boost::random::uniform_int_distribution<> index_dist(0, chars.size() - 1); for(int i = 0; i < 8; ++i) { std::cout << chars[index_dist(rng)]; } std::cout << std::endl; } </pre> <div class="calloutlist"><table border="0" summary="Callout list"> <tr> <td width="5%" valign="top" align="left"><a name="boost_random.tutorial.generating_a_random_password.c1"></a><a href="#boost_random.tutorial.generating_a_random_password.c0"><img src="../../../doc/src/images/callouts/1.png" alt="1" border="0"></a> </td> <td valign="top" align="left"> We first define the characters that we're going to allow. This is pretty much just the characters on a standard keyboard. </td> </tr> <tr> <td width="5%" valign="top" align="left"><a name="boost_random.tutorial.generating_a_random_password.c3"></a><a href="#boost_random.tutorial.generating_a_random_password.c2"><img src="../../../doc/src/images/callouts/2.png" alt="2" border="0"></a> </td> <td valign="top" align="left"> We use <code class="computeroutput"><a class="link" href="../boost/random/random_device.html" title="Class random_device">random_device</a></code> as a source of entropy, since we want passwords that are not predictable. </td> </tr> <tr> <td width="5%" valign="top" align="left"><a name="boost_random.tutorial.generating_a_random_password.c5"></a><a href="#boost_random.tutorial.generating_a_random_password.c4"><img src="../../../doc/src/images/callouts/3.png" alt="3" border="0"></a> </td> <td valign="top" align="left"> Finally we select 8 random characters from the string and print them to cout. </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 Copyright © 2009, 2010 Steven Watanabe 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>) </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>