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boost_fusion/doc/html/fusion/algorithm/iteration/functions/fold.html
Christopher Schmidt dab67207cf associative iterators & views (Fixes #3473) 
[SVN r57242]
2009-10-30 00:19:19 +00:00

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<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
<title>fold</title>
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</div>
<div class="section" title="fold">
<div class="titlepage"><div><div><h5 class="title">
<a name="fusion.algorithm.iteration.functions.fold"></a><a class="link" href="fold.html" title="fold">fold</a>
</h5></div></div></div>
<a name="fusion.algorithm.iteration.functions.fold.description"></a><h6>
<a name="id750659"></a>
<a class="link" href="fold.html#fusion.algorithm.iteration.functions.fold.description">Description</a>
</h6>
<p>
For a sequence <code class="computeroutput"><span class="identifier">seq</span></code>, initial
state <code class="computeroutput"><span class="identifier">initial_state</span></code>,
and binary function object or function pointer <code class="computeroutput"><span class="identifier">f</span></code>,
fold returns the result of the repeated application of binary <code class="computeroutput"><span class="identifier">f</span></code> to the result of the previous <code class="computeroutput"><span class="identifier">f</span></code> invocation (<code class="computeroutput"><span class="identifier">inital_state</span></code>
if it is the first call) and each element of <code class="computeroutput"><span class="identifier">seq</span></code>.
</p>
<a name="fusion.algorithm.iteration.functions.fold.synopsis"></a><h6>
<a name="id750729"></a>
<a class="link" href="fold.html#fusion.algorithm.iteration.functions.fold.synopsis">Synopsis</a>
</h6>
<pre class="programlisting"><span class="keyword">template</span><span class="special">&lt;</span>
<span class="keyword">typename</span> <span class="identifier">Sequence</span><span class="special">,</span>
<span class="keyword">typename</span> <span class="identifier">State</span><span class="special">,</span>
<span class="keyword">typename</span> <span class="identifier">F</span>
<span class="special">&gt;</span>
<span class="keyword">typename</span> <a class="link" href="../metafunctions/fold.html" title="fold"><code class="computeroutput"><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">fold</span></code></a><span class="special">&lt;</span><span class="identifier">Sequence</span><span class="special">,</span> <span class="identifier">State</span><span class="special">,</span> <span class="identifier">F</span><span class="special">&gt;::</span><span class="identifier">type</span> <span class="identifier">fold</span><span class="special">(</span>
<span class="identifier">Sequence</span><span class="special">&amp;</span> <span class="identifier">seq</span><span class="special">,</span> <span class="identifier">State</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">initial_state</span><span class="special">,</span> <span class="identifier">F</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">f</span><span class="special">);</span>
</pre>
<div class="table">
<a name="id750913"></a><p class="title"><b>Table 1.37. Parameters</b></p>
<div class="table-contents"><table class="table" summary="Parameters">
<colgroup>
<col>
<col>
<col>
</colgroup>
<thead><tr>
<th>
<p>
Parameter
</p>
</th>
<th>
<p>
Requirement
</p>
</th>
<th>
<p>
Description
</p>
</th>
</tr></thead>
<tbody>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">seq</span></code>
</p>
</td>
<td>
<p>
A model of <a class="link" href="../../../sequence/concepts/forward_sequence.html" title="Forward Sequence">Forward
Sequence</a>, <code class="computeroutput"><span class="identifier">f</span><span class="special">(</span><span class="identifier">s</span><span class="special">,</span><span class="identifier">e</span><span class="special">)</span></code> must be a valid expression for
current state <code class="computeroutput"><span class="identifier">s</span></code>,
and each element <code class="computeroutput"><span class="identifier">e</span></code>
in <code class="computeroutput"><span class="identifier">seq</span></code>
</p>
</td>
<td>
<p>
Operation's argument
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">initial_state</span></code>
</p>
</td>
<td>
<p>
Any type
</p>
</td>
<td>
<p>
Initial state
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">f</span></code>
</p>
</td>
<td>
<p>
<code class="computeroutput"><a href="http://www.boost.org/libs/utility/utility.htm#result_of" target="_top"><code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">result_of</span></code></a><span class="special">&lt;</span><span class="identifier">F</span><span class="special">(</span><span class="identifier">S</span><span class="special">,</span><span class="identifier">E</span><span class="special">)&gt;::</span><span class="identifier">type</span></code> is the return type of <code class="computeroutput"><span class="identifier">f</span><span class="special">(</span><span class="identifier">s</span><span class="special">,</span><span class="identifier">e</span><span class="special">)</span></code>
current state <code class="computeroutput"><span class="identifier">s</span></code>
of type <code class="computeroutput"><span class="identifier">S</span></code>, and
for each element <code class="computeroutput"><span class="identifier">e</span></code>
of type <code class="computeroutput"><span class="identifier">E</span></code> in <code class="computeroutput"><span class="identifier">seq</span></code>
</p>
</td>
<td>
<p>
Operation's argument
</p>
</td>
</tr>
</tbody>
</table></div>
</div>
<br class="table-break"><a name="fusion.algorithm.iteration.functions.fold.expression_semantics"></a><h6>
<a name="id751215"></a>
<a class="link" href="fold.html#fusion.algorithm.iteration.functions.fold.expression_semantics">Expression
Semantics</a>
</h6>
<pre class="programlisting"><span class="identifier">fold</span><span class="special">(</span><span class="identifier">seq</span><span class="special">,</span> <span class="identifier">initial_state</span><span class="special">,</span> <span class="identifier">f</span><span class="special">);</span>
</pre>
<p>
<span class="bold"><strong>Return type</strong></span>: Any type
</p>
<p>
<span class="bold"><strong>Semantics</strong></span>: Equivalent to <code class="computeroutput"><span class="identifier">f</span><span class="special">(...</span> <span class="identifier">f</span><span class="special">(</span><span class="identifier">f</span><span class="special">(</span><span class="identifier">initial_state</span><span class="special">,</span><span class="identifier">e1</span><span class="special">),</span><span class="identifier">e2</span><span class="special">)</span> <span class="special">...</span><span class="identifier">eN</span><span class="special">)</span></code> where <code class="computeroutput"><span class="identifier">e1</span>
<span class="special">...</span><span class="identifier">eN</span></code>
are the elements of <code class="computeroutput"><span class="identifier">seq</span></code>.
</p>
<a name="fusion.algorithm.iteration.functions.fold.complexity"></a><h6>
<a name="id751366"></a>
<a class="link" href="fold.html#fusion.algorithm.iteration.functions.fold.complexity">Complexity</a>
</h6>
<p>
Linear, exactly <code class="computeroutput"><a class="link" href="../../../sequence/intrinsic/metafunctions/size.html" title="size"><code class="computeroutput"><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">size</span></code></a><span class="special">&lt;</span><span class="identifier">Sequence</span><span class="special">&gt;::</span><span class="identifier">value</span></code> applications of <code class="computeroutput"><span class="identifier">f</span></code>.
</p>
<a name="fusion.algorithm.iteration.functions.fold.header"></a><h6>
<a name="id751424"></a>
<a class="link" href="fold.html#fusion.algorithm.iteration.functions.fold.header">Header</a>
</h6>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">fusion</span><span class="special">/</span><span class="identifier">algorithm</span><span class="special">/</span><span class="identifier">iteration</span><span class="special">/</span><span class="identifier">fold</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">fusion</span><span class="special">/</span><span class="identifier">include</span><span class="special">/</span><span class="identifier">fold</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
</pre>
<a name="fusion.algorithm.iteration.functions.fold.example"></a><h6>
<a name="id751539"></a>
<a class="link" href="fold.html#fusion.algorithm.iteration.functions.fold.example">Example</a>
</h6>
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">make_string</span>
<span class="special">{</span>
<span class="keyword">typedef</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="identifier">result_type</span><span class="special">;</span>
<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">&gt;</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="keyword">operator</span><span class="special">()(</span><span class="keyword">const</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">&amp;</span> <span class="identifier">str</span><span class="special">,</span> <span class="keyword">const</span> <span class="identifier">T</span><span class="special">&amp;</span> <span class="identifier">t</span><span class="special">)</span> <span class="keyword">const</span>
<span class="special">{</span>
<span class="keyword">return</span> <span class="identifier">str</span> <span class="special">+</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">lexical_cast</span><span class="special">&lt;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">&gt;(</span><span class="identifier">t</span><span class="special">);</span>
<span class="special">}</span>
<span class="special">};</span>
<span class="special">...</span>
<span class="keyword">const</span> <a class="link" href="../../../container/vector.html" title="vector"><code class="computeroutput"><span class="identifier">vector</span></code></a><span class="special">&lt;</span><span class="keyword">int</span><span class="special">,</span><span class="keyword">int</span><span class="special">&gt;</span> <span class="identifier">vec</span><span class="special">(</span><span class="number">1</span><span class="special">,</span><span class="number">2</span><span class="special">);</span>
<span class="identifier">assert</span><span class="special">(</span><a class="link" href="fold.html" title="fold"><code class="computeroutput"><span class="identifier">fold</span></code></a><span class="special">(</span><span class="identifier">vec</span><span class="special">,</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">(</span><span class="string">""</span><span class="special">),</span> <span class="identifier">make_string</span><span class="special">())</span> <span class="special">==</span> <span class="string">"12"</span><span class="special">);</span>
</pre>
</div>
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<td align="right"><div class="copyright-footer">Copyright © 2001-2007 Joel de Guzman, Dan Marsden, Tobias
Schwinger<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>)
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