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  26 <div class="titlepage"><div><div><h4 class="title">
  27 <a name="math_toolkit.stat_tut.weg.error_eg"></a><a class="link" href="error_eg.html" title="Error Handling Example">Error Handling
  28         Example</a>
  29 </h4></div></div></div>
  30 <p>
  31           See <a class="link" href="../../error_handling.html" title="Error Handling">error handling documentation</a>
  32           for a detailed explanation of the mechanism of handling errors, including
  33           the common "bad" arguments to distributions and functions, and
  34           how to use <a class="link" href="../../../policy.html" title="Chapter&#160;15.&#160;Policies: Controlling Precision, Error Handling etc">Policies</a> to control it.
  35         </p>
  36 <p>
  37           But, by default, <span class="bold"><strong>exceptions will be raised</strong></span>,
  38           for domain errors, pole errors, numeric overflow, and internal evaluation
  39           errors. To avoid the exceptions from getting thrown and instead get an
  40           appropriate value returned, usually a NaN (domain errors pole errors or
  41           internal errors), or infinity (from overflow), you need to change the policy.
  42         </p>
  43 <p>
  44           The following example demonstrates the effect of setting the macro BOOST_MATH_DOMAIN_ERROR_POLICY
  45           when an invalid argument is encountered. For the purposes of this example,
  46           we'll pass a negative degrees of freedom parameter to the student's t distribution.
  47         </p>
  48 <p>
  49           Since we know that this is a single file program we could just add:
  50         </p>
  51 <pre class="programlisting"><span class="preprocessor">#define</span> <span class="identifier">BOOST_MATH_DOMAIN_ERROR_POLICY</span> <span class="identifier">ignore_error</span>
  52 </pre>
  53 <p>
  54           to the top of the source file to change the default policy to one that
  55           simply returns a NaN when a domain error occurs. Alternatively we could
  56           use:
  57         </p>
  58 <pre class="programlisting"><span class="preprocessor">#define</span> <span class="identifier">BOOST_MATH_DOMAIN_ERROR_POLICY</span> <span class="identifier">errno_on_error</span>
  59 </pre>
  60 <p>
  61           To ensure the <code class="computeroutput"><span class="special">::</span><span class="identifier">errno</span></code>
  62           is set when a domain error occurs as well as returning a NaN.
  63         </p>
  64 <p>
  65           This is safe provided the program consists of a single translation unit
  66           <span class="emphasis"><em>and</em></span> we place the define <span class="emphasis"><em>before</em></span>
  67           any #includes. Note that should we add the define after the includes then
  68           it will have no effect! A warning such as:
  69         </p>
  70 <pre class="programlisting">warning C4005: 'BOOST_MATH_OVERFLOW_ERROR_POLICY' : macro redefinition</pre>
  71 <p>
  72           is a certain sign that it will <span class="emphasis"><em>not</em></span> have the desired
  73           effect.
  74         </p>
  75 <p>
  76           We'll begin our sample program with the needed includes:
  77         </p>
  78 <pre class="programlisting">   <span class="preprocessor">#define</span> <span class="identifier">BOOST_MATH_DOMAIN_ERROR_POLICY</span> <span class="identifier">ignore_error</span>
  79
  80 <span class="comment">// Boost</span>
  81 <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">students_t</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</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>  <span class="comment">// Probability of students_t(df, t).</span>
  83
  84 <span class="comment">// std</span>
  85 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">iostream</span><span class="special">&gt;</span>
  86    <span class="keyword">using</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span><span class="special">;</span>
  87    <span class="keyword">using</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
  88
  89 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">stdexcept</span><span class="special">&gt;</span>
  90
  91
  92 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">cstddef</span><span class="special">&gt;</span>
  93    <span class="comment">// using ::errno</span>
  94 </pre>
  95 <p>
  96           Next we'll define the program's main() to call the student's t distribution
  97           with an invalid degrees of freedom parameter, the program is set up to
  98           handle either an exception or a NaN:
  99         </p>
 100 <pre class="programlisting"><span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
 101 <span class="special">{</span>
 102    <span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"Example error handling using Student's t function. "</span> <span class="special">&lt;&lt;</span> <span class="identifier">endl</span><span class="special">;</span>
 103    <span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"BOOST_MATH_DOMAIN_ERROR_POLICY is set to: "</span>
 104       <span class="special">&lt;&lt;</span> <span class="identifier">BOOST_STRINGIZE</span><span class="special">(</span><span class="identifier">BOOST_MATH_DOMAIN_ERROR_POLICY</span><span class="special">)</span> <span class="special">&lt;&lt;</span> <span class="identifier">endl</span><span class="special">;</span>
 105
 106    <span class="keyword">double</span> <span class="identifier">degrees_of_freedom</span> <span class="special">=</span> <span class="special">-</span><span class="number">1</span><span class="special">;</span> <span class="comment">// A bad argument!</span>
 107    <span class="keyword">double</span> <span class="identifier">t</span> <span class="special">=</span> <span class="number">10</span><span class="special">;</span>
 108
 109    <span class="keyword">try</span>
 110    <span class="special">{</span>
 111       <span class="identifier">errno</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span> <span class="comment">// Clear/reset.</span>
 112       <span class="identifier">students_t</span> <span class="identifier">dist</span><span class="special">(</span><span class="identifier">degrees_of_freedom</span><span class="special">);</span> <span class="comment">// exception is thrown here if enabled.</span>
 113       <span class="keyword">double</span> <span class="identifier">p</span> <span class="special">=</span> <span class="identifier">cdf</span><span class="special">(</span><span class="identifier">dist</span><span class="special">,</span> <span class="identifier">t</span><span class="special">);</span>
 114       <span class="comment">// Test for error reported by other means:</span>
 115       <span class="keyword">if</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">isnan</span><span class="special">)(</span><span class="identifier">p</span><span class="special">))</span>
 116       <span class="special">{</span>
 117          <span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"cdf returned a NaN!"</span> <span class="special">&lt;&lt;</span> <span class="identifier">endl</span><span class="special">;</span>
 118          <span class="keyword">if</span> <span class="special">(</span><span class="identifier">errno</span> <span class="special">!=</span> <span class="number">0</span><span class="special">)</span>
 119          <span class="special">{</span> <span class="comment">// So errno has been set.</span>
 120            <span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"errno is set to: "</span> <span class="special">&lt;&lt;</span> <span class="identifier">errno</span> <span class="special">&lt;&lt;</span> <span class="identifier">endl</span><span class="special">;</span>
 121          <span class="special">}</span>
 122       <span class="special">}</span>
 123       <span class="keyword">else</span>
 124          <span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"Probability of Student's t is "</span> <span class="special">&lt;&lt;</span> <span class="identifier">p</span> <span class="special">&lt;&lt;</span> <span class="identifier">endl</span><span class="special">;</span>
 125    <span class="special">}</span>
 126    <span class="keyword">catch</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">exception</span><span class="special">&amp;</span> <span class="identifier">e</span><span class="special">)</span>
 127    <span class="special">{</span>
 128       <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span>
 129          <span class="string">"\n"</span><span class="string">"Message from thrown exception was:\n   "</span> <span class="special">&lt;&lt;</span> <span class="identifier">e</span><span class="special">.</span><span class="identifier">what</span><span class="special">()</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
 130    <span class="special">}</span>
 131    <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
 132 <span class="special">}</span> <span class="comment">// int main()</span>
 133 </pre>
 134 <p>
 135           Here's what the program output looks like with a default build (one that
 136           <span class="bold"><strong>does throw exceptions</strong></span>):
 137         </p>
 138 <pre class="programlisting">Example error handling using Student's t function.
 139 BOOST_MATH_DOMAIN_ERROR_POLICY is set to: throw_on_error
 140
 141 Message from thrown exception was:
 142    Error in function boost::math::students_t_distribution&lt;double&gt;::students_t_distribution:
 143    Degrees of freedom argument is -1, but must be &gt; 0 !
 144 </pre>
 145 <p>
 146           Alternatively let's build with:
 147         </p>
 148 <pre class="programlisting"><span class="preprocessor">#define</span> <span class="identifier">BOOST_MATH_DOMAIN_ERROR_POLICY</span> <span class="identifier">ignore_error</span>
 149 </pre>
 150 <p>
 151           Now the program output is:
 152         </p>
 153 <pre class="programlisting">Example error handling using Student's t function.
 154 BOOST_MATH_DOMAIN_ERROR_POLICY is set to: ignore_error
 155 cdf returned a NaN!
 156 </pre>
 157 <p>
 158           And finally let's build with:
 159         </p>
 160 <pre class="programlisting"><span class="preprocessor">#define</span> <span class="identifier">BOOST_MATH_DOMAIN_ERROR_POLICY</span> <span class="identifier">errno_on_error</span>
 161 </pre>
 162 <p>
 163           Which gives the output show errno:
 164         </p>
 165 <pre class="programlisting">Example error handling using Student's t function.
 166 BOOST_MATH_DOMAIN_ERROR_POLICY is set to: errno_on_error
 167 cdf returned a NaN!
 168 errno is set to: 33
 169 </pre>
 170 <div class="caution"><table border="0" summary="Caution">
 171 <tr>
 172 <td rowspan="2" align="center" valign="top" width="25"><img alt="[Caution]" src="../../../../../../../doc/src/images/caution.png"></td>
 173 <th align="left">Caution</th>
 174 </tr>
 175 <tr><td align="left" valign="top">
 176 <p>
 177             If throwing of exceptions is enabled (the default) but you do <span class="bold"><strong>not</strong></span> have try &amp; catch block, then the program
 178             will terminate with an uncaught exception and probably abort.
 179           </p>
 180 <p>
 181             Therefore to get the benefit of helpful error messages, enabling <span class="bold"><strong>all exceptions and using try &amp; catch</strong></span> is recommended
 182             for most applications.
 183           </p>
 184 <p>
 185             However, for simplicity, the is not done for most examples.
 186           </p>
 187 </td></tr>
 188 </table></div>
 189 </div>
 190 <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
 191 <td align="left"></td>
 192 <td align="right"><div class="copyright-footer">Copyright &#169; 2006-2010, 2012-2014 Nikhar Agrawal,
 193       Anton Bikineev, Paul A. Bristow, Marco Guazzone, Christopher Kormanyos, Hubert
 194       Holin, Bruno Lalande, John Maddock, Jeremy Murphy, Johan R&#229;de, Gautam Sewani,
 195       Benjamin Sobotta, Thijs van den Berg, Daryle Walker and Xiaogang Zhang<p>
 196         Distributed under the Boost Software License, Version 1.0. (See accompanying
 197         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>)
 198       </p>
 199 </div></td>
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