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<title>Mathematically Undefined Function Policies</title>
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<div class="titlepage"><div><div><h3 class="title">
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<a name="math_toolkit.pol_ref.assert_undefined"></a><a class="link" href="assert_undefined.html" title="Mathematically Undefined Function Policies">Mathematically
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Undefined Function Policies</a>
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</h3></div></div></div>
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<p>
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There are some functions that are generic (they are present for all the statistical
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distributions supported) but which may be mathematically undefined for certain
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distributions, but defined for others.
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</p>
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<p>
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For example, the Cauchy distribution does not have a meaningful mean, so
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what should
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</p>
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<pre class="programlisting"><span class="identifier">mean</span><span class="special">(</span><span class="identifier">cauchy</span><span class="special"><>());</span>
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</pre>
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<p>
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return, and should such an expression even compile at all?
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</p>
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<p>
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The default behaviour is for all such functions to not compile at all - in
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fact they will raise a <a href="http://www.boost.org/libs/static_assert/index.html" target="_top">static
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assertion</a> - but by changing the policy we can have them return the
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result of a domain error instead (which may well throw an exception, depending
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on the error handling policy).
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</p>
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<p>
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This behaviour is controlled by the <code class="computeroutput"><span class="identifier">assert_undefined</span><span class="special"><></span></code> policy:
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</p>
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<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">namespace</span> <span class="identifier">policies</span> <span class="special">{</span>
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<span class="keyword">template</span> <span class="special"><</span><span class="keyword">bool</span> <span class="identifier">b</span><span class="special">></span>
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<span class="keyword">class</span> <span class="identifier">assert_undefined</span><span class="special">;</span>
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<span class="special">}}}</span> <span class="comment">//namespaces</span>
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</pre>
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<p>
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For example:
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</p>
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<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">cauchy</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
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<span class="keyword">using</span> <span class="keyword">namespace</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>
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<span class="keyword">using</span> <span class="keyword">namespace</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">math</span><span class="special">;</span>
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<span class="comment">// This will not compile, cauchy has no mean!</span>
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<span class="keyword">double</span> <span class="identifier">m1</span> <span class="special">=</span> <span class="identifier">mean</span><span class="special">(</span><span class="identifier">cauchy</span><span class="special">());</span>
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<span class="comment">// This will compile, but raises a domain error!</span>
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<span class="keyword">double</span> <span class="identifier">m2</span> <span class="special">=</span> <span class="identifier">mean</span><span class="special">(</span><span class="identifier">cauchy_distribution</span><span class="special"><</span><span class="keyword">double</span><span class="special">,</span> <span class="identifier">policy</span><span class="special"><</span><span class="identifier">assert_undefined</span><span class="special"><</span><span class="keyword">false</span><span class="special">></span> <span class="special">></span> <span class="special">>());</span>
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</pre>
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<p>
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<code class="computeroutput"><span class="identifier">policy</span><span class="special"><</span><span class="identifier">assert_undefined</span><span class="special"><</span><span class="keyword">false</span><span class="special">></span></code>
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behaviour can also be obtained by defining the macro
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</p>
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<pre class="programlisting"><span class="preprocessor">#define</span> <span class="identifier">BOOST_MATH_ASSERT_UNDEFINED_POLICY</span> <span class="keyword">false</span>
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</pre>
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<p>
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at the head of the file - see <a class="link" href="policy_defaults.html" title="Using Macros to Change the Policy Defaults">Using
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Macros to Change the Policy Defaults</a>.
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</p>
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<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
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<td align="left"></td>
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<td align="right"><div class="copyright-footer">Copyright © 2006-2019 Nikhar
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Agrawal, Anton Bikineev, Paul A. Bristow, Marco Guazzone, Christopher Kormanyos,
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Hubert Holin, Bruno Lalande, John Maddock, Jeremy Murphy, Matthew Pulver, Johan
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Råde, Gautam Sewani, Benjamin Sobotta, Nicholas Thompson, Thijs van den Berg,
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Daryle Walker and Xiaogang Zhang<p>
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Distributed under the Boost Software License, Version 1.0. (See accompanying
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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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