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  26 <div class="titlepage"><div><div><h4 class="title">
  27 <a name="math_toolkit.stat_tut.overview.objects"></a><a class="link" href="objects.html" title="Distributions are Objects">Distributions
  28         are Objects</a>
  29 </h4></div></div></div>
  30 <p>
  31           Each kind of distribution in this library is a class type - an object.
  32         </p>
  33 <p>
  34           <a class="link" href="../../../policy.html" title="Chapter&#160;15.&#160;Policies: Controlling Precision, Error Handling etc">Policies</a> provide fine-grained control of
  35           the behaviour of these classes, allowing the user to customise behaviour
  36           such as how errors are handled, or how the quantiles of discrete distribtions
  37           behave.
  38         </p>
  39 <div class="tip"><table border="0" summary="Tip">
  40 <tr>
  41 <td rowspan="2" align="center" valign="top" width="25"><img alt="[Tip]" src="../../../../../../../doc/src/images/tip.png"></td>
  42 <th align="left">Tip</th>
  43 </tr>
  44 <tr><td align="left" valign="top"><p>
  45             If you are familiar with statistics libraries using functions, and 'Distributions
  46             as Objects' seem alien, see <a class="link" href="../weg/nag_library.html" title="Comparison with C, R, FORTRAN-style Free Functions">the
  47             comparison to other statistics libraries.</a>
  48           </p></td></tr>
  49 </table></div>
  50 <p>
  51           Making distributions class types does two things:
  52         </p>
  53 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
  54 <li class="listitem">
  55               It encapsulates the kind of distribution in the C++ type system; so,
  56               for example, Students-t distributions are always a different C++ type
  57               from Chi-Squared distributions.
  58             </li>
  59 <li class="listitem">
  60               The distribution objects store any parameters associated with the distribution:
  61               for example, the Students-t distribution has a <span class="emphasis"><em>degrees of
  62               freedom</em></span> parameter that controls the shape of the distribution.
  63               This <span class="emphasis"><em>degrees of freedom</em></span> parameter has to be provided
  64               to the Students-t object when it is constructed.
  65             </li>
  66 </ul></div>
  67 <p>
  68           Although the distribution classes in this library are templates, there
  69           are typedefs on type <span class="emphasis"><em>double</em></span> that mostly take the usual
  70           name of the distribution (except where there is a clash with a function
  71           of the same name: beta and gamma, in which case using the default template
  72           arguments - <code class="computeroutput"><span class="identifier">RealType</span> <span class="special">=</span>
  73           <span class="keyword">double</span></code> - is nearly as convenient).
  74           Probably 95% of uses are covered by these typedefs:
  75         </p>
  76 <pre class="programlisting"><span class="comment">// using namespace boost::math; // Avoid potential ambiguity with names in std &lt;random&gt;</span>
  77 <span class="comment">// Safer to declare specific functions with using statement(s):</span>
  78
  79 <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">beta_distribution</span><span class="special">;</span>
  80 <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>
  81 <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">students_t</span><span class="special">;</span>
  82
  83 <span class="comment">// Construct a students_t distribution with 4 degrees of freedom:</span>
  84 <span class="identifier">students_t</span> <span class="identifier">d1</span><span class="special">(</span><span class="number">4</span><span class="special">);</span>
  85
  86 <span class="comment">// Construct a double-precision beta distribution</span>
  87 <span class="comment">// with parameters a = 10, b = 20</span>
  88 <span class="identifier">beta_distribution</span><span class="special">&lt;&gt;</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&lt;&gt; suffix !</span>
  89 </pre>
  90 <p>
  91           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
  92           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>
  93           appended, for example: <a class="link" href="../../dist_ref/dists/students_t_dist.html" title="Students t Distribution">Students
  94           t Distribution</a> or <a class="link" href="../../dist_ref/dists/binomial_dist.html" title="Binomial Distribution">Binomial
  95           Distribution</a>:
  96         </p>
  97 <pre class="programlisting"><span class="comment">// Construct a students_t distribution, of float type,</span>
  98 <span class="comment">// with 4 degrees of freedom:</span>
  99 <span class="identifier">students_t_distribution</span><span class="special">&lt;</span><span class="keyword">float</span><span class="special">&gt;</span> <span class="identifier">d3</span><span class="special">(</span><span class="number">4</span><span class="special">);</span>
 100
 101 <span class="comment">// Construct a binomial distribution, of long double type,</span>
 102 <span class="comment">// with probability of success 0.3</span>
 103 <span class="comment">// and 20 trials in total:</span>
 104 <span class="identifier">binomial_distribution</span><span class="special">&lt;</span><span class="keyword">long</span> <span class="keyword">double</span><span class="special">&gt;</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>
 105 </pre>
 106 <p>
 107           The parameters passed to the distributions can be accessed via getter member
 108           functions:
 109         </p>
 110 <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>
 111 </pre>
 112 <p>
 113           This is all well and good, but not very useful so far. What we often want
 114           is to be able to calculate the <span class="emphasis"><em>cumulative distribution functions</em></span>
 115           and <span class="emphasis"><em>quantiles</em></span> etc for these distributions.
 116         </p>
 117 </div>
 118 <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
 119 <td align="left"></td>
 120 <td align="right"><div class="copyright-footer">Copyright &#169; 2006-2010, 2012-2014 Nikhar Agrawal,
 121       Anton Bikineev, Paul A. Bristow, Marco Guazzone, Christopher Kormanyos, Hubert
 122       Holin, Bruno Lalande, John Maddock, Jeremy Murphy, Johan R&#229;de, Gautam Sewani,
 123       Benjamin Sobotta, Thijs van den Berg, Daryle Walker and Xiaogang Zhang<p>
 124         Distributed under the Boost Software License, Version 1.0. (See accompanying
 125         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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