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<html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>Chapter 16. Sequences</title><meta name="generator" content="DocBook XSL Stylesheets V1.75.2" /><meta name="keywords" content=" ISO C++ , library " /><link rel="home" href="../spine.html" title="The GNU C++ Library Documentation" /><link rel="up" href="containers.html" title="Part VII. Containers" /><link rel="prev" href="containers.html" title="Part VII. Containers" /><link rel="next" href="vector.html" title="vector" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Chapter 16. Sequences</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="containers.html">Prev</a> </td><th width="60%" align="center">Part VII.
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Containers
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</th><td width="20%" align="right"> <a accesskey="n" href="vector.html">Next</a></td></tr></table><hr /></div><div class="chapter" title="Chapter 16. Sequences"><div class="titlepage"><div><div><h2 class="title"><a id="manual.containers.sequences"></a>Chapter 16. Sequences</h2></div></div></div><div class="toc"><p><b>Table of Contents</b></p><dl><dt><span class="sect1"><a href="sequences.html#containers.sequences.list">list</a></span></dt><dd><dl><dt><span class="sect2"><a href="sequences.html#sequences.list.size">list::size() is O(n)</a></span></dt></dl></dd><dt><span class="sect1"><a href="vector.html">vector</a></span></dt><dd><dl><dt><span class="sect2"><a href="vector.html#sequences.vector.management">Space Overhead Management</a></span></dt></dl></dd></dl></div><div class="sect1" title="list"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="containers.sequences.list"></a>list</h2></div></div></div><div class="sect2" title="list::size() is O(n)"><div class="titlepage"><div><div><h3 class="title"><a id="sequences.list.size"></a>list::size() is O(n)</h3></div></div></div><p>
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Yes it is, and that's okay. This is a decision that we preserved
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when we imported SGI's STL implementation. The following is
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quoted from <a class="ulink" href="http://www.sgi.com/tech/stl/FAQ.html" target="_top">their FAQ</a>:
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</p><div class="blockquote"><blockquote class="blockquote"><p>
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The size() member function, for list and slist, takes time
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proportional to the number of elements in the list. This was a
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deliberate tradeoff. The only way to get a constant-time
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size() for linked lists would be to maintain an extra member
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variable containing the list's size. This would require taking
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extra time to update that variable (it would make splice() a
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linear time operation, for example), and it would also make the
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list larger. Many list algorithms don't require that extra
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word (algorithms that do require it might do better with
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vectors than with lists), and, when it is necessary to maintain
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an explicit size count, it's something that users can do
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themselves.
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</p><p>
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This choice is permitted by the C++ standard. The standard says
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that size() <span class="quote">“<span class="quote">should</span>”</span> be constant time, and
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<span class="quote">“<span class="quote">should</span>”</span> does not mean the same thing as
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<span class="quote">“<span class="quote">shall</span>”</span>. This is the officially recommended ISO
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wording for saying that an implementation is supposed to do
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something unless there is a good reason not to.
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</p><p>
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One implication of linear time size(): you should never write
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</p><pre class="programlisting">
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if (L.size() == 0)
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...
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</pre><p>
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Instead, you should write
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</p><pre class="programlisting">
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if (L.empty())
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...
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</pre></blockquote></div></div></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="containers.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="containers.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="vector.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Part VII.
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Containers
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</td><td width="20%" align="center"><a accesskey="h" href="../spine.html">Home</a></td><td width="40%" align="right" valign="top"> vector</td></tr></table></div></body></html>
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