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| 4 | <title>Setting Policies for Distributions on an Ad Hoc Basis</title> | |
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| 24 | </div> | |
| 25 | <div class="section"> | |
| 26 | <div class="titlepage"><div><div><h3 class="title"> | |
| 27 | <a name="math_toolkit.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 | |
| 28 | Policies for Distributions on an Ad Hoc Basis</a> | |
| 29 | </h3></div></div></div> | |
| 30 | <p> | |
| 31 | All of the statistical distributions in this library are class templates | |
| 32 | that accept two template parameters: real type (float, double ...) and policy | |
| 33 | (how to handle exceptional events), both with sensible defaults, for example: | |
| 34 | </p> | |
| 35 | <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> | |
| 36 | ||
| 37 | <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> | |
| 38 | <span class="keyword">class</span> <span class="identifier">fisher_f_distribution</span><span class="special">;</span> | |
| 39 | ||
| 40 | <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> | |
| 41 | ||
| 42 | <span class="special">}}</span> | |
| 43 | </pre> | |
| 44 | <p> | |
| 45 | This policy gets used by all the accessor functions that accept a distribution | |
| 46 | as an argument, and forwarded to all the functions called by these. So if | |
| 47 | 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 | |
| 48 | policies. | |
| 49 | </p> | |
| 50 | <p> | |
| 51 | However, say for example we wanted to evaluate the quantile of the binomial | |
| 52 | distribution at float precision, without internal promotion to double, and | |
| 53 | with the result rounded to the <span class="emphasis"><em>nearest</em></span> integer, then | |
| 54 | here's how it can be done: | |
| 55 | </p> | |
| 56 | <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> | |
| 57 | <span class="keyword">using</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> | |
| 58 | ||
| 59 | <span class="comment">// Begin by defining a policy type, that gives the behaviour we want:</span> | |
| 60 | ||
| 61 | <span class="comment">//using namespace boost::math::policies; or explicitly</span> | |
| 62 | <span class="keyword">using</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="identifier">policy</span><span class="special">;</span> | |
| 63 | ||
| 64 | <span class="keyword">using</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="identifier">promote_float</span><span class="special">;</span> | |
| 65 | <span class="keyword">using</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="identifier">discrete_quantile</span><span class="special">;</span> | |
| 66 | <span class="keyword">using</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="identifier">integer_round_nearest</span><span class="special">;</span> | |
| 67 | ||
| 68 | <span class="keyword">typedef</span> <span class="identifier">policy</span><span class="special"><</span> | |
| 69 | <span class="identifier">promote_float</span><span class="special"><</span><span class="keyword">false</span><span class="special">>,</span> <span class="comment">// Do not promote to double.</span> | |
| 70 | <span class="identifier">discrete_quantile</span><span class="special"><</span><span class="identifier">integer_round_nearest</span><span class="special">></span> <span class="comment">// Round result to nearest integer.</span> | |
| 71 | <span class="special">></span> <span class="identifier">mypolicy</span><span class="special">;</span> | |
| 72 | <span class="comment">//</span> | |
| 73 | <span class="comment">// Then define a new distribution that uses it:</span> | |
| 74 | <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> | |
| 75 | ||
| 76 | <span class="comment">// And now use it to get the quantile:</span> | |
| 77 | ||
| 78 | <span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span> | |
| 79 | <span class="special">{</span> | |
| 80 | <span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"quantile(mybinom(200, 0.25), 0.05) is: "</span> <span class="special"><<</span> | |
| 81 | <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">endl</span><span class="special">;</span> | |
| 82 | <span class="special">}</span> | |
| 83 | </pre> | |
| 84 | <p> | |
| 85 | Which outputs: | |
| 86 | </p> | |
| 87 | <pre class="programlisting">quantile is: 40</pre> | |
| 88 | </div> | |
| 89 | <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> | |
| 90 | <td align="left"></td> | |
| 91 | <td align="right"><div class="copyright-footer">Copyright © 2006-2010, 2012-2014 Nikhar Agrawal, | |
| 92 | Anton Bikineev, Paul A. Bristow, Marco Guazzone, Christopher Kormanyos, Hubert | |
| 93 | Holin, Bruno Lalande, John Maddock, Jeremy Murphy, Johan Råde, Gautam Sewani, | |
| 94 | Benjamin Sobotta, Thijs van den Berg, Daryle Walker and Xiaogang Zhang<p> | |
| 95 | Distributed under the Boost Software License, Version 1.0. (See accompanying | |
| 96 | 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>) | |
| 97 | </p> | |
| 98 | </div></td> | |
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