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<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>

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