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4 | <title>Policies Have Sensible Defaults</title> | |
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24 | </div> | |
25 | <div class="section"> | |
26 | <div class="titlepage"><div><div><h3 class="title"> | |
27 | <a name="math_toolkit.pol_tutorial.policy_tut_defaults"></a><a class="link" href="policy_tut_defaults.html" title="Policies Have Sensible Defaults">Policies | |
28 | Have Sensible Defaults</a> | |
29 | </h3></div></div></div> | |
30 | <p> | |
31 | Most of the time you can just ignore the policy framework. | |
32 | </p> | |
33 | <p> | |
34 | <span class="emphasis"><em>*The defaults for the various policies are as follows, if these | |
35 | work OK for you then you can stop reading now!</em></span> | |
36 | </p> | |
37 | <div class="variablelist"> | |
38 | <p class="title"><b></b></p> | |
39 | <dl class="variablelist"> | |
40 | <dt><span class="term">Domain Error</span></dt> | |
41 | <dd><p> | |
42 | Throws a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">domain_error</span></code> exception. | |
43 | </p></dd> | |
44 | <dt><span class="term">Pole Error</span></dt> | |
45 | <dd><p> | |
46 | 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. | |
47 | </p></dd> | |
48 | <dt><span class="term">Overflow Error</span></dt> | |
49 | <dd><p> | |
50 | Throws a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">overflow_error</span></code> exception. | |
51 | </p></dd> | |
52 | <dt><span class="term">Underflow</span></dt> | |
53 | <dd><p> | |
54 | Ignores the underflow, and returns zero. | |
55 | </p></dd> | |
56 | <dt><span class="term">Denormalised Result</span></dt> | |
57 | <dd><p> | |
58 | Ignores the fact that the result is denormalised, and returns it. | |
59 | </p></dd> | |
60 | <dt><span class="term">Rounding Error</span></dt> | |
61 | <dd><p> | |
62 | 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. | |
63 | </p></dd> | |
64 | <dt><span class="term">Internal Evaluation Error</span></dt> | |
65 | <dd><p> | |
66 | 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. | |
67 | </p></dd> | |
68 | <dt><span class="term">Indeterminate Result Error</span></dt> | |
69 | <dd><p> | |
70 | Returns a result that depends on the function where the error occurred. | |
71 | </p></dd> | |
72 | <dt><span class="term">Promotion of float to double</span></dt> | |
73 | <dd><p> | |
74 | Does occur by default - gives full float precision results. | |
75 | </p></dd> | |
76 | <dt><span class="term">Promotion of double to long double</span></dt> | |
77 | <dd><p> | |
78 | Does occur by default if long double offers more precision than double. | |
79 | </p></dd> | |
80 | <dt><span class="term">Precision of Approximation Used</span></dt> | |
81 | <dd><p> | |
82 | By default uses an approximation that will result in the lowest level | |
83 | of error for the type of the result. | |
84 | </p></dd> | |
85 | <dt><span class="term">Behaviour of Discrete Quantiles</span></dt> | |
86 | <dd> | |
87 | <p> | |
88 | The quantile function will by default return an integer result that | |
89 | has been <span class="emphasis"><em>rounded outwards</em></span>. That is to say lower | |
90 | quantiles (where the probability is less than 0.5) are rounded downward, | |
91 | and upper quantiles (where the probability is greater than 0.5) are | |
92 | rounded upwards. This behaviour ensures that if an X% quantile is requested, | |
93 | then <span class="emphasis"><em>at least</em></span> the requested coverage will be present | |
94 | in the central region, and <span class="emphasis"><em>no more than</em></span> the requested | |
95 | coverage will be present in the tails. | |
96 | </p> | |
97 | <p> | |
98 | This behaviour can be changed so that the quantile functions are rounded | |
99 | differently, or even return a real-valued result using <a class="link" href="../pol_overview.html" title="Policy Overview">Policies</a>. | |
100 | It is strongly recommended that you read the tutorial <a class="link" href="understand_dis_quant.html" title="Understanding Quantiles of Discrete Distributions">Understanding | |
101 | Quantiles of Discrete Distributions</a> before using the quantile | |
102 | function on a discrete distribution. The <a class="link" href="../pol_ref/discrete_quant_ref.html" title="Discrete Quantile Policies">reference | |
103 | docs</a> describe how to change the rounding policy for these distributions. | |
104 | </p> | |
105 | </dd> | |
106 | </dl> | |
107 | </div> | |
108 | <p> | |
109 | What's more, if you define your own policy type, then it automatically inherits | |
110 | the defaults for any policies not explicitly set, so given: | |
111 | </p> | |
112 | <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> | |
113 | <span class="comment">//</span> | |
114 | <span class="comment">// Define a policy that sets ::errno on overflow, and does</span> | |
115 | <span class="comment">// not promote double to long double internally:</span> | |
116 | <span class="comment">//</span> | |
117 | <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> | |
118 | </pre> | |
119 | <p> | |
120 | then <code class="computeroutput"><span class="identifier">mypolicy</span></code> defines a policy | |
121 | where only the overflow error handling and <code class="computeroutput"><span class="keyword">double</span></code>-promotion | |
122 | policies differ from the defaults. | |
123 | </p> | |
124 | </div> | |
125 | <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> | |
126 | <td align="left"></td> | |
127 | <td align="right"><div class="copyright-footer">Copyright © 2006-2010, 2012-2014 Nikhar Agrawal, | |
128 | Anton Bikineev, Paul A. Bristow, Marco Guazzone, Christopher Kormanyos, Hubert | |
129 | Holin, Bruno Lalande, John Maddock, Jeremy Murphy, Johan Råde, Gautam Sewani, | |
130 | Benjamin Sobotta, Thijs van den Berg, Daryle Walker and Xiaogang Zhang<p> | |
131 | Distributed under the Boost Software License, Version 1.0. (See accompanying | |
132 | 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>) | |
133 | </p> | |
134 | </div></td> | |
135 | </tr></table> | |
136 | <hr> | |
137 | <div class="spirit-nav"> | |
138 | <a accesskey="p" href="what_is_a_policy.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../pol_tutorial.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="policy_usage.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a> | |
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