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<link rel="start" href="../index.html" title="Chapter 1. Fusion 2.0">
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<link rel="prev" href="sequences/operators/comparison/greater_than_equal.html" title="greater
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than equal">
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<a accesskey="p" href="sequences/operators/comparison/greater_than_equal.html"><img src="../images/prev.png" alt="Prev"></a><a accesskey="u" href="../index.html"><img src="../images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../images/home.png" alt="Home"></a><a accesskey="n" href="algorithms/iteration.html"><img src="../images/next.png" alt="Next"></a>
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<div class="section" lang="en">
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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
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<a name="fusion.algorithms"></a><a href="algorithms.html" title="Algorithms">Algorithms</a></h2></div></div></div>
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<div class="toc"><dl>
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<dt><span class="section"><a href="algorithms/iteration.html">Iteration</a></span></dt>
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<dd><dl>
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<dt><span class="section"><a href="algorithms/iteration/functions.html">Functions</a></span></dt>
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<dt><span class="section"><a href="algorithms/iteration/metafunctions.html">Metafunctions</a></span></dt>
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</dl></dd>
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<dt><span class="section"><a href="algorithms/query.html">Query</a></span></dt>
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<dd><dl>
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<dt><span class="section"><a href="algorithms/query/functions.html">Functions</a></span></dt>
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<dt><span class="section"><a href="algorithms/query/metafunctions.html">Metafunctions</a></span></dt>
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</dl></dd>
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<dt><span class="section"><a href="algorithms/transformation.html">Transformation</a></span></dt>
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<dd><dl>
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<dt><span class="section"><a href="algorithms/transformation/functions.html">Functions</a></span></dt>
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<dt><span class="section"><a href="algorithms/transformation/metafunctions.html">Metafunctions</a></span></dt>
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</dl></dd>
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</dl></div>
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<a name="fusion.algorithms.lazy_evaluation"></a><h3>
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<a name="id1113801"></a>
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<a href="algorithms.html#fusion.algorithms.lazy_evaluation">Lazy Evaluation</a>
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</h3>
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<p>
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Unlike <a href="http://www.boost.org/libs/mpl/index.html" target="_top">MPL</a>, Fusion
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algorithms are lazy and non sequence-type preserving. What does that mean?
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It means that when you operate on a sequence through a Fusion algorithm that
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returns a sequence, the sequence returned may not be of the same class as the
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original. This is by design. Runtime efficiency is given a high priority. Like
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<a href="http://www.boost.org/libs/mpl/index.html" target="_top">MPL</a>, and unlike
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<a href="http://en.wikipedia.org/wiki/Standard_Template_Library" target="_top">STL</a>,
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fusion algorithms are functional in nature such that algorithms are non mutating
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(no side effects). However, due to the high cost of returning full sequences
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such as vectors and lists, <span class="emphasis"><em>Views</em></span> are returned from Fusion
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algorithms instead. For example, the <a href="algorithms/transformation/functions/transform.html" title="transform"><code class="computeroutput"><span class="identifier">transform</span></code></a> algorithm does not actually
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return a transformed version of the original sequence. <a href="algorithms/transformation/functions/transform.html" title="transform"><code class="computeroutput"><span class="identifier">transform</span></code></a> returns a <a href="sequences/views/transform_view.html" title="transform_view"><code class="computeroutput"><span class="identifier">transform_view</span></code></a>. This view holds a
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reference to the original sequence plus the transform function. Iteration over
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the <a href="sequences/views/transform_view.html" title="transform_view"><code class="computeroutput"><span class="identifier">transform_view</span></code></a>
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will apply the transform function over the sequence elements on demand. This
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<span class="emphasis"><em>lazy</em></span> evaluation scheme allows us to chain as many algorithms
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as we want without incurring a high runtime penalty.
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</p>
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<a name="fusion.algorithms.sequence_extension"></a><h3>
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<a name="id1113934"></a>
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<a href="algorithms.html#fusion.algorithms.sequence_extension">Sequence Extension</a>
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</h3>
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<p>
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The <span class="emphasis"><em>lazy</em></span> evaluation scheme where <a href="algorithms.html" title="Algorithms">Algorithms</a>
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return <a href="sequences/views.html" title="Views">Views</a> also allows operations
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such as <a href="algorithms/transformation/functions/push_back.html" title="push_back"><code class="computeroutput"><span class="identifier">push_back</span></code></a> to be totally generic. In
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Fusion, <a href="algorithms/transformation/functions/push_back.html" title="push_back"><code class="computeroutput"><span class="identifier">push_back</span></code></a> is actually a generic algorithm
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that works on all sequences. Given an input sequence <code class="computeroutput"><span class="identifier">s</span></code>
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and a value <code class="computeroutput"><span class="identifier">x</span></code>, Fusion's <a href="algorithms/transformation/functions/push_back.html" title="push_back"><code class="computeroutput"><span class="identifier">push_back</span></code></a> algorithm simply returns
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a <a href="sequences/views/joint_view.html" title="joint_view"><code class="computeroutput"><span class="identifier">joint_view</span></code></a>:
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a view that holds a reference to the original sequence <code class="computeroutput"><span class="identifier">s</span></code>
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and the value <code class="computeroutput"><span class="identifier">x</span></code>. Functions
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that were once sequence specific and need to be implemented N times over N
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different sequences are now implemented only once. That is to say that Fusion
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sequences are cheaply extensible. However, an important caveat is that the
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result of a sequence extending operation like <a href="algorithms/transformation/functions/push_back.html" title="push_back"><code class="computeroutput"><span class="identifier">push_back</span></code></a> does not retain the properties
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of the original sequence such as associativity of <span class="underline">_set</span>_s.
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To regain the original sequence, <a href="sequences/conversion/functions.html" title="Functions">Conversion</a>
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functions are provided. You may use one of the <a href="sequences/conversion/functions.html" title="Functions">Conversion</a>
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functions to convert back to the original sequence type.
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</p>
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<a name="fusion.algorithms.header"></a><h3>
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<a name="id1114134"></a>
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<a href="algorithms.html#fusion.algorithms.header">Header</a>
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</h3>
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<pre class="programlisting">
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<span class="preprocessor">#include</span> <span class="special"><</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">hpp</span><span class="special">></span>
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</pre>
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</div>
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<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
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<td align="left"></td>
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<td align="right"><small>Copyright © 2001-2007 Joel de Guzman, Dan Marsden, Tobias
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Schwinger</small></td>
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</tr></table>
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