1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
|
<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 BGCOLOR="#ffffff" LINK="#0000ee" TEXT="#000000" VLINK="#551a8b"
ALINK="#ff0000">
<IMG SRC="CorpID.gif"
ALT="SGI" HEIGHT="43" WIDTH="151">
<!--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 partially
orders 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 as
the 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 of
the 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 one
element is less than another. The first version compares
objects using <tt>operator<</tt>, and the second compares objects using
a <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 nonstandard
backward-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 that
neither the range <tt>[first, nth)</tt> nor the range <tt>[nth, last)</tt> is be
sorted: 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 that
sense, <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 complexity
of <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-->
<HR SIZE="6">
<A href="http://www.sgi.com/"><IMG SRC="surf.gif" HEIGHT="54" WIDTH="54"
ALT="[Silicon Surf]"></A>
<A HREF="index.html"><IMG SRC="stl_home.gif"
HEIGHT="54" WIDTH="54" ALT="[STL Home]"></A>
<BR>
<FONT SIZE="-2">
<A href="http://www.sgi.com/Misc/sgi_info.html" TARGET="_top">Copyright ©
1999 Silicon Graphics, Inc.</A> All Rights Reserved.</FONT>
<FONT SIZE="-3"><a href="http://www.sgi.com/Misc/external.list.html" TARGET="_top">TrademarkInformation</A>
</FONT>
<P>
</BODY>
</HTML>
|
