<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=US-ASCII">
<title>Internal Promotion Policies</title>
<link rel="stylesheet" href="../../../../../../../../doc/src/boostbook.css" type="text/css">
<meta name="generator" content="DocBook XSL Stylesheets V1.74.0">
<link rel="home" href="../../../index.html" title="Math Toolkit">
<link rel="up" href="../pol_ref.html" title="Policy Reference">
<link rel="prev" href="error_handling_policies.html" title="Error Handling Policies">
<link rel="next" href="assert_undefined.html" title="Mathematically Undefined Function Policies">
</head>
<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF">
<table cellpadding="2" width="100%"><tr>
<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../../../../boost.png"></td>
<td align="center"><a href="../../../../../../../../index.html">Home</a></td>
<td align="center"><a href="../../../../../../../../libs/libraries.htm">Libraries</a></td>
<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td>
<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td>
<td align="center"><a href="../../../../../../../../more/index.htm">More</a></td>
</tr></table>
<hr>
<div class="spirit-nav">
<a accesskey="p" href="error_handling_policies.html"><img src="../../../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../pol_ref.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="assert_undefined.html"><img src="../../../../../../../../doc/src/images/next.png" alt="Next"></a>
</div>
<div class="section" lang="en">
<div class="titlepage"><div><div><h4 class="title">
<a name="math_toolkit.policy.pol_ref.internal_promotion"></a><a class="link" href="internal_promotion.html" title="Internal Promotion Policies"> Internal
Promotion Policies</a>
</h4></div></div></div>
<p>
Normally when evaluating a function at say <code class="computeroutput"><span class="keyword">float</span></code>
precision, maximal accuracy is assured by conducting the calculation at
<code class="computeroutput"><span class="keyword">double</span></code> precision internally,
and then rounding the result. There are two policies that effect whether
internal promotion takes place or not:
</p>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th>
<p>
Policy
</p>
</th>
<th>
<p>
Meaning
</p>
</th>
</tr></thead>
<tbody>
<tr>
<td>
<p>
<code class="computeroutput"><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="identifier">promote_float</span><span class="special"><</span><span class="identifier">B</span><span class="special">></span></code>
</p>
</td>
<td>
<p>
Indicates whether <code class="computeroutput"><span class="keyword">float</span></code>
arguments should be promoted to <code class="computeroutput"><span class="keyword">double</span></code>
precision internally: defaults to <code class="computeroutput"><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="identifier">promote_float</span><span class="special"><</span><span class="keyword">true</span><span class="special">></span></code>
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><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="identifier">promote_double</span><span class="special"><</span><span class="identifier">B</span><span class="special">></span></code>
</p>
</td>
<td>
<p>
Indicates whether <code class="computeroutput"><span class="keyword">double</span></code>
arguments should be promoted to <code class="computeroutput"><span class="keyword">long</span>
<span class="keyword">double</span></code> precision internally:
defaults to <code class="computeroutput"><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="identifier">promote_double</span><span class="special"><</span><span class="keyword">true</span><span class="special">></span></code>
</p>
</td>
</tr>
</tbody>
</table></div>
<a name="math_toolkit.policy.pol_ref.internal_promotion.examples"></a><h5>
<a name="id1264764"></a>
<a class="link" href="internal_promotion.html#math_toolkit.policy.pol_ref.internal_promotion.examples">Examples</a>
</h5>
<p>
Suppose we want <code class="computeroutput"><span class="identifier">tgamma</span></code>
to be evaluated without internal promotion to <code class="computeroutput"><span class="keyword">long</span>
<span class="keyword">double</span></code>, then we could use:
</p>
<p>
</p>
<p>
</p>
<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">special_functions</span><span class="special">/</span><span class="identifier">gamma</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
<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="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="comment">// Define a policy:
</span><span class="keyword">typedef</span> <span class="identifier">policy</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">my_policy</span><span class="special">;</span>
<span class="comment">// Call the function:
</span><span class="keyword">double</span> <span class="identifier">t1</span> <span class="special">=</span> <span class="identifier">tgamma</span><span class="special">(</span><span class="identifier">some_value</span><span class="special">,</span> <span class="identifier">my_policy</span><span class="special">());</span>
<span class="comment">// Alternatively we could use make_policy and define everything at the call site:
</span><span class="keyword">double</span> <span class="identifier">t2</span> <span class="special">=</span> <span class="identifier">tgamma</span><span class="special">(</span><span class="identifier">some_value</span><span class="special">,</span> <span class="identifier">make_policy</span><span class="special">(</span><span class="identifier">promote_double</span><span class="special"><</span><span class="keyword">false</span><span class="special">>()));</span>
</pre>
<p>
</p>
<p>
</p>
<p>
Alternatively, suppose we want a distribution to perform calculations without
promoting <code class="computeroutput"><span class="keyword">float</span></code> to <code class="computeroutput"><span class="keyword">double</span></code>, then we could use:
</p>
<p>
</p>
<p>
</p>
<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">normal</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
<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="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="comment">// Define a policy:
</span><span class="keyword">typedef</span> <span class="identifier">policy</span><span class="special"><</span>
<span class="identifier">promote_float</span><span class="special"><</span><span class="keyword">false</span><span class="special">></span>
<span class="special">></span> <span class="identifier">my_policy</span><span class="special">;</span>
<span class="comment">// Define the distribution:
</span><span class="keyword">typedef</span> <span class="identifier">normal_distribution</span><span class="special"><</span><span class="keyword">float</span><span class="special">,</span> <span class="identifier">my_policy</span><span class="special">></span> <span class="identifier">my_norm</span><span class="special">;</span>
<span class="comment">// Get a quantile:
</span><span class="keyword">float</span> <span class="identifier">q</span> <span class="special">=</span> <span class="identifier">quantile</span><span class="special">(</span><span class="identifier">my_norm</span><span class="special">(),</span> <span class="number">0.05f</span><span class="special">);</span>
</pre>
<p>
</p>
<p>
</p>
</div>
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
<td align="left"></td>
<td align="right"><div class="copyright-footer">Copyright © 2006 , 2007, 2008, 2009 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>)
</p>
</div></td>
</tr></table>
<hr>
<div class="spirit-nav">
<a accesskey="p" href="error_handling_policies.html"><img src="../../../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../pol_ref.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="assert_undefined.html"><img src="../../../../../../../../doc/src/images/next.png" alt="Next"></a>
</div>
</body>
</html>
