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<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. 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It is provided "as is" without express or implied warranty. -- --><Head><Title>random_sample</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>random_sample</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</tt> is an overloaded name; there are actually two<tt>random_sample</tt> functions.<pre>template <class <A href="InputIterator.html">InputIterator</A>, class <A href="RandomAccessIterator.html">RandomAccessIterator</A>>Random AccessIterator random_sample(InputIterator first, InputIterator last, RandomAccessIterator ofirst, RandomAccessIterator olast) template <class <A href="InputIterator.html">InputIterator</A>, class <A href="RandomAccessIterator.html">RandomAccessIterator</A>, class <A href="RandomNumberGenerator.html">RandomNumberGenerator</A>>random_sample(InputIterator first, InputIterator last, RandomAccessIterator ofirst, RandomAccessIterator olast, RandomNumberGenerator& rand) </pre> <h3>Description</h3><tt>Random_sample</tt> randomly copies a sample of the elements fromthe range <tt>[first, last)</tt> into the range <tt>[ofirst, olast)</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 might appear in any order: relativeorder within the input range is not guaranteed to be preserved. <A href="#2">[2]</A><P><tt>Random_sample</tt> copies <tt>n</tt> elements from <tt>[first, last)</tt>to <tt>[ofirst, olast)</tt>, where <tt>n</tt> is <tt>min(last - first, olast - ofirst)</tt>.The return value is <tt>ofirst + n</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>InputIterator</tt> is a model of <A href="InputIterator.html">Input Iterator</A><LI><tt>RandomAccessIterator</tt> is a model of <A href="RandomAccessIterator.html">Random Access Iterator</A><LI><tt>RandomAccessIterator</tt> is mutable.<LI><tt>InputIterator</tt>'s value type is convertible to <tt>RandomAccessIterator</tt>'s value type.</UL>For the second version:<UL><LI><tt>InputIterator</tt> is a model of <A href="InputIterator.html">Input Iterator</A><LI><tt>RandomAccessIterator</tt> is a model of <A href="RandomAccessIterator.html">Random Access Iterator</A><LI><tt>RandomAccessIterator</tt> is mutable.<LI><tt>RandomNumberGenerator</tt> is a model of <A href="RandomNumberGenerator.html">Random Number Generator</A><LI><tt>InputIterator</tt>'s value type is convertible to <tt>RandomAccessIterator</tt>'s value type.<LI><tt>RandomAccessIterator</tt>'s distance type 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>[ofirst, olast)</tt> is a valid range.<LI><tt>[first, last)</tt> and <tt>[ofirst, olast)</tt> do not overlap.<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 are copiedfrom the input range to the output range.<h3>Example</h3><pre>int main(){ const int N = 10; const int n = 4; int A[] = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10}; int B[n]; random_sample(A, A+N, B, B+n); <A href="copy.html">copy</A>(B, B + n, <A href="ostream_iterator.html">ostream_iterator</A><int>(cout, " ")); // The printed value might be 1 6 3 5, // or any of 5039 other possibilities.}</pre><h3>Notes</h3><P><A name="1">[1]</A>This is "Algorithm R" 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 Alan Waterman. Note that thereare <tt>N! / n! / (N - n)!</tt> ways of selecting a sample of <tt>n</tt> elementsfrom a range of <tt>N</tt> elements. <tt>Random_sample</tt> yields uniformlydistributed results; that is, the probability of selecting anyparticular element is <tt>n / N</tt>, and the probability of any particularsampling (not considering order of elements) is <tt>n! * (N - n)! / N!</tt>. <P><A name="2">[2]</A>If preservation of the relative ordering within the input rangeis important for your application, you should use <tt><A href="random_sample_n.html">random_sample_n</A></tt>instead. The main restriction of <tt><A href="random_sample_n.html">random_sample_n</A></tt> is that theinput range must consist of <A href="ForwardIterator.html">Forward Iterators</A>, rather than <A href="InputIterator.html">Input Iterators</A>.<h3>See also</h3><tt><A href="random_shuffle.html">random_shuffle</A></tt>, <tt><A href="random_sample_n.html">random_sample_n</A></tt>, <A href="RandomNumberGenerator.html">Random Number Generator</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> ⌨️ 快捷键说明
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