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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. 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 Number Generator</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 Number Generator</H1><Table CellPadding=0 CellSpacing=0 width=100%><TR><TD Align=left><Img src = "functors.gif" Alt="" WIDTH = "194" HEIGHT = "38" ></TD><TD Align=right><Img src = "concept.gif" Alt="" WIDTH = "194" HEIGHT = "38" ></TD></TR><TR><TD Align=left VAlign=top><b>Category</b>: functors</TD><TD Align=right VAlign=top><b>Component type</b>: concept</TD></TR></Table><h3>Description</h3>A Random Number Generator is a <A href="functors.html">function object</A> that can be used to generate a random sequence of integers. That is: if <tt>f</tt> is a RandomNumber Generator and <tt>N</tt> is a positive integer, then <tt>f(N)</tt> will returnan integer less than <tt>N</tt> and greater than or equal to <tt>0</tt>. If <tt>f</tt> is called many times with the same value of <tt>N</tt>, it will yield a sequenceof numbers that is uniformly distributed <A href="#1">[1]</A> in the range <tt>[0, N)</tt>. <A href="#2">[2]</A><h3>Refinement of</h3><A href="UnaryFunction.html">Unary Function</A><h3>Associated types</h3><Table border><TR><TD VAlign=top>Argument type</TD><TD VAlign=top>The type of the Random Number Generator's argument. This must be an integral type.</TD></TR><TR><TD VAlign=top>Result type</TD><TD VAlign=top>The type returned when the Random Number Generator is called. It must be the same as the argument type.</TD></tr></table><h3>Notation</h3><Table><TR><TD VAlign=top><tt>F</tt></TD><TD VAlign=top>A type that is a model of Random Number Generator.</TD></TR><TR><TD VAlign=top><tt>Integer</tt></TD><TD VAlign=top>The argument type of <tt>F</tt>.</TD></TR><TR><TD VAlign=top><tt>f</tt></TD><TD VAlign=top>Object of type <tt>F</tt>.</TD></TR><TR><TD VAlign=top><tt>N</tt></TD><TD VAlign=top>Object of type <tt>Integer</tt></TD></tr></table><h3>Definitions</h3>The <i>domain</i> of a Random Number Generator (<i>i.e.</i> the setof permissible values for its argument) is the set of numbers that aregreater than zero and less than some maximum value.<P>The <i>range</i> of a Random Number Generator is the set of nonnegativeintegers that are less than the Random Number Generator's argument.<h3>Valid expressions</h3>None, except for those defined by <A href="UnaryFunction.html">Unary Function</A>.<h3>Expression semantics</h3><Table border><TR><TH>Name</TH><TH>Expression</TH><TH>Precondition</TH><TH>Semantics</TH><TH>Postcondition</TH></TR><TR><TD VAlign=top>Function call</TD><TD VAlign=top><tt>f(N)</tt></TD><TD VAlign=top><tt>N</tt> is positive.</TD><TD VAlign=top>Returns a pseudo-random number of type <tt>Integer</tt>. <A href="#2">[2]</A></TD><TD VAlign=top>The return value is less than <tt>N</tt>, and greater than or equal to 0.</TD></tr></table><h3>Complexity guarantees</h3><h3>Invariants</h3><Table border><TR><TD VAlign=top>Uniformity</TD><TD VAlign=top>In the limit as <tt>f</tt> is called many times with the same argument<tt>N</tt>, every integer in the range <tt>[0, N)</tt> will appear an equal numberof times.</TD></tr></table><h3>Models</h3><h3>Notes</h3><P><A name="1">[1]</A>Uniform distribution means that all of the numbers in the range<tt>[0, N)</tt> appear with equal frequency. Or, to put it differently,the probability for obtaining any particular value is <tt>1/N</tt>.<P><A name="2">[2]</A>Random number generators are a very subtle subject: a good randomnumber generator must satisfy many statistical properties beyonduniform distribution. See section 3.4 of Knuth for a discussion ofwhat it means for a sequence to be random, and section 3.2 for severalalgorithms that may be used to write random number generators.(D. E. Knuth, <i>The Art of ComputerProgramming. Volume 2: Seminumerical Algorithms</i>, third edition. Addison-Wesley, 1998.)<h3>See also</h3><!--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> ?? 快捷鍵說明
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