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<title>Setting Policies for Distributions on an Ad Hoc Basis</title>
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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>
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<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">&lt;</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">&lt;&gt;</span> <span class="special">&gt;</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">&lt;&gt;</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">&lt;</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">&gt;</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">&lt;</span>
<span class="identifier">promote_float</span><span class="special">&lt;</span><span class="keyword">false</span><span class="special">&gt;,</span> <span class="comment">// Do not promote to double.
</span> <span class="identifier">discrete_quantile</span><span class="special">&lt;</span><span class="identifier">integer_round_nearest</span><span class="special">&gt;</span> <span class="comment">// Round result to nearest integer.
</span><span class="special">&gt;</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">&lt;</span><span class="keyword">float</span><span class="special">,</span> <span class="identifier">mypolicy</span><span class="special">&gt;</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">&lt;&lt;</span> <span class="string">"quantile(mybinom(200, 0.25), 0.05) is: "</span> <span class="special">&lt;&lt;</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">&lt;&lt;</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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Distributed under the Boost Software License, Version 1.0. (See accompanying
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