00001 #include "MusimatChapter9.h" 00002 MusimatChapter9Section(C090703a) { 00003 Print("*** Seeding the Random Number Generator ***"); 00004 /***************************************************************************** 00005 Seeding the Random Number Generator 00006 00007 Unlike the natural sources of randomness, LCRandom() will always produce the 00008 same sequence with the same initial parameters. 00009 00010 Different sets of pseudorandom 00011 sequences can be generated by varying the initial value of x, as with the 00012 SeedRandom() function defined below. 00013 *****************************************************************************/ 00014 para1(); // Step into this function to continue. 00015 } 00016 00017 Integer lcRandom(); 00018 extern Integer lc_x; 00019 00020 /***************************************************************************** 00021 seedRandom() is a simple function to seed the random number generator. 00022 This function is a copy of SeedRandom() defined in Random.cpp, reproduced here 00023 for dydactic reasons (to keep things simple). 00024 lc_x is a global Integer, again for dydactic reasons. 00025 *****************************************************************************/ 00026 00027 Void seedRandom(Integer s) { 00028 lc_x = s; // set global variable lc_x to seed s 00029 } 00030 00031 Static Void para1() { 00032 /***************************************************************************** 00033 Below is an example of generating random numbers from a seed value of 100, 00034 resetting the seed to 100, and generating the same three random numbers. 00035 Repeatability is why this is called a pseudorandom number generator. 00036 *****************************************************************************/ 00037 00038 Print("*** Three random numbers based on seed 100 ***"); 00039 seedRandom( 100 ); 00040 For (Integer i = 0; i < 3; i++ ) { 00041 Print( lcRandom() ); 00042 } 00043 Print("Same three random numbers after resetting the seed to 100:"); 00044 seedRandom( 100 ); 00045 For (Integer i = 0; i < 3; i++ ) { 00046 Print( lcRandom() ); 00047 } 00048 00049 /***************************************************************************** 00050 The seedRandom() function allows us to set the initial value of x. If we initialize x to a 00051 parameter such as the current time in seconds from some fixed moment, then we 00052 start at a different place in the pseudorandom cycle each time (although, of 00053 course, this is finite, too, because the sequence length is necessarily limited). 00054 *****************************************************************************/ 00055 00056 Print("LCRandom seeded with current time in seconds:"); 00057 seedRandom( Time() ); 00058 For (Integer i = 0; i < 3; i++ ) { 00059 Print( lcRandom() ); 00060 } 00061 00062 /***************************************************************************** 00063 The linear congruential method is simple and efficient, but it is hardly the 00064 best source of random values. Even ignoring the fact that it repeats, its 00065 uniformity is not wonderful. Knuth (1973, vol. 2) cautioned, "Random number 00066 generators should not be chosen at random." For superior techniques, see 00067 Press et al. (1988, 210). However, this method is very simple to implement 00068 and has the advantage over natural random processes of providing the same 00069 pseudorandom sequence if seeded with the same values. 00070 *****************************************************************************/ 00071 } 00072 00074 /* $Revision: 1.3 $ $Date: 2006/09/05 08:02:45 $ $Author: dgl $ $Name: $ $Id: C090703a.cpp,v 1.3 2006/09/05 08:02:45 dgl Exp $ */ 00075 // The Musimat Tutorial © 2006 Gareth Loy 00076 // Derived from Chapter 9 and Appendix B of "Musimathics Vol. 1" © 2006 Gareth Loy 00077 // and published exclusively by The MIT Press. 00078 // This program is released WITHOUT ANY WARRANTY; without even the implied 00079 // warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. 00080 // For information on usage and redistribution, and for a DISCLAIMER OF ALL 00081 // WARRANTIES, see the file, "LICENSE.txt," in this distribution. 00082 // "Musimathics" is available here: http://mitpress.mit.edu/catalog/item/default.asp?ttype=2&tid=10916 00083 // Gareth Loy's Musimathics website: http://www.musimathics.com/ 00084 // The Musimat website: http://www.musimat.com/ 00085 // This program is released under the terms of the GNU General Public License 00086 // available here: http://www.gnu.org/licenses/gpl.txt
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