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<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="math_toolkit.stat_tut.weg.error_eg"></a><a class="link" href="error_eg.html" title="Error Handling Example">Error Handling
Example</a>
</h4></div></div></div>
<p>
See <a class="link" href="../../error_handling.html" title="Error Handling">error handling documentation</a>
for a detailed explanation of the mechanism of handling errors, including
the common "bad" arguments to distributions and functions, and
how to use <a class="link" href="../../../policy.html" title="Chapter 14. Policies: Controlling Precision, Error Handling etc">Policies</a> to control it.
</p>
<p>
But, by default, <span class="bold"><strong>exceptions will be raised</strong></span>,
for domain errors, pole errors, numeric overflow, and internal evaluation
errors. To avoid the exceptions from getting thrown and instead get an
appropriate value returned, usually a NaN (domain errors pole errors or
internal errors), or infinity (from overflow), you need to change the policy.
</p>
<p>
The following example demonstrates the effect of setting the macro BOOST_MATH_DOMAIN_ERROR_POLICY
when an invalid argument is encountered. For the purposes of this example,
we'll pass a negative degrees of freedom parameter to the student's t distribution.
</p>
<p>
Since we know that this is a single file program we could just add:
</p>
<pre class="programlisting"><span class="preprocessor">#define</span> <span class="identifier">BOOST_MATH_DOMAIN_ERROR_POLICY</span> <span class="identifier">ignore_error</span>
</pre>
<p>
to the top of the source file to change the default policy to one that
simply returns a NaN when a domain error occurs. Alternatively we could
use:
</p>
<pre class="programlisting"><span class="preprocessor">#define</span> <span class="identifier">BOOST_MATH_DOMAIN_ERROR_POLICY</span> <span class="identifier">errno_on_error</span>
</pre>
<p>
To ensure the <code class="computeroutput"><span class="special">::</span><span class="identifier">errno</span></code>
is set when a domain error occurs as well as returning a NaN.
</p>
<p>
This is safe provided the program consists of a single translation unit
<span class="emphasis"><em>and</em></span> we place the define <span class="emphasis"><em>before</em></span>
any #includes. Note that should we add the define after the includes then
it will have no effect! A warning such as:
</p>
<pre class="programlisting">warning C4005: 'BOOST_MATH_OVERFLOW_ERROR_POLICY' : macro redefinition</pre>
<p>
is a certain sign that it will <span class="emphasis"><em>not</em></span> have the desired
effect.
</p>
<p>
We'll begin our sample program with the needed includes:
</p>
<pre class="programlisting"> <span class="preprocessor">#define</span> <span class="identifier">BOOST_MATH_DOMAIN_ERROR_POLICY</span> <span class="identifier">ignore_error</span>
<span class="comment">// Boost</span>
<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">students_t</span><span class="special">.</span><span class="identifier">hpp</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">students_t</span><span class="special">;</span> <span class="comment">// Probability of students_t(df, t).</span>
<span class="comment">// std</span>
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">iostream</span><span class="special">></span>
<span class="keyword">using</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span><span class="special">;</span>
<span class="keyword">using</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">stdexcept</span><span class="special">></span>
<span class="keyword">using</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">exception</span><span class="special">;</span>
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">cstddef</span><span class="special">></span>
<span class="comment">// using ::errno</span>
</pre>
<p>
Next we'll define the program's main() to call the student's t distribution
with an invalid degrees of freedom parameter, the program is set up to
handle either an exception or a NaN:
</p>
<pre class="programlisting"><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"><<</span> <span class="string">"Example error handling using Student's t function. "</span> <span class="special"><<</span> <span class="identifier">endl</span><span class="special">;</span>
<span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"BOOST_MATH_DOMAIN_ERROR_POLICY is set to: "</span>
<span class="special"><<</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"><<</span> <span class="identifier">endl</span><span class="special">;</span>
<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>
<span class="keyword">double</span> <span class="identifier">t</span> <span class="special">=</span> <span class="number">10</span><span class="special">;</span>
<span class="keyword">try</span>
<span class="special">{</span>
<span class="identifier">errno</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span> <span class="comment">// Clear/reset.</span>
<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>
<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>
<span class="comment">// Test for error reported by other means:</span>
<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>
<span class="special">{</span>
<span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"cdf returned a NaN!"</span> <span class="special"><<</span> <span class="identifier">endl</span><span class="special">;</span>
<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>
<span class="special">{</span> <span class="comment">// So errno has been set.</span>
<span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"errno is set to: "</span> <span class="special"><<</span> <span class="identifier">errno</span> <span class="special"><<</span> <span class="identifier">endl</span><span class="special">;</span>
<span class="special">}</span>
<span class="special">}</span>
<span class="keyword">else</span>
<span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"Probability of Student's t is "</span> <span class="special"><<</span> <span class="identifier">p</span> <span class="special"><<</span> <span class="identifier">endl</span><span class="special">;</span>
<span class="special">}</span>
<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">&</span> <span class="identifier">e</span><span class="special">)</span>
<span class="special">{</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span>
<span class="string">"\n"</span><span class="string">"Message from thrown exception was:\n "</span> <span class="special"><<</span> <span class="identifier">e</span><span class="special">.</span><span class="identifier">what</span><span class="special">()</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
<span class="special">}</span>
<span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
<span class="special">}</span> <span class="comment">// int main()</span>
</pre>
<p>
Here's what the program output looks like with a default build (one that
<span class="bold"><strong>does throw exceptions</strong></span>):
</p>
<pre class="programlisting">Example error handling using Student's t function.
BOOST_MATH_DOMAIN_ERROR_POLICY is set to: throw_on_error
Message from thrown exception was:
Error in function boost::math::students_t_distribution<double>::students_t_distribution:
Degrees of freedom argument is -1, but must be > 0 !
</pre>
<p>
Alternatively let's build with:
</p>
<pre class="programlisting"><span class="preprocessor">#define</span> <span class="identifier">BOOST_MATH_DOMAIN_ERROR_POLICY</span> <span class="identifier">ignore_error</span>
</pre>
<p>
Now the program output is:
</p>
<pre class="programlisting">Example error handling using Student's t function.
BOOST_MATH_DOMAIN_ERROR_POLICY is set to: ignore_error
cdf returned a NaN!
</pre>
<p>
And finally let's build with:
</p>
<pre class="programlisting"><span class="preprocessor">#define</span> <span class="identifier">BOOST_MATH_DOMAIN_ERROR_POLICY</span> <span class="identifier">errno_on_error</span>
</pre>
<p>
Which gives the output show errno:
</p>
<pre class="programlisting">Example error handling using Student's t function.
BOOST_MATH_DOMAIN_ERROR_POLICY is set to: errno_on_error
cdf returned a NaN!
errno is set to: 33
</pre>
<div class="caution"><table border="0" summary="Caution">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Caution]" src="../../../../../../../doc/src/images/caution.png"></td>
<th align="left">Caution</th>
</tr>
<tr><td align="left" valign="top">
<p>
If throwing of exceptions is enabled (the default) but you do <span class="bold"><strong>not</strong></span> have try & catch block, then the program
will terminate with an uncaught exception and probably abort.
</p>
<p>
Therefore to get the benefit of helpful error messages, enabling <span class="bold"><strong>all exceptions and using try & catch</strong></span> is recommended
for most applications.
</p>
<p>
However, for simplicity, the is not done for most examples.
</p>
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<td align="left"></td>
<td align="right"><div class="copyright-footer">Copyright © 2006-2010, 2012-2014 Nikhar Agrawal,
Anton Bikineev, Paul A. Bristow, Marco Guazzone, Christopher Kormanyos, Hubert
Holin, Bruno Lalande, John Maddock, Johan Råde, Gautam Sewani, Benjamin Sobotta,
Thijs van den Berg, Daryle Walker and Xiaogang Zhang<p>
Distributed under the Boost Software License, Version 1.0. (See accompanying
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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