3 <meta http-equiv="Content-Type" content="text/html; charset=US-ASCII">
4 <title>Precision Policies</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 2.5.2">
8 <link rel="up" href="../pol_ref.html" title="Policy Reference">
9 <link rel="prev" href="discrete_quant_ref.html" title="Discrete Quantile Policies">
10 <link rel="next" href="iteration_pol.html" title="Iteration Limits Policies">
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>
22 <div class="spirit-nav">
23 <a accesskey="p" href="discrete_quant_ref.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="iteration_pol.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a>
26 <div class="titlepage"><div><div><h3 class="title">
27 <a name="math_toolkit.pol_ref.precision_pol"></a><a class="link" href="precision_pol.html" title="Precision Policies">Precision Policies</a>
28 </h3></div></div></div>
30 There are two equivalent policies that effect the <span class="emphasis"><em>working precision</em></span>
31 used to calculate results, these policies both default to 0 - meaning calculate
32 to the maximum precision available in the type being used - but can be set
33 to other values to cause lower levels of precision to be used. One might
34 want to trade precision for evaluation speed.
36 <pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">boost</span><span class="special">{</span> <span class="keyword">namespace</span> <span class="identifier">math</span><span class="special">{</span> <span class="keyword">namespace</span> <span class="identifier">policies</span><span class="special">{</span>
38 <span class="keyword">template</span> <span class="special"><</span><span class="keyword">int</span> <span class="identifier">N</span><span class="special">></span>
39 <span class="identifier">digits10</span><span class="special">;</span>
41 <span class="keyword">template</span> <span class="special"><</span><span class="keyword">int</span> <span class="identifier">N</span><span class="special">></span>
42 <span class="identifier">digits2</span><span class="special">;</span>
44 <span class="special">}}}</span> <span class="comment">// namespaces</span>
47 As you would expect, <span class="emphasis"><em>digits10</em></span> specifies the number of
48 decimal digits to use, and <span class="emphasis"><em>digits2</em></span> the number of binary
49 digits. Internally, whichever is used, the precision is always converted
50 to <span class="emphasis"><em>binary digits</em></span>.
53 These policies are specified at compile-time, because many of the special
54 functions use compile-time-dispatch to select which approximation to use
55 based on the precision requested and the numeric type being used.
58 For example we could calculate <code class="computeroutput"><span class="identifier">tgamma</span></code>
59 to approximately 5 decimal digits using:
61 <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>
62 <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">tgamma</span><span class="special">;</span>
63 <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">policies</span><span class="special">::</span><span class="identifier">policy</span><span class="special">;</span>
64 <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">policies</span><span class="special">::</span><span class="identifier">digits10</span><span class="special">;</span>
66 <span class="keyword">typedef</span> <span class="identifier">policy</span><span class="special"><</span><span class="identifier">digits10</span><span class="special"><</span><span class="number">5</span><span class="special">></span> <span class="special">></span> <span class="identifier">my_pol_5</span><span class="special">;</span> <span class="comment">// Define a new, non-default, policy</span>
67 <span class="comment">// to calculate tgamma to accuracy of approximately 5 decimal digits.</span>
70 Or again using helper function <code class="computeroutput"><span class="identifier">make_policy</span></code>:
72 <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>
73 <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">tgamma</span><span class="special">;</span>
75 <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>
77 <span class="keyword">double</span> <span class="identifier">t</span> <span class="special">=</span> <span class="identifier">tgamma</span><span class="special">(</span><span class="number">12</span><span class="special">,</span> <span class="identifier">policy</span><span class="special"><</span><span class="identifier">digits10</span><span class="special"><</span><span class="number">5</span><span class="special">></span> <span class="special">>());</span> <span class="comment">// Concise make_policy.</span>
80 And for a quantile of a distribution to approximately 25-bit precision:
82 <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>
83 <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">normal_distribution</span><span class="special">;</span>
85 <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>
87 <span class="keyword">const</span> <span class="keyword">int</span> <span class="identifier">bits</span> <span class="special">=</span> <span class="number">25</span><span class="special">;</span> <span class="comment">// approximate precision.</span>
89 <span class="keyword">double</span> <span class="identifier">q</span> <span class="special">=</span> <span class="identifier">quantile</span><span class="special">(</span>
90 <span class="identifier">normal_distribution</span><span class="special"><</span><span class="keyword">double</span><span class="special">,</span> <span class="identifier">policy</span><span class="special"><</span><span class="identifier">digits2</span><span class="special"><</span><span class="identifier">bits</span><span class="special">></span> <span class="special">></span> <span class="special">>(),</span>
91 <span class="number">0.05</span><span class="special">);</span> <span class="comment">// 5% quantile.</span>
94 <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
95 <td align="left"></td>
96 <td align="right"><div class="copyright-footer">Copyright © 2006-2010, 2012-2014 Nikhar Agrawal,
97 Anton Bikineev, Paul A. Bristow, Marco Guazzone, Christopher Kormanyos, Hubert
98 Holin, Bruno Lalande, John Maddock, Jeremy Murphy, Johan Råde, Gautam Sewani,
99 Benjamin Sobotta, Thijs van den Berg, Daryle Walker and Xiaogang Zhang<p>
100 Distributed under the Boost Software License, Version 1.0. (See accompanying
101 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>)
106 <div class="spirit-nav">
107 <a accesskey="p" href="discrete_quant_ref.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="iteration_pol.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a>