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<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1" />
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<meta name="AUTHOR" content="pme@gcc.gnu.org (Phil Edwards)" />
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<meta name="KEYWORDS" content="HOWTO, libstdc++, GCC, g++, libg++, STL" />
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<meta name="DESCRIPTION" content="HOWTO for the libstdc++ chapter 25." />
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<meta name="GENERATOR" content="vi and eight fingers" />
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<title>libstdc++-v3 HOWTO: Chapter 25: Algorithms</title>
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<link rel="Start" href="../documentation.html" type="text/html"
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title="GNU C++ Standard Library" />
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<link rel="Prev" href="../24_iterators/howto.html" type="text/html"
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title="Iterators" />
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title="Numerics" />
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</head>
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<body>
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<h1 class="centered"><a name="top">Chapter 25: Algorithms</a></h1>
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<p>Chapter 25 deals with the generalized subroutines for automatically
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transforming lemmings into gold.
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</p>
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<hr />
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<h1>Contents</h1>
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<ul>
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<li><a href="#1">Prerequisites</a></li>
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<li><a href="#2">Special <code>swap</code>s</a></li>
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</ul>
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<hr />
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<h2><a name="1">Prerequisites</a></h2>
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<p>The neatest accomplishment of the algorithms chapter is that all the
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work is done via iterators, not containers directly. This means two
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important things:
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</p>
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<ol>
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<li>Anything that behaves like an iterator can be used in one of
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these algorithms. Raw pointers make great candidates, thus
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built-in arrays are fine containers, as well as your own iterators.
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</li>
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<li>The algorithms do not (and cannot) affect the container as a
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whole; only the things between the two iterator endpoints. If
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you pass a range of iterators only enclosing the middle third of
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a container, then anything outside that range is inviolate.
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</li>
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</ol>
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<p>Even strings can be fed through the algorithms here, although the
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string class has specialized versions of many of these functions (for
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example, <code>string::find()</code>). Most of the examples on this
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page will use simple arrays of integers as a playground for
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algorithms, just to keep things simple.
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<a name="Nsize">The use of <strong>N</strong></a> as a size in the
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examples is to keep things easy to read but probably won't be valid
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code. You can use wrappers such as those described in the
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<a href="../23_containers/howto.html">containers chapter</a> to keep
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real code readable.
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</p>
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<p>The single thing that trips people up the most is the definition of
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<em>range</em> used with iterators; the famous
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"past-the-end" rule that everybody loves to hate. The
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<a href="../24_iterators/howto.html#2">iterators chapter</a> of this
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document has a complete explanation of this simple rule that seems to
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cause so much confusion. Once you get <em>range</em> into your head
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(it's not that hard, honest!), then the algorithms are a cakewalk.
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</p>
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<p>Return <a href="#top">to top of page</a> or
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<a href="../faq/index.html">to the FAQ</a>.
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</p>
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<hr />
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<h2><a name="2">Special <code>swap</code>s</a></h2>
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<p>If you call <code> std::swap(x,y); </code> where x and y are standard
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containers, then the call will automatically be replaced by a call to
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<code> x.swap(y); </code> instead.
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</p>
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<p>This allows member functions of each container class to take over, and
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containers' swap functions should have O(1) complexity according to
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the standard. (And while "should" allows implementations to
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behave otherwise and remain compliant, this implementation does in
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fact use constant-time swaps.) This should not be surprising, since
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for two containers of the same type to swap contents, only some
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internal pointers to storage need to be exchanged.
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</p>
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<p>Return <a href="#top">to top of page</a> or
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<a href="../faq/index.html">to the FAQ</a>.
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</p>
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<hr />
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<p class="fineprint"><em>
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See <a href="../17_intro/license.html">license.html</a> for copying conditions.
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Comments and suggestions are welcome, and may be sent to
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<a href="mailto:libstdc++@gcc.gnu.org">the libstdc++ mailing list</a>.
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