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  26 <div class="titlepage"><div><div><h3 class="title">
  27 <a name="the_boost_algorithm_library.Misc.gather"></a><a class="link" href="gather.html" title="gather">gather</a>
  28 </h3></div></div></div>
  29 <p>
  30         The header file 'boost/algorithm/gather.hpp' contains two variants of a single
  31         algorithm, <code class="computeroutput"><span class="identifier">gather</span></code>.
  32       </p>
  33 <p>
  34         <code class="computeroutput"><span class="identifier">gather</span><span class="special">()</span></code>
  35         takes a collection of elements defined by a pair of iterators and moves the
  36         ones satisfying a predicate to them to a position (called the pivot) within
  37         the sequence. The algorithm is stable. The result is a pair of iterators
  38         that contains the items that satisfy the predicate.
  39       </p>
  40 <h5>
  41 <a name="the_boost_algorithm_library.Misc.gather.h0"></a>
  42         <span class="phrase"><a name="the_boost_algorithm_library.Misc.gather.interface"></a></span><a class="link" href="gather.html#the_boost_algorithm_library.Misc.gather.interface">Interface</a>
  43       </h5>
  44 <p>
  45         The function <code class="computeroutput"><span class="identifier">gather</span></code> returns
  46         a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span></code> of iterators that denote the elements
  47         that satisfy the predicate.
  48       </p>
  49 <p>
  50         There are two versions; one takes two iterators, and the other takes a range.
  51       </p>
  52 <p>
  53 </p>
  54 <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">algorithm</span> <span class="special">{</span>
  55
  56 <span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">BidirectionalIterator</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Pred</span><span class="special">&gt;</span>
  57 <span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special">&lt;</span><span class="identifier">BidirectionalIterator</span><span class="special">,</span><span class="identifier">BidirectionalIterator</span><span class="special">&gt;</span>
  58 <span class="identifier">gather</span> <span class="special">(</span> <span class="identifier">BidirectionalIterator</span> <span class="identifier">first</span><span class="special">,</span> <span class="identifier">BidirectionalIterator</span> <span class="identifier">last</span><span class="special">,</span> <span class="identifier">BidirectionalIterator</span> <span class="identifier">pivot</span><span class="special">,</span> <span class="identifier">Pred</span> <span class="identifier">pred</span> <span class="special">);</span>
  59
  60 <span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">BidirectionalRange</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Pred</span><span class="special">&gt;</span>
  61 <span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">range_iterator</span><span class="special">&lt;</span><span class="keyword">const</span> <span class="identifier">BidirectionalRange</span><span class="special">&gt;::</span><span class="identifier">type</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">range_iterator</span><span class="special">&lt;</span><span class="keyword">const</span> <span class="identifier">BidirectionalRange</span><span class="special">&gt;::</span><span class="identifier">type</span><span class="special">&gt;</span>
  62 <span class="identifier">gather</span> <span class="special">(</span> <span class="keyword">const</span> <span class="identifier">BidirectionalRange</span> <span class="special">&amp;</span><span class="identifier">range</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">range_iterator</span><span class="special">&lt;</span><span class="keyword">const</span> <span class="identifier">BidirectionalRange</span><span class="special">&gt;::</span><span class="identifier">type</span> <span class="identifier">pivot</span><span class="special">,</span> <span class="identifier">Pred</span> <span class="identifier">pred</span> <span class="special">);</span>
  63
  64 <span class="special">}}</span>
  65 </pre>
  66 <p>
  67       </p>
  68 <h5>
  69 <a name="the_boost_algorithm_library.Misc.gather.h1"></a>
  70         <span class="phrase"><a name="the_boost_algorithm_library.Misc.gather.examples"></a></span><a class="link" href="gather.html#the_boost_algorithm_library.Misc.gather.examples">Examples</a>
  71       </h5>
  72 <p>
  73         Given an sequence containing:
  74 </p>
  75 <pre class="programlisting"><span class="number">0</span> <span class="number">1</span> <span class="number">2</span> <span class="number">3</span> <span class="number">4</span> <span class="number">5</span> <span class="number">6</span> <span class="number">7</span> <span class="number">8</span> <span class="number">9</span>
  76 </pre>
  77 <p>
  78       </p>
  79 <p>
  80         a call to gather ( arr, arr + 10, arr + 4, IsEven ) will result in:
  81       </p>
  82 <p>
  83 </p>
  84 <pre class="programlisting"><span class="number">1</span> <span class="number">3</span> <span class="number">0</span> <span class="number">2</span> <span class="number">4</span> <span class="number">6</span> <span class="number">8</span> <span class="number">5</span> <span class="number">7</span> <span class="number">9</span>
  85     <span class="special">|---|-----|</span>
  86   <span class="identifier">first</span> <span class="special">|</span>  <span class="identifier">second</span>
  87       <span class="identifier">pivot</span>
  88 </pre>
  89 <p>
  90         where <code class="computeroutput"><span class="identifier">first</span></code> and <code class="computeroutput"><span class="identifier">second</span></code> are the fields of the pair that
  91         is returned by the call.
  92       </p>
  93 <h5>
  94 <a name="the_boost_algorithm_library.Misc.gather.h2"></a>
  95         <span class="phrase"><a name="the_boost_algorithm_library.Misc.gather.iterator_requirements"></a></span><a class="link" href="gather.html#the_boost_algorithm_library.Misc.gather.iterator_requirements">Iterator
  96         Requirements</a>
  97       </h5>
  98 <p>
  99         <code class="computeroutput"><span class="identifier">gather</span></code> work on bidirectional
 100         iterators or better. This requirement comes from the usage of <code class="computeroutput"><span class="identifier">stable_partition</span></code>, which requires bidirectional
 101         iterators. Some standard libraries (libstdc++ and libc++, for example) have
 102         implementations of <code class="computeroutput"><span class="identifier">stable_partition</span></code>
 103         that work with forward iterators. If that is the case, then <code class="computeroutput"><span class="identifier">gather</span></code> will work with forward iterators
 104         as well.
 105       </p>
 106 <h5>
 107 <a name="the_boost_algorithm_library.Misc.gather.h3"></a>
 108         <span class="phrase"><a name="the_boost_algorithm_library.Misc.gather.storage_requirements"></a></span><a class="link" href="gather.html#the_boost_algorithm_library.Misc.gather.storage_requirements">Storage
 109         Requirements</a>
 110       </h5>
 111 <p>
 112         <code class="computeroutput"><span class="identifier">gather</span></code> uses <code class="computeroutput"><span class="identifier">stable_partition</span></code>, which will attempt to
 113         allocate temporary memory, but will work in-situ if there is none available.
 114       </p>
 115 <h5>
 116 <a name="the_boost_algorithm_library.Misc.gather.h4"></a>
 117         <span class="phrase"><a name="the_boost_algorithm_library.Misc.gather.complexity"></a></span><a class="link" href="gather.html#the_boost_algorithm_library.Misc.gather.complexity">Complexity</a>
 118       </h5>
 119 <p>
 120         If there is sufficient memory available, the run time is linear: <code class="computeroutput"><span class="identifier">O</span><span class="special">(</span><span class="identifier">N</span><span class="special">)</span></code>
 121       </p>
 122 <p>
 123         If there is not any memory available, then the run time is <code class="computeroutput"><span class="identifier">O</span><span class="special">(</span><span class="identifier">N</span>
 124         <span class="identifier">log</span> <span class="identifier">N</span><span class="special">)</span></code>.
 125       </p>
 126 <h5>
 127 <a name="the_boost_algorithm_library.Misc.gather.h5"></a>
 128         <span class="phrase"><a name="the_boost_algorithm_library.Misc.gather.exception_safety"></a></span><a class="link" href="gather.html#the_boost_algorithm_library.Misc.gather.exception_safety">Exception
 129         Safety</a>
 130       </h5>
 131 <h5>
 132 <a name="the_boost_algorithm_library.Misc.gather.h6"></a>
 133         <span class="phrase"><a name="the_boost_algorithm_library.Misc.gather.notes"></a></span><a class="link" href="gather.html#the_boost_algorithm_library.Misc.gather.notes">Notes</a>
 134       </h5>
 135 </div>
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 137 <td align="left"></td>
 138 <td align="right"><div class="copyright-footer">Copyright &#169; 2010-2012 Marshall Clow<p>
 139         Distributed under the Boost Software License, Version 1.0. (See accompanying
 140         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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