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| <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: real type (float, double ...) and |
| policy (how to handle exceptional events), 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="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> |
| |
| <span class="comment">// Begin by defining a policy type, that gives the behaviour we want: |
| </span> |
| <span class="comment">//using namespace boost::math::policies; or explicitly |
| </span><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> |
| |
| <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> |
| <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> |
| <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> |
| |
| <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="comment">// Do not promote to double. |
| </span> <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><span class="special">></span> <span class="identifier">mypolicy</span><span class="special">;</span> |
| <span class="comment">// |
| </span><span class="comment">// Then define a new distribution that uses it: |
| </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">// And now use it to get the quantile: |
| </span> |
| <span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span> |
| <span class="special">{</span> |
| <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> |
| <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> |
| <span class="special">}</span> |
| |
| </pre> |
| <p> |
| </p> |
| <p> |
| </p> |
| <p> |
| Which outputs: |
| </p> |
| <pre class="programlisting">quantile is: 40</pre> |
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| 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>) |
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