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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
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31 Each kind of distribution in this library is a class type - an object,
32 with member functions.
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40 If you are familiar with statistics libraries using functions, and 'Distributions
41 as Objects' seem alien, see <a class="link" href="../weg/nag_library.html" title="Comparison with C, R, FORTRAN-style Free Functions">the
42 comparison to other statistics libraries.</a>
46 <a class="link" href="../../../policy.html" title="Chapter 20. Policies: Controlling Precision, Error Handling etc">Policies</a> provide optional fine-grained control
47 of the behaviour of these classes, allowing the user to customise behaviour
48 such as how errors are handled, or how the quantiles of discrete distributions
52 Making distributions class types does two things:
54 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
56 It encapsulates the kind of distribution in the C++ type system; so,
57 for example, Students-t distributions are always a different C++ type
58 from Chi-Squared distributions.
61 The distribution objects store any parameters associated with the distribution:
62 for example, the Students-t distribution has a <span class="emphasis"><em>degrees of
63 freedom</em></span> parameter that controls the shape of the distribution.
64 This <span class="emphasis"><em>degrees of freedom</em></span> parameter has to be provided
65 to the Students-t object when it is constructed.
69 Although the distribution classes in this library are templates, there
70 are typedefs on type <span class="emphasis"><em>double</em></span> that mostly take the usual
71 name of the distribution (except where there is a clash with a function
72 of the same name: beta and gamma, in which case using the default template
73 arguments - <code class="computeroutput"><span class="identifier">RealType</span> <span class="special">=</span>
74 <span class="keyword">double</span></code> - is nearly as convenient).
75 Probably 95% of uses are covered by these typedefs:
77 <pre class="programlisting"><span class="comment">// using namespace boost::math; // Avoid potential ambiguity with names in std <random></span>
78 <span class="comment">// Safer to declare specific functions with using statement(s):</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">beta_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">binomial_distribution</span><span class="special">;</span>
82 <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>
84 <span class="comment">// Construct a students_t distribution with 4 degrees of freedom:</span>
85 <span class="identifier">students_t</span> <span class="identifier">d1</span><span class="special">(</span><span class="number">4</span><span class="special">);</span>
87 <span class="comment">// Construct a double-precision beta distribution</span>
88 <span class="comment">// with parameters a = 10, b = 20</span>
89 <span class="identifier">beta_distribution</span><span class="special"><></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<> suffix !</span>
92 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
93 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>
94 appended, for example: <a class="link" href="../../dist_ref/dists/students_t_dist.html" title="Students t Distribution">Students
95 t Distribution</a> or <a class="link" href="../../dist_ref/dists/binomial_dist.html" title="Binomial Distribution">Binomial
98 <pre class="programlisting"><span class="comment">// Construct a students_t distribution, of float type,</span>
99 <span class="comment">// with 4 degrees of freedom:</span>
100 <span class="identifier">students_t_distribution</span><span class="special"><</span><span class="keyword">float</span><span class="special">></span> <span class="identifier">d3</span><span class="special">(</span><span class="number">4</span><span class="special">);</span>
102 <span class="comment">// Construct a binomial distribution, of long double type,</span>
103 <span class="comment">// with probability of success 0.3</span>
104 <span class="comment">// and 20 trials in total:</span>
105 <span class="identifier">binomial_distribution</span><span class="special"><</span><span class="keyword">long</span> <span class="keyword">double</span><span class="special">></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>
108 The parameters passed to the distributions can be accessed via getter member
111 <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>
114 This is all well and good, but not very useful so far. What we often want
115 is to be able to calculate the <span class="emphasis"><em>cumulative distribution functions</em></span>
116 and <span class="emphasis"><em>quantiles</em></span> etc for these distributions.
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121 <td align="right"><div class="copyright-footer">Copyright © 2006-2019 Nikhar
122 Agrawal, Anton Bikineev, Paul A. Bristow, Marco Guazzone, Christopher Kormanyos,
123 Hubert Holin, Bruno Lalande, John Maddock, Jeremy Murphy, Matthew Pulver, Johan
124 Råde, Gautam Sewani, Benjamin Sobotta, Nicholas Thompson, Thijs van den Berg,
125 Daryle Walker and Xiaogang Zhang<p>
126 Distributed under the Boost Software License, Version 1.0. (See accompanying
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