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|  | <title>Continued Fraction Evaluation</title> | ||
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|  | </div> | ||
|  | <div class="section"> | ||
|  | <div class="titlepage"><div><div><h3 class="title"> | ||
|  | <a name="math_toolkit.internals.cf"></a><a class="link" href="cf.html" title="Continued Fraction Evaluation">Continued Fraction Evaluation</a> | ||
|  | </h3></div></div></div> | ||
|  | <h5> | ||
|  | <a name="math_toolkit.internals.cf.h0"></a> | ||
|  |         <span class="phrase"><a name="math_toolkit.internals.cf.synopsis"></a></span><a class="link" href="cf.html#math_toolkit.internals.cf.synopsis">Synopsis</a> | ||
|  |       </h5> | ||
|  | <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">tools</span><span class="special">/</span><span class="identifier">fraction</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> | ||
|  | </pre> | ||
|  | <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">tools</span><span class="special">{</span> | ||
|  | 
 | ||
|  | <span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">Gen</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">U</span><span class="special">></span> | ||
|  | <span class="keyword">typename</span> <span class="identifier">detail</span><span class="special">::</span><span class="identifier">fraction_traits</span><span class="special"><</span><span class="identifier">Gen</span><span class="special">>::</span><span class="identifier">result_type</span> | ||
|  |    <span class="identifier">continued_fraction_b</span><span class="special">(</span><span class="identifier">Gen</span><span class="special">&</span> <span class="identifier">g</span><span class="special">,</span> <span class="keyword">const</span> <span class="identifier">U</span><span class="special">&</span> <span class="identifier">tolerance</span><span class="special">,</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">uintmax_t</span><span class="special">&</span> <span class="identifier">max_terms</span><span class="special">)</span> | ||
|  | 
 | ||
|  | <span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">Gen</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">U</span><span class="special">></span> | ||
|  | <span class="keyword">typename</span> <span class="identifier">detail</span><span class="special">::</span><span class="identifier">fraction_traits</span><span class="special"><</span><span class="identifier">Gen</span><span class="special">>::</span><span class="identifier">result_type</span> | ||
|  |    <span class="identifier">continued_fraction_b</span><span class="special">(</span><span class="identifier">Gen</span><span class="special">&</span> <span class="identifier">g</span><span class="special">,</span> <span class="keyword">const</span> <span class="identifier">U</span><span class="special">&</span> <span class="identifier">tolerance</span><span class="special">)</span> | ||
|  | 
 | ||
|  | <span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">Gen</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">U</span><span class="special">></span> | ||
|  | <span class="keyword">typename</span> <span class="identifier">detail</span><span class="special">::</span><span class="identifier">fraction_traits</span><span class="special"><</span><span class="identifier">Gen</span><span class="special">>::</span><span class="identifier">result_type</span> | ||
|  |    <span class="identifier">continued_fraction_a</span><span class="special">(</span><span class="identifier">Gen</span><span class="special">&</span> <span class="identifier">g</span><span class="special">,</span> <span class="keyword">const</span> <span class="identifier">U</span><span class="special">&</span> <span class="identifier">tolerance</span><span class="special">,</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">uintmax_t</span><span class="special">&</span> <span class="identifier">max_terms</span><span class="special">)</span> | ||
|  | 
 | ||
|  | <span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">Gen</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">U</span><span class="special">></span> | ||
|  | <span class="keyword">typename</span> <span class="identifier">detail</span><span class="special">::</span><span class="identifier">fraction_traits</span><span class="special"><</span><span class="identifier">Gen</span><span class="special">>::</span><span class="identifier">result_type</span> | ||
|  |    <span class="identifier">continued_fraction_a</span><span class="special">(</span><span class="identifier">Gen</span><span class="special">&</span> <span class="identifier">g</span><span class="special">,</span> <span class="keyword">const</span> <span class="identifier">U</span><span class="special">&</span> <span class="identifier">tolerance</span><span class="special">)</span> | ||
|  | 
 | ||
|  | <span class="comment">//</span> | ||
|  | <span class="comment">// These interfaces are present for legacy reasons, and are now deprecated:</span> | ||
|  | <span class="comment">//</span> | ||
|  | <span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">Gen</span><span class="special">></span> | ||
|  | <span class="keyword">typename</span> <span class="identifier">detail</span><span class="special">::</span><span class="identifier">fraction_traits</span><span class="special"><</span><span class="identifier">Gen</span><span class="special">>::</span><span class="identifier">result_type</span> | ||
|  |    <span class="identifier">continued_fraction_b</span><span class="special">(</span><span class="identifier">Gen</span><span class="special">&</span> <span class="identifier">g</span><span class="special">,</span> <span class="keyword">int</span> <span class="identifier">bits</span><span class="special">);</span> | ||
|  | 
 | ||
|  | <span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">Gen</span><span class="special">></span> | ||
|  | <span class="keyword">typename</span> <span class="identifier">detail</span><span class="special">::</span><span class="identifier">fraction_traits</span><span class="special"><</span><span class="identifier">Gen</span><span class="special">>::</span><span class="identifier">result_type</span> | ||
|  |    <span class="identifier">continued_fraction_b</span><span class="special">(</span><span class="identifier">Gen</span><span class="special">&</span> <span class="identifier">g</span><span class="special">,</span> <span class="keyword">int</span> <span class="identifier">bits</span><span class="special">,</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">uintmax_t</span><span class="special">&</span> <span class="identifier">max_terms</span><span class="special">);</span> | ||
|  | 
 | ||
|  | <span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">Gen</span><span class="special">></span> | ||
|  | <span class="keyword">typename</span> <span class="identifier">detail</span><span class="special">::</span><span class="identifier">fraction_traits</span><span class="special"><</span><span class="identifier">Gen</span><span class="special">>::</span><span class="identifier">result_type</span> | ||
|  |    <span class="identifier">continued_fraction_a</span><span class="special">(</span><span class="identifier">Gen</span><span class="special">&</span> <span class="identifier">g</span><span class="special">,</span> <span class="keyword">int</span> <span class="identifier">bits</span><span class="special">);</span> | ||
|  | 
 | ||
|  | <span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">Gen</span><span class="special">></span> | ||
|  | <span class="keyword">typename</span> <span class="identifier">detail</span><span class="special">::</span><span class="identifier">fraction_traits</span><span class="special"><</span><span class="identifier">Gen</span><span class="special">>::</span><span class="identifier">result_type</span> | ||
|  |    <span class="identifier">continued_fraction_a</span><span class="special">(</span><span class="identifier">Gen</span><span class="special">&</span> <span class="identifier">g</span><span class="special">,</span> <span class="keyword">int</span> <span class="identifier">bits</span><span class="special">,</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">uintmax_t</span><span class="special">&</span> <span class="identifier">max_terms</span><span class="special">);</span> | ||
|  | 
 | ||
|  | <span class="special">}}}</span> <span class="comment">// namespaces</span> | ||
|  | </pre> | ||
|  | <h5> | ||
|  | <a name="math_toolkit.internals.cf.h1"></a> | ||
|  |         <span class="phrase"><a name="math_toolkit.internals.cf.description"></a></span><a class="link" href="cf.html#math_toolkit.internals.cf.description">Description</a> | ||
|  |       </h5> | ||
|  | <p> | ||
|  |         <a href="http://en.wikipedia.org/wiki/Continued_fraction" target="_top">Continued fractions | ||
|  |         are a common method of approximation. </a> These functions all evaluate | ||
|  |         the continued fraction described by the <span class="emphasis"><em>generator</em></span> type | ||
|  |         argument. The functions with an "_a" suffix evaluate the fraction: | ||
|  |       </p> | ||
|  | <p> | ||
|  |         <span class="inlinemediaobject"><img src="../../../equations/fraction2.svg"></span> | ||
|  |       </p> | ||
|  | <p> | ||
|  |         and those with a "_b" suffix evaluate the fraction: | ||
|  |       </p> | ||
|  | <p> | ||
|  |         <span class="inlinemediaobject"><img src="../../../equations/fraction1.svg"></span> | ||
|  |       </p> | ||
|  | <p> | ||
|  |         This latter form is somewhat more natural in that it corresponds with the | ||
|  |         usual definition of a continued fraction, but note that the first <span class="emphasis"><em>a</em></span> | ||
|  |         value returned by the generator is discarded. Further, often the first <span class="emphasis"><em>a</em></span> | ||
|  |         and <span class="emphasis"><em>b</em></span> values in a continued fraction have different | ||
|  |         defining equations to the remaining terms, which may make the "_a" | ||
|  |         suffixed form more appropriate. | ||
|  |       </p> | ||
|  | <p> | ||
|  |         The generator type should be a function object which supports the following | ||
|  |         operations: | ||
|  |       </p> | ||
|  | <div class="informaltable"><table class="table"> | ||
|  | <colgroup> | ||
|  | <col> | ||
|  | <col> | ||
|  | </colgroup> | ||
|  | <thead><tr> | ||
|  | <th> | ||
|  |                 <p> | ||
|  |                   Expression | ||
|  |                 </p> | ||
|  |               </th> | ||
|  | <th> | ||
|  |                 <p> | ||
|  |                   Description | ||
|  |                 </p> | ||
|  |               </th> | ||
|  | </tr></thead> | ||
|  | <tbody> | ||
|  | <tr> | ||
|  | <td> | ||
|  |                 <p> | ||
|  |                   Gen::result_type | ||
|  |                 </p> | ||
|  |               </td> | ||
|  | <td> | ||
|  |                 <p> | ||
|  |                   The type that is the result of invoking operator(). This can be | ||
|  |                   either an arithmetic type, or a std::pair<> of arithmetic | ||
|  |                   types. | ||
|  |                 </p> | ||
|  |               </td> | ||
|  | </tr> | ||
|  | <tr> | ||
|  | <td> | ||
|  |                 <p> | ||
|  |                   g() | ||
|  |                 </p> | ||
|  |               </td> | ||
|  | <td> | ||
|  |                 <p> | ||
|  |                   Returns an object of type Gen::result_type. | ||
|  |                 </p> | ||
|  |                 <p> | ||
|  |                   Each time this operator is called then the next pair of <span class="emphasis"><em>a</em></span> | ||
|  |                   and <span class="emphasis"><em>b</em></span> values is returned. Or, if result_type | ||
|  |                   is an arithmetic type, then the next <span class="emphasis"><em>b</em></span> value | ||
|  |                   is returned and all the <span class="emphasis"><em>a</em></span> values are assumed | ||
|  |                   to 1. | ||
|  |                 </p> | ||
|  |               </td> | ||
|  | </tr> | ||
|  | </tbody> | ||
|  | </table></div> | ||
|  | <p> | ||
|  |         In all the continued fraction evaluation functions the <span class="emphasis"><em>tolerance</em></span> | ||
|  |         parameter is the precision desired in the result, evaluation of the fraction | ||
|  |         will continue until the last term evaluated leaves the relative error in | ||
|  |         the result less than <span class="emphasis"><em>tolerance</em></span>. The deprecated interfaces | ||
|  |         take a number of digits precision here, internally they just convert this | ||
|  |         to a tolerance and forward call. | ||
|  |       </p> | ||
|  | <p> | ||
|  |         If the optional <span class="emphasis"><em>max_terms</em></span> parameter is specified then | ||
|  |         no more than <span class="emphasis"><em>max_terms</em></span> calls to the generator will be | ||
|  |         made, and on output, <span class="emphasis"><em>max_terms</em></span> will be set to actual | ||
|  |         number of calls made. This facility is particularly useful when profiling | ||
|  |         a continued fraction for convergence. | ||
|  |       </p> | ||
|  | <h5> | ||
|  | <a name="math_toolkit.internals.cf.h2"></a> | ||
|  |         <span class="phrase"><a name="math_toolkit.internals.cf.implementation"></a></span><a class="link" href="cf.html#math_toolkit.internals.cf.implementation">Implementation</a> | ||
|  |       </h5> | ||
|  | <p> | ||
|  |         Internally these algorithms all use the modified Lentz algorithm: refer to | ||
|  |         Numeric Recipes in C++, W. H. Press et all, chapter 5, (especially 5.2 Evaluation | ||
|  |         of continued fractions, p 175 - 179) for more information, also Lentz, W.J. | ||
|  |         1976, Applied Optics, vol. 15, pp. 668-671. | ||
|  |       </p> | ||
|  | <h5> | ||
|  | <a name="math_toolkit.internals.cf.h3"></a> | ||
|  |         <span class="phrase"><a name="math_toolkit.internals.cf.examples"></a></span><a class="link" href="cf.html#math_toolkit.internals.cf.examples">Examples</a> | ||
|  |       </h5> | ||
|  | <p> | ||
|  |         The <a href="http://en.wikipedia.org/wiki/Golden_ratio" target="_top">golden ratio phi | ||
|  |         = 1.618033989...</a> can be computed from the simplest continued fraction | ||
|  |         of all: | ||
|  |       </p> | ||
|  | <p> | ||
|  |         <span class="inlinemediaobject"><img src="../../../equations/fraction3.svg"></span> | ||
|  |       </p> | ||
|  | <p> | ||
|  |         We begin by defining a generator function: | ||
|  |       </p> | ||
|  | <pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> | ||
|  | <span class="keyword">struct</span> <span class="identifier">golden_ratio_fraction</span> | ||
|  | <span class="special">{</span> | ||
|  |    <span class="keyword">typedef</span> <span class="identifier">T</span> <span class="identifier">result_type</span><span class="special">;</span> | ||
|  | 
 | ||
|  |    <span class="identifier">result_type</span> <span class="keyword">operator</span><span class="special">()</span> | ||
|  |    <span class="special">{</span> | ||
|  |       <span class="keyword">return</span> <span class="number">1</span><span class="special">;</span> | ||
|  |    <span class="special">}</span> | ||
|  | <span class="special">};</span> | ||
|  | </pre> | ||
|  | <p> | ||
|  |         The golden ratio can then be computed to double precision using: | ||
|  |       </p> | ||
|  | <pre class="programlisting"><span class="identifier">continued_fraction_a</span><span class="special">(</span> | ||
|  |    <span class="identifier">golden_ratio_fraction</span><span class="special"><</span><span class="keyword">double</span><span class="special">>(),</span> | ||
|  |    <span class="identifier">std</span><span class="special">::</span><span class="identifier">numeric_limits</span><span class="special"><</span><span class="keyword">double</span><span class="special">>::</span><span class="identifier">epsilon</span><span class="special">());</span> | ||
|  | </pre> | ||
|  | <p> | ||
|  |         It's more usual though to have to define both the <span class="emphasis"><em>a</em></span>'s | ||
|  |         and the <span class="emphasis"><em>b</em></span>'s when evaluating special functions by continued | ||
|  |         fractions, for example the tan function is defined by: | ||
|  |       </p> | ||
|  | <p> | ||
|  |         <span class="inlinemediaobject"><img src="../../../equations/fraction4.svg"></span> | ||
|  |       </p> | ||
|  | <p> | ||
|  |         So its generator object would look like: | ||
|  |       </p> | ||
|  | <pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> | ||
|  | <span class="keyword">struct</span> <span class="identifier">tan_fraction</span> | ||
|  | <span class="special">{</span> | ||
|  | <span class="keyword">private</span><span class="special">:</span> | ||
|  |    <span class="identifier">T</span> <span class="identifier">a</span><span class="special">,</span> <span class="identifier">b</span><span class="special">;</span> | ||
|  | <span class="keyword">public</span><span class="special">:</span> | ||
|  |    <span class="identifier">tan_fraction</span><span class="special">(</span><span class="identifier">T</span> <span class="identifier">v</span><span class="special">)</span> | ||
|  |       <span class="special">:</span> <span class="identifier">a</span><span class="special">(-</span><span class="identifier">v</span><span class="special">*</span><span class="identifier">v</span><span class="special">),</span> <span class="identifier">b</span><span class="special">(-</span><span class="number">1</span><span class="special">)</span> | ||
|  |    <span class="special">{}</span> | ||
|  | 
 | ||
|  |    <span class="keyword">typedef</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">result_type</span><span class="special">;</span> | ||
|  | 
 | ||
|  |    <span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">operator</span><span class="special">()()</span> | ||
|  |    <span class="special">{</span> | ||
|  |       <span class="identifier">b</span> <span class="special">+=</span> <span class="number">2</span><span class="special">;</span> | ||
|  |       <span class="keyword">return</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">make_pair</span><span class="special">(</span><span class="identifier">a</span><span class="special">,</span> <span class="identifier">b</span><span class="special">);</span> | ||
|  |    <span class="special">}</span> | ||
|  | <span class="special">};</span> | ||
|  | </pre> | ||
|  | <p> | ||
|  |         Notice that if the continuant is subtracted from the <span class="emphasis"><em>b</em></span> | ||
|  |         terms, as is the case here, then all the <span class="emphasis"><em>a</em></span> terms returned | ||
|  |         by the generator will be negative. The tangent function can now be evaluated | ||
|  |         using: | ||
|  |       </p> | ||
|  | <pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> | ||
|  | <span class="identifier">T</span> <span class="identifier">tan</span><span class="special">(</span><span class="identifier">T</span> <span class="identifier">a</span><span class="special">)</span> | ||
|  | <span class="special">{</span> | ||
|  |    <span class="identifier">tan_fraction</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">fract</span><span class="special">(</span><span class="identifier">a</span><span class="special">);</span> | ||
|  |    <span class="keyword">return</span> <span class="identifier">a</span> <span class="special">/</span> <span class="identifier">continued_fraction_b</span><span class="special">(</span><span class="identifier">fract</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">numeric_limits</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">epsilon</span><span class="special">());</span> | ||
|  | <span class="special">}</span> | ||
|  | </pre> | ||
|  | <p> | ||
|  |         Notice that this time we're using the "_b" suffixed version to | ||
|  |         evaluate the fraction: we're removing the leading <span class="emphasis"><em>a</em></span> | ||
|  |         term during fraction evaluation as it's different from all the others. | ||
|  |       </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 © 2006-2010, 2012-2014 Nikhar Agrawal, | ||
|  |       Anton Bikineev, Paul A. Bristow, Marco Guazzone, Christopher Kormanyos, Hubert | ||
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