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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> | |
29 | <p> | |
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. | |
35 | </p> | |
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> | |
37 | ||
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> | |
40 | ||
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> | |
43 | ||
44 | <span class="special">}}}</span> <span class="comment">// namespaces</span> | |
45 | </pre> | |
46 | <p> | |
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>. | |
51 | </p> | |
52 | <p> | |
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. | |
56 | </p> | |
57 | <p> | |
58 | For example we could calculate <code class="computeroutput"><span class="identifier">tgamma</span></code> | |
59 | to approximately 5 decimal digits using: | |
60 | </p> | |
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> | |
65 | ||
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> | |
68 | </pre> | |
69 | <p> | |
70 | Or again using helper function <code class="computeroutput"><span class="identifier">make_policy</span></code>: | |
71 | </p> | |
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> | |
74 | ||
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> | |
76 | ||
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> | |
78 | </pre> | |
79 | <p> | |
80 | And for a quantile of a distribution to approximately 25-bit precision: | |
81 | </p> | |
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> | |
84 | ||
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> | |
86 | ||
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> | |
88 | ||
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> | |
92 | </pre> | |
93 | </div> | |
94 | <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> | |
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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>) | |
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