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| <div class="titlepage"><div><div><h4 class="title"> |
| <a name="math_toolkit.policy.pol_tutorial.policy_tut_defaults"></a><a class="link" href="policy_tut_defaults.html" title="Policies Have Sensible Defaults"> |
| Policies Have Sensible Defaults</a> |
| </h4></div></div></div> |
| <p> |
| Most of the time you can just ignore the policy framework. |
| </p> |
| <p> |
| <span class="emphasis"><em>*The defaults for the various policies are as follows, if these |
| work OK for you then you can stop reading now!</em></span> |
| </p> |
| <div class="variablelist"> |
| <p class="title"><b></b></p> |
| <dl> |
| <dt><span class="term">Domain Error</span></dt> |
| <dd><p> |
| Throws a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">domain_error</span></code> exception. |
| </p></dd> |
| <dt><span class="term">Pole Error</span></dt> |
| <dd><p> |
| Occurs when a function is evaluated at a pole: throws a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">domain_error</span></code> exception. |
| </p></dd> |
| <dt><span class="term">Overflow Error</span></dt> |
| <dd><p> |
| Throws a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">overflow_error</span></code> exception. |
| </p></dd> |
| <dt><span class="term">Underflow</span></dt> |
| <dd><p> |
| Ignores the underflow, and returns zero. |
| </p></dd> |
| <dt><span class="term">Denormalised Result</span></dt> |
| <dd><p> |
| Ignores the fact that the result is denormalised, and returns it. |
| </p></dd> |
| <dt><span class="term">Rounding Error</span></dt> |
| <dd><p> |
| Throws a <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">math</span><span class="special">::</span><span class="identifier">rounding_error</span></code> exception. |
| </p></dd> |
| <dt><span class="term">Internal Evaluation Error</span></dt> |
| <dd><p> |
| Throws a <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">math</span><span class="special">::</span><span class="identifier">evaluation_error</span></code> exception. |
| </p></dd> |
| <dt><span class="term">Indeterminate Result Error</span></dt> |
| <dd><p> |
| Returns a result that depends on the function where the error occurred. |
| </p></dd> |
| <dt><span class="term">Promotion of float to double</span></dt> |
| <dd><p> |
| Does occur by default - gives full float precision results. |
| </p></dd> |
| <dt><span class="term">Promotion of double to long double</span></dt> |
| <dd><p> |
| Does occur by default if long double offers more precision than double. |
| </p></dd> |
| <dt><span class="term">Precision of Approximation Used</span></dt> |
| <dd><p> |
| By default uses an approximation that will result in the lowest level |
| of error for the type of the result. |
| </p></dd> |
| <dt><span class="term">Behaviour of Discrete Quantiles</span></dt> |
| <dd> |
| <p> |
| The quantile function will by default return an integer result that |
| has been <span class="emphasis"><em>rounded outwards</em></span>. That is to say lower |
| quantiles (where the probability is less than 0.5) are rounded downward, |
| and upper quantiles (where the probability is greater than 0.5) are |
| rounded upwards. This behaviour ensures that if an X% quantile is |
| requested, then <span class="emphasis"><em>at least</em></span> the requested coverage |
| will be present in the central region, and <span class="emphasis"><em>no more than</em></span> |
| the requested coverage will be present in the tails. |
| </p> |
| <p> |
| This behaviour can be changed so that the quantile functions are |
| rounded differently, or even return a real-valued result using <a class="link" href="../pol_overview.html" title="Policy Overview">Policies</a>. It is |
| strongly recommended that you read the tutorial <a class="link" href="understand_dis_quant.html" title="Understanding Quantiles of Discrete Distributions">Understanding |
| Quantiles of Discrete Distributions</a> before using the quantile |
| function on a discrete distribution. The <a class="link" href="../pol_ref/discrete_quant_ref.html" title="Discrete Quantile Policies">reference |
| docs</a> describe how to change the rounding policy for these |
| distributions. |
| </p> |
| </dd> |
| </dl> |
| </div> |
| <p> |
| What's more, if you define your own policy type, then it automatically |
| inherits the defaults for any policies not explicitly set, so given: |
| </p> |
| <pre class="programlisting"><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> |
| <span class="comment">// |
| </span><span class="comment">// Define a policy that sets ::errno on overflow, and does |
| </span><span class="comment">// not promote double to long double internally: |
| </span><span class="comment">// |
| </span><span class="keyword">typedef</span> <span class="identifier">policy</span><span class="special"><</span><span class="identifier">domain_error</span><span class="special"><</span><span class="identifier">errno_on_error</span><span class="special">>,</span> <span class="identifier">promote_double</span><span class="special"><</span><span class="keyword">false</span><span class="special">></span> <span class="special">></span> <span class="identifier">mypolicy</span><span class="special">;</span> |
| </pre> |
| <p> |
| then <code class="computeroutput"><span class="identifier">mypolicy</span></code> defines a |
| policy where only the overflow error handling and <code class="computeroutput"><span class="keyword">double</span></code>-promotion |
| policies differ from the defaults. |
| </p> |
| </div> |
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| <td align="right"><div class="copyright-footer">Copyright © 2006 , 2007, 2008, 2009, 2010 John Maddock, Paul A. Bristow, |
| Hubert Holin, Xiaogang Zhang, Bruno Lalande, Johan Råde, Gautam Sewani and |
| Thijs van den Berg<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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