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166 lines
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<HTML>
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<HEAD>
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<TITLE>nth_element()</TITLE>
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<LINK REL=StyleSheet HREF="../rw.css" TYPE="text/css" TITLE="Apache stdcxx Stylesheet"></HEAD>
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<BODY BGCOLOR=#FFFFFF>
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<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>
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<H2>nth_element()</H2>
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<P><B>Library:</B> <A HREF="2-9.html">Algorithms</A></P>
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<PRE><HR><B><I>Function</I></B><HR></PRE>
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<UL>
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<LI><A HREF="#sec1">Local Index</A></LI>
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<LI><A HREF="#sec2">Summary</A></LI>
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<LI><A HREF="#sec3">Synopsis</A></LI>
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<LI><A HREF="#sec4">Description</A></LI>
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<LI><A HREF="#sec5">Complexity</A></LI>
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<LI><A HREF="#sec6">Example</A></LI>
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<LI><A HREF="#sec7">See Also</A></LI>
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<LI><A HREF="#sec8">Standards Conformance</A></LI>
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</UL>
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<A NAME="sec1"><H3>Local Index</H3></A>
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No Entries
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<A NAME="sec2"><H3>Summary</H3></A>
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<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>
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<A NAME="sec3"><H3>Synopsis</H3></A>
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<PRE>#include <algorithm>
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namespace std {
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template <class RandomAccessIterator>
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void nth_element(RandomAccessIterator start,
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RandomAccessIterator nth,
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RandomAccessIterator finish);
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template <class RandomAccessIterator, class Compare>
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void nth_element(RandomAccessIterator start,
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RandomAccessIterator nth,
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RandomAccessIterator finish,
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Compare comp);
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}
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</PRE>
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<A NAME="sec4"><H3>Description</H3></A>
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<P>The <SAMP>nth_element()</SAMP> algorithm rearranges a collection according to either <SAMP>operator<()</SAMP> or the function object <SAMP>comp</SAMP>. After the algorithm is applied, the follwoing hold:</P>
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<UL>
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<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>
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<LI><P CLASS="LIST">All elements prior to the <SAMP>nth</SAMP> position would also precede that position in an ordered sequence</P></LI>
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<LI><P CLASS="LIST">All elements following the <SAMP>nth</SAMP> position would also follow that position in an ordered sequence</P></LI>
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</UL>
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<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 > *j)</SAMP> or <SAMP>comp(*i, *j) == false</SAMP>. </P>
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<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>
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<A NAME="sec5"><H3>Complexity</H3></A>
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<P>The algorithm is linear, on average, where <SAMP>N</SAMP> is the size of the range <SAMP>[start, finish)</SAMP>.</P>
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<A NAME="sec6"><H3>Example</H3></A>
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<UL><PRE>//
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// nthelem.cpp
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//
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#include <algorithm> // for swap
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#include <vector> // for vector
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#include <iostream> // for cout, endl
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template<class RandomAccessIterator>
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void quik_sort (RandomAccessIterator start, RandomAccessIterator end)
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{
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size_t dist = end - start;
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// Stop condition for recursion.
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if (dist > 2) {
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// Use nth_element to do all the work for quik_sort.
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std::nth_element (start, start + (dist / 2), end);
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// Recursive calls to each remaining unsorted portion.
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quik_sort (start, start + (dist / 2 - 1));
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quik_sort (start + (dist / 2 + 1), end);
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}
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if (2 == dist && *(end - 1) < *start)
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std::swap (start, end);
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}
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int main ()
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{
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typedef std::vector<int, std::allocator<int> > Vector;
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// Initialize a vector using an array of integers.
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Vector::value_type arr[] = { 37, 12, 2, -5, 14,
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1, 0, -1, 14, 32 };
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Vector v (arr + 0, arr + sizeof arr / sizeof *arr);
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// Print the initial vector.
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std::cout << "The unsorted values are: \n ";
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for (Vector::iterator i = v.begin (); i != v.end (); ++i)
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std::cout << *i << ", ";
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std::cout << "\n\n";
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// Use the new sort algorithm.
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quik_sort (v.begin (), v.end ());
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// Output the sorted vector.
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std::cout << "The sorted values are: \n ";
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for (Vector::iterator i = v.begin (); i != v.end (); ++i)
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std::cout << *i << ", ";
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std::cout << std::endl << std::endl;
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return 0;
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}
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Program Output:
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The unsorted values are:
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37, 12, 2, -5, 14, 1, 0, -1, 14, 32,
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The sorted values are:
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-5, -1, 0, 1, 2, 12, 14, 14, 32, 37,
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</PRE></UL>
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<UL><PRE></PRE></UL>
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<A NAME="sec7"><H3>See Also</H3></A>
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<P><A HREF="algorithms.html">Algorithms</A></P>
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<A NAME="sec8"><H3>Standards Conformance</H3></A>
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<P><I>ISO/IEC 14882:1998 -- International Standard for Information Systems -- Programming Language C++, Section 25.3.2</I></P>
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<BR>
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