📄 nth_element.html
字号:
<HTML><!-- -- Copyright (c) 1996-1999 -- Silicon Graphics Computer Systems, Inc. -- -- Permission to use, copy, modify, distribute and sell this software -- and its documentation for any purpose is hereby granted without fee, -- provided that the above copyright notice appears in all copies and -- that both that copyright notice and this permission notice appear -- in supporting documentation. Silicon Graphics makes no -- representations about the suitability of this software for any -- purpose. It is provided "as is" without express or implied warranty. -- -- Copyright (c) 1994 -- Hewlett-Packard Company -- -- Permission to use, copy, modify, distribute and sell this software -- and its documentation for any purpose is hereby granted without fee, -- provided that the above copyright notice appears in all copies and -- that both that copyright notice and this permission notice appear -- in supporting documentation. Hewlett-Packard Company makes no -- representations about the suitability of this software for any -- purpose. It is provided "as is" without express or implied warranty. -- --><Head><Title>nth_element</Title><!-- Generated by htmldoc --></HEAD><BODY TEXT="#000000" LINK="#006600" ALINK="#003300" VLINK="#7C7F87" BGCOLOR="#FFFFFF"><A HREF="/"><IMG SRC="/images/common/sgilogo_small.gif" ALT="SGI Logo" WIDTH="80" HEIGHT="72" BORDER="0"></A><P><!--end header--><BR Clear><H1>nth_element</H1><Table CellPadding=0 CellSpacing=0 width=100%><TR><TD Align=left><Img src = "algorithms.gif" Alt="" WIDTH = "194" HEIGHT = "38" ></TD><TD Align=right><Img src = "function.gif" Alt="" WIDTH = "194" HEIGHT = "38" ></TD></TR><TR><TD Align=left VAlign=top><b>Category</b>: algorithms</TD><TD Align=right VAlign=top><b>Component type</b>: function</TD></TR></Table><h3>Prototype</h3><tt>Nth_element</tt> is an overloaded name; there are actually two <tt>nth_element</tt>functions.<pre>template <class <A href="RandomAccessIterator.html">RandomAccessIterator</A>>void nth_element(RandomAccessIterator first, RandomAccessIterator nth, RandomAccessIterator last);template <class <A href="RandomAccessIterator.html">RandomAccessIterator</A>, class <A href="StrictWeakOrdering.html">StrictWeakOrdering</A>>void nth_element(RandomAccessIterator first, RandomAccessIterator nth, RandomAccessIterator last, StrictWeakOrdering comp);</pre> <h3>Description</h3><tt>Nth_element</tt> is similar to <tt><A href="partial_sort.html">partial_sort</A></tt>, in that it partiallyorders a range of elements: it arranges the range <tt>[first, last)</tt> such that the element pointed to by the iterator <tt>nth</tt> is the same asthe element that would be in that position if the entire range <tt>[first, last)</tt> had been sorted. Additionally, none of the elements in the range <tt>[nth, last)</tt> is less than any ofthe elements in the range <tt>[first, nth)</tt>. <A href="#1">[1]</A><P>The two versions of <tt>nth_element</tt> differ in how they define whether oneelement is less than another. The first version comparesobjects using <tt>operator<</tt>, and the second compares objects usinga <A href="functors.html">function object</A> <tt>comp</tt>.<P>The postcondition for the first version of <tt>nth_element</tt> is as follows.There exists no iterator <tt>i</tt> in the range <tt>[first, nth)</tt> such that<tt>*nth < *i</tt>, and there exists no iterator <tt>j</tt> in the range <tt>[nth + 1, last)</tt>such that <tt>*j < *nth</tt>.<P>The postcondition for the second version of <tt>nth_element</tt> is as follows.There exists no iterator <tt>i</tt> in the range <tt>[first, nth)</tt> such that<tt>comp(*nth, *i)</tt> is <tt>true</tt>, and there exists no iterator <tt>j</tt> in the range <tt>[nth + 1, last)</tt> such that <tt>comp(*j, *nth)</tt> is <tt>true</tt>.<h3>Definition</h3>Defined in the standard header <A href="algorithm">algorithm</A>, and in the nonstandardbackward-compatibility header <A href="algo.h">algo.h</A>.<h3>Requirements on types</h3>For the first version, the one that takes three arguments:<UL><LI><tt>RandomAccessIterator</tt> is a model of <A href="RandomAccessIterator.html">Random Access Iterator</A>.<LI><tt>RandomAccessIterator</tt> is mutable.<LI><tt>RandomAccessIterator</tt>'s value type is <A href="LessThanComparable.html">LessThan Comparable</A>.<LI>The ordering relation on <tt>RandomAccessIterator</tt>'s value type is a <i>strict weak ordering</i>, as defined in the <A href="LessThanComparable.html">LessThan Comparable</A> requirements.</UL>For the second version, the one that takes four arguments:<UL><LI><tt>RandomAccessIterator</tt> is a model of <A href="RandomAccessIterator.html">Random Access Iterator</A>.<LI><tt>RandomAccessIterator</tt> is mutable.<LI><tt>StrictWeakOrdering</tt> is a model of <A href="StrictWeakOrdering.html">Strict Weak Ordering</A>.<LI><tt>RandomAccessIterator</tt>'s value type is convertible to <tt>StrictWeakOrdering</tt>'s argument type.</UL><h3>Preconditions</h3><UL><LI><tt>[first, nth)</tt> is a valid range.<LI><tt>[nth, last)</tt> is a valid range.</UL>(It follows from these two conditions that <tt>[first, last)</tt> is a valid range.)<h3>Complexity</h3>On average, linear in <tt>last - first</tt>. <A href="#2">[2]</A><h3>Example</h3><pre>int A[] = {7, 2, 6, 11, 9, 3, 12, 10, 8, 4, 1, 5};const int N = sizeof(A) / sizeof(int);nth_element(A, A + 6, A + N);copy(A, A + N, ostream_iterator<int>(cout, " "));// The printed result is "5 2 6 1 4 3 7 8 9 10 11 12".</pre><h3>Notes</h3><P><A name="1">[1]</A>The way in which this differs from <tt><A href="partial_sort.html">partial_sort</A></tt> is thatneither the range <tt>[first, nth)</tt> nor the range <tt>[nth, last)</tt> is besorted: it is simply guaranteed that none of the elements in <tt>[nth,last)</tt> is less than any of the elements in <tt>[first, nth)</tt>. In thatsense, <tt>nth_element</tt> is more similar to <tt><A href="partition.html">partition</A></tt> than to<tt><A href="sort.html">sort</A></tt>. <tt>Nth_element</tt> does less work than <tt><A href="partial_sort.html">partial_sort</A></tt>, so,reasonably enough, it is faster. That's the main reason to use<tt>nth_element</tt> instead of <tt><A href="partial_sort.html">partial_sort</A></tt>.<P><A name="2">[2]</A>Note that this is significantly less than the run-time complexityof <tt><A href="partial_sort.html">partial_sort</A></tt>.<h3>See also</h3><tt><A href="partial_sort.html">partial_sort</A></tt>,<tt><A href="partition.html">partition</A></tt>,<tt><A href="sort.html">sort</A></tt>,<A href="StrictWeakOrdering.html">StrictWeakOrdering</A>,<A href="LessThanComparable.html">LessThan Comparable</A><!-- start footer --><!-- Footer Begins --><STYLE TYPE="text/css"><!--TD.footer, TD.footer A{ font-family: Arial, helvetica, sans-serif; font-size: 8pt;}A.home {font-family: Arial, helvetica, sans-serif;}--></STYLE><P><A CLASS="home" HREF="index.html">STL Home</A><P><TABLE WIDTH="600" CELLPADDING="0" CELLPADDING="0" BORDER="0"> <TR> <TD ALIGN="RIGHT" CLASS="footer"><A HREF="/company_info/terms.html" TARGET="_top">terms of use</A> | <A HREF="/company_info/privacy.html" TARGET="_top">privacy policy</A></TD> <TD ALIGN="CENTER" CLASS="footer"> | </TD> <TD ALIGN="LEFT" CLASS="footer"><A HREF="/cgi-bin/feedback/" TARGET="_top">contact us</A></TD> </TR><TR> <TD ALIGN="RIGHT" CLASS="footer">Copyright © 1993-2003 Silicon Graphics, Inc. All rights reserved.</TD> <TD ALIGN="CENTER" CLASS="footer"> | </TD> <TD ALIGN="LEFT" CLASS="footer"><A HREF="/company_info/trademarks/" TARGET="_top">Trademark Information</A></TD> </TR></TABLE><!-- Footer Ends --><!-- end footer --><P></BODY></HTML> ⌨️ 快捷键说明
复制代码
Ctrl + C
搜索代码
Ctrl + F
全屏模式
F11
切换主题
Ctrl + Shift + D
显示快捷键
?
增大字号
Ctrl + =
减小字号
Ctrl + -