1<html> 2<head> 3<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 4<title>Error Handling Example</title> 5<link rel="stylesheet" href="../../../math.css" type="text/css"> 6<meta name="generator" content="DocBook XSL Stylesheets V1.79.1"> 7<link rel="home" href="../../../index.html" title="Math Toolkit 2.12.0"> 8<link rel="up" href="../weg.html" title="Worked Examples"> 9<link rel="prev" href="nccs_eg/nccs_power_eg.html" title="Tables of the power function of the chi2 test."> 10<link rel="next" href="find_eg.html" title="Find Location and Scale Examples"> 11</head> 12<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> 13<table cellpadding="2" width="100%"><tr> 14<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../../../boost.png"></td> 15<td align="center"><a href="../../../../../../../index.html">Home</a></td> 16<td align="center"><a href="../../../../../../../libs/libraries.htm">Libraries</a></td> 17<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> 18<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> 19<td align="center"><a href="../../../../../../../more/index.htm">More</a></td> 20</tr></table> 21<hr> 22<div class="spirit-nav"> 23<a accesskey="p" href="nccs_eg/nccs_power_eg.html"><img src="../../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../weg.html"><img src="../../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../../index.html"><img src="../../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="find_eg.html"><img src="../../../../../../../doc/src/images/next.png" alt="Next"></a> 24</div> 25<div class="section"> 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 21. 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"><</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> 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"><</span><span class="identifier">iostream</span><span class="special">></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"><</span><span class="identifier">stdexcept</span><span class="special">></span> 90 91 92<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">cstddef</span><span class="special">></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"><<</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> 103 <span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"BOOST_MATH_DOMAIN_ERROR_POLICY is set to: "</span> 104 <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> 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"><<</span> <span class="string">"cdf returned a NaN!"</span> <span class="special"><<</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"><<</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> 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"><<</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> 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">&</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"><<</span> 129 <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> 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. 139BOOST_MATH_DOMAIN_ERROR_POLICY is set to: throw_on_error 140 141Message from thrown exception was: 142 Error in function boost::math::students_t_distribution<double>::students_t_distribution: 143 Degrees of freedom argument is -1, but must be > 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. 154BOOST_MATH_DOMAIN_ERROR_POLICY is set to: ignore_error 155cdf 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. 166BOOST_MATH_DOMAIN_ERROR_POLICY is set to: errno_on_error 167cdf returned a NaN! 168errno 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 & 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 & 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 © 2006-2019 Nikhar 193 Agrawal, Anton Bikineev, Paul A. Bristow, Marco Guazzone, Christopher Kormanyos, 194 Hubert Holin, Bruno Lalande, John Maddock, Jeremy Murphy, Matthew Pulver, Johan 195 Råde, Gautam Sewani, Benjamin Sobotta, Nicholas Thompson, Thijs van den Berg, 196 Daryle Walker and Xiaogang Zhang<p> 197 Distributed under the Boost Software License, Version 1.0. (See accompanying 198 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>) 199 </p> 200</div></td> 201</tr></table> 202<hr> 203<div class="spirit-nav"> 204<a accesskey="p" href="nccs_eg/nccs_power_eg.html"><img src="../../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../weg.html"><img src="../../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../../index.html"><img src="../../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="find_eg.html"><img src="../../../../../../../doc/src/images/next.png" alt="Next"></a> 205</div> 206</body> 207</html> 208
