📄 random_sample_n.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>random_sample_n</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>random_sample_n</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>Random_sample_n</tt> is an overloaded name; there are actually two<tt>random_sample_n</tt> functions.<pre>template <class <A href="ForwardIterator.html">ForwardIterator</A>, class <A href="OutputIterator.html">OutputIterator</A>, class Distance>OutputIterator random_sample_n(ForwardIterator first, ForwardIterator last, OutputIterator out, Distance n)template <class <A href="ForwardIterator.html">ForwardIterator</A>, class <A href="OutputIterator.html">OutputIterator</A>, class Distance, class <A href="RandomNumberGenerator.html">RandomNumberGenerator</A>>OutputIterator random_sample_n(ForwardIterator first, ForwardIterator last, OutputIterator out, Distance n, RandomNumberGenerator& rand)</pre> <h3>Description</h3><tt>Random_sample_n</tt> randomly copies a sample of the elements fromthe range <tt>[first, last)</tt> into the range <tt>[out, out + n)</tt>.Each element in the input range appears at most once in the outputrange, and samples are chosen with uniform probability. <A href="#1">[1]</A> Elements in the output range appear in the same relative orderas their relative order within the input range. <A href="#2">[2]</A><P><tt>Random_sample</tt> copies <tt>m</tt> elements from <tt>[first, last)</tt>to <tt>[out, out + m)</tt>, where <tt>m</tt> is <tt>min(last - first, n)</tt>.The return value is <tt>out + m</tt>.<P>The first version uses an internal random number generator, and thesecond uses a <A href="RandomNumberGenerator.html">Random Number Generator</A>, a special kind of<A href="functors.html">function object</A>, that is explicitly passed as an argument.<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>.This function is an SGI extension; it is not part of the C++standard.<h3>Requirements on types</h3>For the first version:<UL><LI><tt>ForwardIterator</tt> is a model of <A href="ForwardIterator.html">Forward Iterator</A><LI><tt>OutputIterator</tt> is a model of <A href="OutputIterator.html">Output Iterator</A><LI><tt>ForwardIterator</tt>'s value type is convertible to a type in <tt>OutputIterator</tt>'s set of value types.<LI><tt>Distance</tt> is an integral type that is large enough to represent the value <tt>last - first</tt>.</UL>For the second version:<UL><LI><tt>ForwardIterator</tt> is a model of <A href="ForwardIterator.html">Forward Iterator</A><LI><tt>OutputIterator</tt> is a model of <A href="OutputIterator.html">Output Iterator</A><LI><tt>RandomNumberGenerator</tt> is a model of <A href="RandomNumberGenerator.html">Random Number Generator</A><LI><tt>Distance</tt> is an integral type that is large enough to represent the value <tt>last - first</tt>.<LI><tt>ForwardIterator</tt>'s value type is convertible to a type in <tt>OutputIterator</tt>'s set of value types.<LI><tt>Distance</tt> is convertible to <tt>RandomNumberGenerator</tt>'s argument type.</UL><h3>Preconditions</h3><UL><LI><tt>[first, last)</tt> is a valid range.<LI><tt>n</tt> is nonnegative.<LI><tt>[first, last)</tt> and <tt>[out, out + n)</tt> do not overlap.<LI>There is enough space to hold all of the elements being copied. More formally, the requirement is that <tt>[out, out + min(n, last - first))</tt> is a valid range.<LI><tt>last - first</tt> is less than <tt>rand</tt>'s maximum value.</UL><h3>Complexity</h3>Linear in <tt>last - first</tt>. At most <tt>last - first</tt> elements from theinput range are examined, and exactly <tt>min(n, last - first)</tt> elementsare copied to the output range.<h3>Example</h3><pre>int main(){ const int N = 10; int A[] = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10}; random_sample_n(A, A+N, ostream_iterator<int>(cout, " "), 4); // The printed value might be 3 5 6 10, // or any of 209 other possibilities.}</pre><h3>Notes</h3><P><A name="1">[1]</A>This is "Algorithm S" from section 3.4.2 of Knuth(D. E. Knuth, <i>The Art of ComputerProgramming. Volume 2: Seminumerical Algorithms</i>, second edition. Addison-Wesley, 1981). Knuth credits C. T. Fan, M. E. Muller, andI. Rezucha (1962) and T. G. Jones (1962).Note that there are <tt>N! / n! / (N - n)!</tt> ways of selecting a sample of<tt>n</tt> elements from a range of <tt>N</tt> elements. <tt>Random_sample_n</tt> yieldsuniformly distributed results; that is, the probability of selectingany particular element is <tt>n / N</tt>, and the probability of anyparticular sampling is <tt>n! * (N - n)! / N!</tt>.<P><A name="2">[2]</A>In contrast, the <tt><A href="random_sample.html">random_sample</A></tt> algorithm does not preserverelative ordering within the input range. The other major distinction between the two algorithms is that <tt>random_sample_n</tt> requires its input range to be <A href="ForwardIterator.html">Forward Iterators</A>and only requires its output range to be <A href="OutputIterator.html">Output Iterators</A>, while<tt><A href="random_sample.html">random_sample</A></tt> only requires its input range to be<A href="InputIterator.html">Input Iterators</A> and requires its output range to be<A href="RandomAccessIterator.html">Random Access Iterators</A>.<h3>See also</h3><tt><A href="random_shuffle.html">random_shuffle</A></tt>, <tt><A href="random_sample.html">random_sample</A></tt>, <A href="RandomNumberGenerator.html">Random Number Generator</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 + -