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								<div class="titlepage"><div><div><h3 class="title">
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								<a name="math_toolkit.internals.cf"></a><a class="link" href="cf.html" title="Continued Fraction Evaluation">Continued Fraction Evaluation</a>
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								</h3></div></div></div>
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								<h5>
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								<a name="math_toolkit.internals.cf.h0"></a>
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								        <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>
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								      </h5>
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								<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>
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								</pre>
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								<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>
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								<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>
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								<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>
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								   <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>
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								<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>
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								<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>
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								   <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>
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								<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>
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								<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>
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								   <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>
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								<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>
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								<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>
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								   <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>
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								<span class="comment">//</span>
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								<span class="comment">// These interfaces are present for legacy reasons, and are now deprecated:</span>
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								<span class="comment">//</span>
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								<span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">Gen</span><span class="special">></span>
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								<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>
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								   <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>
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								<span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">Gen</span><span class="special">></span>
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								<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>
							 | 
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								   <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>
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								<span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">Gen</span><span class="special">></span>
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								<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>
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								   <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>
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								<span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">Gen</span><span class="special">></span>
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								<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>
							 | 
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								   <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>
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								<span class="special">}}}</span> <span class="comment">// namespaces</span>
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								</pre>
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								<h5>
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								<a name="math_toolkit.internals.cf.h1"></a>
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								        <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>
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								      </h5>
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								<p>
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								        <a href="http://en.wikipedia.org/wiki/Continued_fraction" target="_top">Continued fractions
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								        are a common method of approximation. </a> These functions all evaluate
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								        the continued fraction described by the <span class="emphasis"><em>generator</em></span> type
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								        argument. The functions with an "_a" suffix evaluate the fraction:
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								      </p>
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								<p>
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								        <span class="inlinemediaobject"><img src="../../../equations/fraction2.svg"></span>
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								      </p>
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								<p>
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								        and those with a "_b" suffix evaluate the fraction:
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								      </p>
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								<p>
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								        <span class="inlinemediaobject"><img src="../../../equations/fraction1.svg"></span>
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								      </p>
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								<p>
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								        This latter form is somewhat more natural in that it corresponds with the
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								        usual definition of a continued fraction, but note that the first <span class="emphasis"><em>a</em></span>
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								        value returned by the generator is discarded. Further, often the first <span class="emphasis"><em>a</em></span>
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								        and <span class="emphasis"><em>b</em></span> values in a continued fraction have different
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								        defining equations to the remaining terms, which may make the "_a"
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								        suffixed form more appropriate.
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								      </p>
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								<p>
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								        The generator type should be a function object which supports the following
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								        operations:
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								      </p>
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								<div class="informaltable"><table class="table">
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								<colgroup>
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								<col>
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								<col>
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								</colgroup>
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								<thead><tr>
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								<th>
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								                <p>
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								                  Expression
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								                </p>
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								              </th>
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								<th>
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								                <p>
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								                  Description
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								                </p>
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								              </th>
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								</tr></thead>
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								<tbody>
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								<tr>
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								<td>
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								                <p>
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								                  Gen::result_type
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								                </p>
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								              </td>
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								<td>
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								                <p>
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								                  The type that is the result of invoking operator(). This can be
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								                  either an arithmetic type, or a std::pair<> of arithmetic
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								                  types.
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								                </p>
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								              </td>
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								</tr>
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								<tr>
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								<td>
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								                <p>
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								                  g()
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								                </p>
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								              </td>
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								<td>
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								                <p>
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								                  Returns an object of type Gen::result_type.
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								                </p>
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								                <p>
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								                  Each time this operator is called then the next pair of <span class="emphasis"><em>a</em></span>
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								                  and <span class="emphasis"><em>b</em></span> values is returned. Or, if result_type
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								                  is an arithmetic type, then the next <span class="emphasis"><em>b</em></span> value
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								                  is returned and all the <span class="emphasis"><em>a</em></span> values are assumed
							 | 
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								                  to 1.
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								                </p>
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								              </td>
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								</tr>
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								</tbody>
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								</table></div>
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								<p>
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								        In all the continued fraction evaluation functions the <span class="emphasis"><em>tolerance</em></span>
							 | 
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								        parameter is the precision desired in the result, evaluation of the fraction
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								        will continue until the last term evaluated leaves the relative error in
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								        the result less than <span class="emphasis"><em>tolerance</em></span>. The deprecated interfaces
							 | 
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								        take a number of digits precision here, internally they just convert this
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								        to a tolerance and forward call.
							 | 
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								      </p>
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								<p>
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								        If the optional <span class="emphasis"><em>max_terms</em></span> parameter is specified then
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								        no more than <span class="emphasis"><em>max_terms</em></span> calls to the generator will be
							 | 
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								        made, and on output, <span class="emphasis"><em>max_terms</em></span> will be set to actual
							 | 
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								        number of calls made. This facility is particularly useful when profiling
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								        a continued fraction for convergence.
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								      </p>
							 | 
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| 
								 | 
							
								<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>
							 | 
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| 
								 | 
							
								      </h5>
							 | 
						||
| 
								 | 
							
								<p>
							 | 
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| 
								 | 
							
								        Internally these algorithms all use the modified Lentz algorithm: refer to
							 | 
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| 
								 | 
							
								        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>
							 | 
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| 
								 | 
							
								<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>
							 | 
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| 
								 | 
							
								        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:
							 | 
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| 
								 | 
							
								      </p>
							 | 
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| 
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								<p>
							 | 
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| 
								 | 
							
								        <span class="inlinemediaobject"><img src="../../../equations/fraction3.svg"></span>
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| 
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								      </p>
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| 
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								<p>
							 | 
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| 
								 | 
							
								        We begin by defining a generator function:
							 | 
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| 
								 | 
							
								      </p>
							 | 
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| 
								 | 
							
								<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>
							 | 
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| 
								 | 
							
								<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
							 | 
						||
| 
								 | 
							
								      Holin, Bruno Lalande, John Maddock, Jeremy Murphy, Johan Råde, Gautam Sewani,
							 | 
						||
| 
								 | 
							
								      Benjamin Sobotta, Thijs van den Berg, Daryle Walker and Xiaogang Zhang<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>
							 | 
						||
| 
								 | 
							
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							 | 
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