<!--
    Licensed to the Apache Software  Foundation (ASF) under one or more
    contributor  license agreements.  See  the NOTICE  file distributed
    with  this  work  for  additional information  regarding  copyright
    ownership.   The ASF  licenses this  file to  you under  the Apache
    License, Version  2.0 (the  License); you may  not use  this file
    except in  compliance with the License.   You may obtain  a copy of
    the License at

    http://www.apache.org/licenses/LICENSE-2.0

    Unless required by applicable law or agreed to in writing, software
    distributed under the  License is distributed on an  "AS IS" BASIS,
    WITHOUT  WARRANTIES OR CONDITIONS  OF ANY  KIND, either  express or
    implied.   See  the License  for  the  specific language  governing
    permissions and limitations under the License.

    Copyright 1999-2007 Rogue Wave Software, Inc.
-->

<HTML>
<HEAD>
<TITLE>nth_element()</TITLE>
<LINK REL=StyleSheet HREF="../rw.css" TYPE="text/css" TITLE="Apache stdcxx Stylesheet"></HEAD>
<BODY BGCOLOR=#FFFFFF>
<A HREF="not-equal-to.html"><IMG SRC="images/bprev.gif" WIDTH=20 HEIGHT=21 ALT="Previous file" BORDER=O></A><A HREF="noframes.html"><IMG SRC="images/btop.gif" WIDTH=56 HEIGHT=21 ALT="Top of Document" BORDER=O></A><A HREF="booktoc.html"><IMG SRC="images/btoc.gif" WIDTH=56 HEIGHT=21 ALT="Contents" BORDER=O></A><A HREF="tindex.html"><IMG SRC="images/bindex.gif" WIDTH=56 HEIGHT=21 ALT="Index page" BORDER=O></A><A HREF="num-get.html"><IMG SRC="images/bnext.gif" WIDTH=25 HEIGHT=21 ALT="Next file" BORDER=O></A><DIV CLASS="DOCUMENTNAME"><B>Apache C++ Standard Library Reference Guide</B></DIV>
<H2>nth_element()</H2>
<P><B>Library:</B>&nbsp;&nbsp;<A HREF="2-9.html">Algorithms</A></P>

<PRE><HR><B><I>Function</I></B><HR></PRE>

<UL>
<LI><A HREF="#sec1">Local Index</A></LI>
<LI><A HREF="#sec2">Summary</A></LI>
<LI><A HREF="#sec3">Synopsis</A></LI>
<LI><A HREF="#sec4">Description</A></LI>
<LI><A HREF="#sec5">Complexity</A></LI>
<LI><A HREF="#sec6">Example</A></LI>
<LI><A HREF="#sec7">See Also</A></LI>
<LI><A HREF="#sec8">Standards Conformance</A></LI>
</UL>
<A NAME="sec1"><H3>Local Index</H3></A>
No Entries
<A NAME="sec2"><H3>Summary</H3></A>
<P>An algorithm that rearranges a collection so that all elements lower in sorted order than the <SAMP>nth</SAMP> element come before it, and all elements higher in sorted order than the <SAMP>nth</SAMP> element come after it</P>
<A NAME="sec3"><H3>Synopsis</H3></A>

<PRE>#include &lt;algorithm&gt;

namespace std {
  template &lt;class RandomAccessIterator&gt;
  void nth_element(RandomAccessIterator start,
                   RandomAccessIterator nth,
                   RandomAccessIterator finish);

  template &lt;class RandomAccessIterator, class Compare&gt;
  void nth_element(RandomAccessIterator start,
                   RandomAccessIterator nth,
                   RandomAccessIterator finish,
                   Compare comp);
}
</PRE>
<A NAME="sec4"><H3>Description</H3></A>
<P>The <SAMP>nth_element()</SAMP> algorithm rearranges a collection according to either  <SAMP>operator&lt;()</SAMP> or the function object <SAMP>comp</SAMP>. After the algorithm is applied, the follwoing hold:</P>
<UL>
<LI><P CLASS="LIST">The element that would be in the <SAMP>nth</SAMP> position if the sequence were completely sorted is in the <SAMP>nth</SAMP> position</P></LI>
<LI><P CLASS="LIST">All elements prior to the <SAMP>nth</SAMP> position would also precede that position in an ordered sequence</P></LI>
<LI><P CLASS="LIST">All elements following the <SAMP>nth</SAMP> position would also follow that position in an ordered sequence</P></LI>
</UL>
<P>That is, for any iterator <SAMP>i</SAMP> in the range <SAMP>[start, nth)</SAMP> and any iterator <SAMP>j</SAMP> in the range <SAMP>[nth, finish)</SAMP>, it holds that <SAMP>!(*i &gt; *j)</SAMP> or <SAMP>comp(*i, *j) == false</SAMP>. </P>
<P>Note that the elements that precede or follow the <SAMP>nth</SAMP> position are not necessarily sorted relative to each other. The <SAMP>nth_element()</SAMP> algorithm does<I> not </I>sort the entire collection.</P>
<A NAME="sec5"><H3>Complexity</H3></A>
<P>The algorithm is linear, on average, where <SAMP>N</SAMP> is the size of the range <SAMP>[start, finish)</SAMP>.</P>
<A NAME="sec6"><H3>Example</H3></A>

<UL><PRE>//
//  nthelem.cpp
//
 
#include &lt;algorithm&gt;   // for swap
#include &lt;vector&gt;      // for vector
#include &lt;iostream&gt;    // for cout, endl


template&lt;class RandomAccessIterator&gt;
void quik_sort (RandomAccessIterator start, RandomAccessIterator end)
{
    size_t dist = end - start;

    // Stop condition for recursion.
    if (dist &gt; 2) {
        // Use nth_element to do all the work for quik_sort.
        std::nth_element (start, start + (dist / 2), end);

        // Recursive calls to each remaining unsorted portion.
        quik_sort (start, start + (dist / 2 - 1));
        quik_sort (start + (dist / 2 + 1), end);
    }

    if (2 == dist &amp;&amp; *(end - 1) &lt; *start)
        std::swap (start, end);
}


int main ()
{
    typedef std::vector&lt;int, std::allocator&lt;int&gt; &gt; Vector;

    // Initialize a vector using an array of integers.
    Vector::value_type arr[] = { 37, 12,  2, -5, 14, 
                                  1,  0, -1, 14, 32 };

    Vector v (arr + 0, arr + sizeof arr / sizeof *arr);

    // Print the initial vector.
    std::cout &lt;&lt; "The unsorted values are: \n     ";

    for (Vector::iterator i = v.begin (); i != v.end (); ++i)
        std::cout &lt;&lt; *i &lt;&lt; ", ";

    std::cout &lt;&lt; "\n\n";

    // Use the new sort algorithm.
    quik_sort (v.begin (), v.end ());

    // Output the sorted vector.
    std::cout &lt;&lt; "The sorted values are: \n     ";
    for (Vector::iterator i = v.begin (); i != v.end (); ++i)
        std::cout &lt;&lt; *i &lt;&lt; ", ";

    std::cout &lt;&lt; std::endl &lt;&lt; std::endl;

    return 0;
}


Program Output:
The unsorted values are: 
     37, 12, 2, -5, 14, 1, 0, -1, 14, 32, 

The sorted values are: 
     -5, -1, 0, 1, 2, 12, 14, 14, 32, 37, 
</PRE></UL>
<UL><PRE></PRE></UL>
<A NAME="sec7"><H3>See Also</H3></A>
<P><A HREF="algorithms.html">Algorithms</A></P>
<A NAME="sec8"><H3>Standards Conformance</H3></A>
<P><I>ISO/IEC 14882:1998 -- International Standard for Information Systems -- Programming Language C++, Section 25.3.2</I></P>

<BR>
<HR>
<A HREF="not-equal-to.html"><IMG SRC="images/bprev.gif" WIDTH=20 HEIGHT=21 ALT="Previous file" BORDER=O></A><A HREF="noframes.html"><IMG SRC="images/btop.gif" WIDTH=56 HEIGHT=21 ALT="Top of Document" BORDER=O></A><A HREF="booktoc.html"><IMG SRC="images/btoc.gif" WIDTH=56 HEIGHT=21 ALT="Contents" BORDER=O></A><A HREF="tindex.html"><IMG SRC="images/bindex.gif" WIDTH=56 HEIGHT=21 ALT="Index page" BORDER=O></A><A HREF="num-get.html"><IMG SRC="images/bnext.gif" WIDTH=20 HEIGHT=21 ALT="Next file" BORDER=O></A>

<!-- Google Analytics tracking code -->
<script src="http://www.google-analytics.com/urchin.js" type="text/javascript">
</script>
<script type="text/javascript">
    _uacct = "UA-1775151-1";
    urchinTracker();
</script>
<!-- end of Google Analytics tracking code -->

</BODY>
</HTML>