<html> <head> <meta http-equiv="Content-Type" content="text/html; charset=US-ASCII"> <title>Setting Policies for Distributions on an Ad Hoc Basis</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="../pol_tutorial.html" title="Policy Tutorial"> <link rel="prev" href="changing_policy_defaults.html" title="Changing the Policy Defaults"> <link rel="next" href="ad_hoc_sf_policies.html" title="Changing the Policy on an Ad Hoc Basis for the Special 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="changing_policy_defaults.html"><img src="../../../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../pol_tutorial.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="ad_hoc_sf_policies.html"><img src="../../../../../../../../doc/src/images/next.png" alt="Next"></a> </div> <div class="section" lang="en"> <div class="titlepage"><div><div><h4 class="title"> <a name="math_toolkit.policy.pol_tutorial.ad_hoc_dist_policies"></a><a class="link" href="ad_hoc_dist_policies.html" title="Setting Policies for Distributions on an Ad Hoc Basis"> Setting Policies for Distributions on an Ad Hoc Basis</a> </h4></div></div></div> <p> All of the statistical distributions in this library are class templates that accept two template parameters, both with sensible defaults, for example: </p> <pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">boost</span><span class="special">{</span> <span class="keyword">namespace</span> <span class="identifier">math</span><span class="special">{</span> <span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">RealType</span> <span class="special">=</span> <span class="keyword">double</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">Policy</span> <span class="special">=</span> <span class="identifier">policies</span><span class="special">::</span><span class="identifier">policy</span><span class="special"><></span> <span class="special">></span> <span class="keyword">class</span> <span class="identifier">fisher_f_distribution</span><span class="special">;</span> <span class="keyword">typedef</span> <span class="identifier">fisher_f_distribution</span><span class="special"><></span> <span class="identifier">fisher_f</span><span class="special">;</span> <span class="special">}}</span> </pre> <p> This policy gets used by all the accessor functions that accept a distribution as an argument, and forwarded to all the functions called by these. So if you use the shorthand-typedef for the distribution, then you get <code class="computeroutput"><span class="keyword">double</span></code> precision arithmetic and all the default policies. </p> <p> However, say for example we wanted to evaluate the quantile of the binomial distribution at float precision, without internal promotion to double, and with the result rounded to the <span class="emphasis"><em>nearest</em></span> integer, then here's how it can be done: </p> <p> </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">math</span><span class="special">/</span><span class="identifier">distributions</span><span class="special">/</span><span class="identifier">binomial</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> <span class="comment">// </span><span class="comment">// Begin by defining a policy type, that gives the </span><span class="comment">// behaviour we want: </span><span class="comment">// </span><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="identifier">policies</span><span class="special">;</span> <span class="keyword">typedef</span> <span class="identifier">policy</span><span class="special"><</span> <span class="identifier">promote_float</span><span class="special"><</span><span class="keyword">false</span><span class="special">>,</span> <span class="identifier">discrete_quantile</span><span class="special"><</span><span class="identifier">integer_round_nearest</span><span class="special">></span> <span class="special">></span> <span class="identifier">mypolicy</span><span class="special">;</span> <span class="comment">// </span><span class="comment">// Then define a distribution that uses it: </span><span class="comment">// </span><span class="keyword">typedef</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">math</span><span class="special">::</span><span class="identifier">binomial_distribution</span><span class="special"><</span><span class="keyword">float</span><span class="special">,</span> <span class="identifier">mypolicy</span><span class="special">></span> <span class="identifier">mybinom</span><span class="special">;</span> <span class="comment">// </span><span class="comment">// And now use it to get the quantile: </span><span class="comment">// </span><span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span> <span class="special">{</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"quantile is: "</span> <span class="special"><<</span> <span class="identifier">quantile</span><span class="special">(</span><span class="identifier">mybinom</span><span class="special">(</span><span class="number">200</span><span class="special">,</span> <span class="number">0.25</span><span class="special">),</span> <span class="number">0.05</span><span class="special">)</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> <span class="special">}</span> </pre> <p> </p> <p> </p> <p> Which outputs: </p> <pre class="programlisting">quantile is: 40</pre> </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="changing_policy_defaults.html"><img src="../../../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../pol_tutorial.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="ad_hoc_sf_policies.html"><img src="../../../../../../../../doc/src/images/next.png" alt="Next"></a> </div> </body> </html>