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# generater big rand number

 P: n/a i use g++ to generater rand number, now i find that the RAND_MAX is 32367 in my computer, how can i make a bigger rand number( the number is wihin in the integer(2^32-1)) Oct 19 '08 #1
8 Replies

 P: n/a On 19 Okt., 09:37, remlostime

 P: n/a remlostime wrote: i use g++ to generater rand number, now i find that the RAND_MAX is 32367 in my computer, how can i make a bigger rand number( the number is wihin in the integer(2^32-1)) By using a better random number generator? There are tons of them if you search the internet. (Curiously it's actually surprisingly difficult to find a good random number generator which is portable, written in C++, very easy to use and doesn't require you to install the entire Boost library to simply use the RNG. For this reason I made this C++ version of the ISAAC rng: http://warp.povusers.org/IsaacRand.zip ) Oct 19 '08 #3

 P: n/a On 19 Oct, 10:43, peter koch

 P: n/a In article <9L*************@read4.inet.fi>, Juha Nieminen

 P: n/a On 19 Okt., 22:03, gw7...@aol.com wrote: On 19 Oct, 10:43, peter koch

 P: n/a On 2008-10-19 16:03:39 -0400, gw****@aol.com said: On 19 Oct, 10:43, peter koch On 19 Okt., 09:37, remlostime >i use g++ to generater rand number, now i find that the RAND_MAX is32367 in my computer, how can i make a bigger rand number( the numberis wihin in the integer(2^32-1)) You have several options: one is to use a library such as boost whichgives you several options to choose different random number generatorsof high quality. Another is to compose your number by calling randseveral times. To illustrate: if your random number only gave valuesfrom 0 to 9, but you needed values from 0 to 99, the solution would myto use rand()*10 + rand(). I am sure you can extrapolate from here ;-) I don't think your second approach will work. For example, suppose your random number generator gives only ten values and produces the sequence: 1, 6, 4, 7, 9, 0, 3, 5, 2, 8, 1, 6, etc then your improved version will only ever give the numbers 16, 64, 47, 79, 90, 3, 35, 52, 28 and 81, instead of the full range from 0 to 99. Well, yes, if the generator produces a sequence of length ten, then there's not much you can do. But producing random digits from 0 to 9 doesn't mean producing a sequence of length ten. Typically a random number generator has a much longer period than that. If the period is long enough, there's no problem tiling the values, as the second approach suggests. -- Pete Roundhouse Consulting, Ltd. (www.versatilecoding.com) Author of "The Standard C++ Library Extensions: a Tutorial and Reference (www.petebecker.com/tr1book) Oct 19 '08 #7

 P: n/a On Oct 19, 11:26 pm, Pete Becker i use g++ to generater rand number, now i find that theRAND_MAX is 32367 in my computer, how can i make a biggerrand number( the number is wihin in the integer(2^32-1)) You have several options: one is to use a library such as boost which gives you several options to choose different random number generators of high quality. Another is to compose your number by calling rand several times. To illustrate: if your random number only gave values from 0 to 9, but you needed values from 0 to 99, the solution would my to use rand()*10 + rand(). I am sure you can extrapolate from here ;-) I don't think your second approach will work. For example, suppose your random number generator gives only ten values and produces the sequence: 1, 6, 4, 7, 9, 0, 3, 5, 2, 8, 1, 6, etc then your improved version will only ever give the numbers 16, 64, 47, 79, 90, 3, 35, 52, 28 and 81, instead of the full range from 0 to 99. Well, yes, if the generator produces a sequence of length ten, then there's not much you can do. But producing random digits from 0 to 9 doesn't mean producing a sequence of length ten. Typically a random number generator has a much longer period than that. If the period is long enough, there's no problem tiling the values, as the second approach suggests. Yes and no. The sequence should be significantly longer than the value one is trying to generate; in other words, the value actually returned by rand() shouldn't represent the entire internal state of the machine. For historical reasons, at in some environments, RAND_MAX is defined as 32767, even though the actual generator uses 31 or 32 bits internally (and has an actual period of around 2^31). If this is the case, then the proposed technique is fine. Similarly, rand() is required to return an int---a number of quality generators use significantly more state internally. In such cases, the technique is also valid. If the generator's period is only RAND_MAX, however, it's likely to be too pretty bad (but depending on the use, maybe "good enough" anyway). -- James Kanze (GABI Software) email:ja*********@gmail.com Conseils en informatique orientée objet/ Beratung in objektorientierter Datenverarbeitung 9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34 Oct 20 '08 #8

 P: n/a blargg wrote: In article <9L*************@read4.inet.fi>, Juha Nieminen (Curiously it's actually surprisingly difficult to find a good randomnumber generator which is portable, written in C++, very easy to use anddoesn't require you to install the entire Boost library to simply usethe RNG. For this reason I made this C++ version of the ISAAC rng: http://warp.povusers.org/IsaacRand.zip ) Agreed; that one seems to assume that unsigned int has exactly 32 bits, no more, no less. Take a look at the ind macro in IsaacRand.cc line 21. That's just one non-portability I found after a quick scan. When I say "portable" I mean things like "doesn't include " and the like. I have seen *way* too many RNGs out there which use whatever non-standard headers and non-standard code. If you are worried that in some system 'int' might not be 32-bit then put some assert() somewhere or whatever. The actual RNG code is not mine. I only wrapped it inside the class and removed everything from the global namespace (the original C code put something like 100 symbols in the global namespace). It fulfills my needs for a fast high-quality RNG in both linux and windows just fine. The same cannot be said from the majority of RNG libraries I have seen. Oct 20 '08 #9

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