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| <TITLE>nth_element()</TITLE> |
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| <H2>nth_element()</H2> |
| <P><B>Library:</B> <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 <algorithm> |
| |
| namespace std { |
| template <class RandomAccessIterator> |
| void nth_element(RandomAccessIterator start, |
| RandomAccessIterator nth, |
| RandomAccessIterator finish); |
| |
| template <class RandomAccessIterator, class Compare> |
| 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<()</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 > *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 <algorithm> // for swap |
| #include <vector> // for vector |
| #include <iostream> // for cout, endl |
| |
| |
| |
| template<class RandomAccessIterator> |
| void quik_sort (RandomAccessIterator start, RandomAccessIterator end) |
| { |
| size_t dist = end - start; |
| |
| // Stop condition for recursion. |
| if (dist > 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 && *(end - 1) < *start) |
| std::swap (start, end); |
| } |
| |
| |
| int main () |
| { |
| typedef std::vector<int, std::allocator<int> > 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 << "The unsorted values are: \n "; |
| |
| for (Vector::iterator i = v.begin (); i != v.end (); ++i) |
| std::cout << *i << ", "; |
| |
| std::cout << "\n\n"; |
| |
| // Use the new sort algorithm. |
| quik_sort (v.begin (), v.end ()); |
| |
| // Output the sorted vector. |
| std::cout << "The sorted values are: \n "; |
| for (Vector::iterator i = v.begin (); i != v.end (); ++i) |
| std::cout << *i << ", "; |
| |
| std::cout << std::endl << 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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