<html> <head> <meta http-equiv="Content-Type" content="text/html; charset=US-ASCII"> <title>Distributions are Objects</title> <link rel="stylesheet" href="../../../../../../../../../doc/src/boostbook.css" type="text/css"> <meta name="generator" content="DocBook XSL Stylesheets V1.74.0"> <link rel="home" href="../../../../index.html" title="Math Toolkit"> <link rel="up" href="../overview.html" title="Overview of Distributions"> <link rel="prev" href="headers.html" title="Headers and Namespaces"> <link rel="next" href="generic.html" title="Generic operations common to all distributions are non-member functions"> </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="headers.html"><img src="../../../../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../overview.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="generic.html"><img src="../../../../../../../../../doc/src/images/next.png" alt="Next"></a> </div> <div class="section" lang="en"> <div class="titlepage"><div><div><h5 class="title"> <a name="math_toolkit.dist.stat_tut.overview.objects"></a><a class="link" href="objects.html" title="Distributions are Objects"> Distributions are Objects</a> </h5></div></div></div> <p> Each kind of distribution in this library is a class type - an object. </p> <p> <a class="link" href="../../../policy.html" title="Policies">Policies</a> provide fine-grained control of the behaviour of these classes, allowing the user to customise behaviour such as how errors are handled, or how the quantiles of discrete distribtions behave. </p> <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"><p> If you are familiar with statistics libraries using functions, and 'Distributions as Objects' seem alien, see <a class="link" href="../weg/nag_library.html" title="Comparison with C, R, FORTRAN-style Free Functions">the comparison to other statistics libraries.</a> </p></td></tr> </table></div> <p> Making distributions class types does two things: </p> <div class="itemizedlist"><ul type="disc"> <li> It encapsulates the kind of distribution in the C++ type system; so, for example, Students-t distributions are always a different C++ type from Chi-Squared distributions. </li> <li> The distribution objects store any parameters associated with the distribution: for example, the Students-t distribution has a <span class="emphasis"><em>degrees of freedom</em></span> parameter that controls the shape of the distribution. This <span class="emphasis"><em>degrees of freedom</em></span> parameter has to be provided to the Students-t object when it is constructed. </li> </ul></div> <p> Although the distribution classes in this library are templates, there are typedefs on type <span class="emphasis"><em>double</em></span> that mostly take the usual name of the distribution (except where there is a clash with a function of the same name: beta and gamma, in which case using the default template arguments - <code class="computeroutput"><span class="identifier">RealType</span> <span class="special">=</span> <span class="keyword">double</span></code> - is nearly as convenient). Probably 95% of uses are covered by these typedefs: </p> <pre class="programlisting"><span class="keyword">using</span> <span class="keyword">namespace</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">math</span><span class="special">;</span> <span class="comment">// Construct a students_t distribution with 4 degrees of freedom: </span><span class="identifier">students_t</span> <span class="identifier">d1</span><span class="special">(</span><span class="number">4</span><span class="special">);</span> <span class="comment">// Construct a double-precision beta distribution </span><span class="comment">// with parameters a = 10, b = 20 </span><span class="identifier">beta_distribution</span><span class="special"><></span> <span class="identifier">d2</span><span class="special">(</span><span class="number">10</span><span class="special">,</span> <span class="number">20</span><span class="special">);</span> <span class="comment">// Note: _distribution<> suffix ! </span></pre> <p> If you need to use the distributions with a type other than <code class="computeroutput"><span class="keyword">double</span></code>, then you can instantiate the template directly: the names of the templates are the same as the <code class="computeroutput"><span class="keyword">double</span></code> typedef but with <code class="computeroutput"><span class="identifier">_distribution</span></code> appended, for example: <a class="link" href="../../dist_ref/dists/students_t_dist.html" title="Students t Distribution">Students t Distribution</a> or <a class="link" href="../../dist_ref/dists/binomial_dist.html" title="Binomial Distribution">Binomial Distribution</a>: </p> <pre class="programlisting"><span class="comment">// Construct a students_t distribution, of float type, </span><span class="comment">// with 4 degrees of freedom: </span><span class="identifier">students_t_distribution</span><span class="special"><</span><span class="keyword">float</span><span class="special">></span> <span class="identifier">d3</span><span class="special">(</span><span class="number">4</span><span class="special">);</span> <span class="comment">// Construct a binomial distribution, of long double type, </span><span class="comment">// with probability of success 0.3 </span><span class="comment">// and 20 trials in total: </span><span class="identifier">binomial_distribution</span><span class="special"><</span><span class="keyword">long</span> <span class="keyword">double</span><span class="special">></span> <span class="identifier">d4</span><span class="special">(</span><span class="number">20</span><span class="special">,</span> <span class="number">0.3</span><span class="special">);</span> </pre> <p> The parameters passed to the distributions can be accessed via getter member functions: </p> <pre class="programlisting"><span class="identifier">d1</span><span class="special">.</span><span class="identifier">degrees_of_freedom</span><span class="special">();</span> <span class="comment">// returns 4.0 </span></pre> <p> This is all well and good, but not very useful so far. What we often want is to be able to calculate the <span class="emphasis"><em>cumulative distribution functions</em></span> and <span class="emphasis"><em>quantiles</em></span> etc for these distributions. </p> </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 © 2006 , 2007, 2008, 2009 John Maddock, Paul A. Bristow, Hubert Holin, Xiaogang Zhang, Bruno Lalande, Johan Råde, Gautam Sewani and Thijs van den Berg<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="headers.html"><img src="../../../../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../overview.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="generic.html"><img src="../../../../../../../../../doc/src/images/next.png" alt="Next"></a> </div> </body> </html>