C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  random_shuffle seed Goto page 1, 2  Next          C++Talk.NET Forum Index -> C++ language (comp.lang.c++) View previous topic :: View next topic   Author Message Peter Ammon Guest Posted: Wed Oct 29, 2003 10:40 pm    Post subject: random_shuffle seed I'm calling random_shuffle without passing in a RandomNumberGenerator and getting the same shuffle every time I restart my program. Apparently I need to seed the internal random number generator. But how? I can't find any way to do that, and if there is none, then how is the internal random number generator useful? Thanks, -peter Back to top Victor Bazarov Guest Posted: Thu Oct 30, 2003 3:08 am    Post subject: Re: random_shuffle seed "Peter Ammon" <peter_ammon (AT) rocketmail (DOT) com> wrote... Quote: I'm calling random_shuffle without passing in a RandomNumberGenerator and getting the same shuffle every time I restart my program. Apparently I need to seed the internal random number generator. But how? I can't find any way to do that, and if there is none, then how is the internal random number generator useful? Read about 'srand' function. V Back to top Chris Theis Guest Posted: Thu Oct 30, 2003 8:00 am    Post subject: Re: random_shuffle seed "Peter Ammon" <peter_ammon (AT) rocketmail (DOT) com> wrote Quote: I'm calling random_shuffle without passing in a RandomNumberGenerator and getting the same shuffle every time I restart my program. Apparently I need to seed the internal random number generator. But how? I can't find any way to do that, and if there is none, then how is the internal random number generator useful? Thanks, -peter Look up the srand() function. Although it seems stupid at first glance, a generator producing "pseudo-random" numbers using always the same seed at startup (e.g. without specifying a different seed every time you run the program), will give you a reproducable sequence of "random" numbers. This is very helpful in case you want to debug the behavior of a simulation for example. BTW you should use srand only once in your program!! HTH Chris Back to top Peter Ammon Guest Posted: Fri Oct 31, 2003 1:05 am    Post subject: Re: random_shuffle seed Victor Bazarov wrote: Quote: "Peter Ammon" <peter_ammon (AT) rocketmail (DOT) com> wrote... I'm calling random_shuffle without passing in a RandomNumberGenerator and getting the same shuffle every time I restart my program. Apparently I need to seed the internal random number generator. But how? I can't find any way to do that, and if there is none, then how is the internal random number generator useful? Read about 'srand' function. V Tried that and no dice. Looking in my headers, I see that random_shuffle uses a template __random_number, which is defined as so: template<typename _Distance> inline _Distance __random_number(_Distance __n) { #ifdef _GLIBCPP_HAVE_DRAND48 return lrand48() % __n; #else return rand() % __n; #endif } and _GLIBCPP_HAVE_DRAND48 is unconditionally defined elsewhere, even when I compile with -ansi -pedantic. I can seed it and get more random results with seed48, but of course this is not portable. So am I to conclude that there is no guarantee that random_shuffle's internal PRNG is rand(), and that there need not be any user accessible way to seed it? This would seriously call into question the utility of random_shuffle without a user supplied RandomNumberGenerator in any portable program Ordinarily I'd call this a QoI issue, but I'm using g++ which is a pretty high Q I. -Peter Back to top Peter Ammon Guest Posted: Fri Oct 31, 2003 1:06 am    Post subject: Re: random_shuffle seed Chris Theis wrote: Quote: "Peter Ammon" <peter_ammon (AT) rocketmail (DOT) com> wrote in message news:bnpfl8$mqb$1 (AT) news (DOT) apple.com... I'm calling random_shuffle without passing in a RandomNumberGenerator and getting the same shuffle every time I restart my program. Apparently I need to seed the internal random number generator. But how? I can't find any way to do that, and if there is none, then how is the internal random number generator useful? Thanks, -peter Look up the srand() function. Although it seems stupid at first glance, a generator producing "pseudo-random" numbers using always the same seed at startup (e.g. without specifying a different seed every time you run the program), will give you a reproducable sequence of "random" numbers. This is very helpful in case you want to debug the behavior of a simulation for example. BTW you should use srand only once in your program!! HTH Chris random_shuffle evidently doesn't have to use rand() as its internal PRNG; see my response to Mr. Bazarov. -Peter Back to top Chris Theis Guest Posted: Fri Oct 31, 2003 8:21 am    Post subject: Re: random_shuffle seed "Peter Ammon" <peter_ammon (AT) rocketmail (DOT) com> wrote Quote: Victor Bazarov wrote: [SNIP] template inline _Distance __random_number(_Distance __n) { #ifdef _GLIBCPP_HAVE_DRAND48 return lrand48() % __n; #else return rand() % __n; #endif } and _GLIBCPP_HAVE_DRAND48 is unconditionally defined elsewhere, even when I compile with -ansi -pedantic. I can seed it and get more random results with seed48, but of course this is not portable. So am I to conclude that there is no guarantee that random_shuffle's internal PRNG is rand(), and that there need not be any user accessible way to seed it? This would seriously call into question the utility of random_shuffle without a user supplied RandomNumberGenerator in any portable program It is enforced by the standard that random shuffle uses a uniform distribution random number generator. It is not guaranteed that this is rand(). Due to the fact rand() is a standard function available on every system it is the practical choice, which is also portable. If your compiler implements the use of a non-standard function like lrand48 it's good for you getting better random numbers, although you'll have to seed it accordingly. IMHO such approaches are arguable because the user should at least be notified that a non portable function is used for a standard algorithm. Anyway I'm not sure whether _GLIBCPP_HAVE_DRAND48 is defined by default even on systems supplying lrand48. This is actually the first case that I've heard that this was troublesome. Generally one should supply an implementation of a reasonable good RNG if good random numbers are required. Regards Chris Back to top DrPizza Guest Posted: Sun Nov 02, 2003 3:45 am    Post subject: Re: random_shuffle seed "Peter Ammon" <peter_ammon (AT) rocketmail (DOT) com> wrote Quote: I'm calling random_shuffle without passing in a RandomNumberGenerator and getting the same shuffle every time I restart my program. Apparently I need to seed the internal random number generator. But how? I can't find any way to do that, and if there is none, then how is the internal random number generator useful? With difficulty. The Standard provides no function to do this, and random_shuffle is forbidden from using std::rand(). C++ "inherits" rand() from C (see [lib.c.math] -- rand isn't defined in the C++ Standard at all, instead a reference is given to the C Standard), and C states that "The implementation shall behave as if no library function calls the rand function". This constraint thus applies to C++, which thus prevents random_shuffle from using rand(). A resolution is proposed. http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/lwg-active.html#395 -- Now Playing: Marc Aurel - Running (dumonde remix) (D I G I T A L L Y - I M P O R T E D - European Trance, Techno, Hi-NRG... we can't define it!) char a[99999],*p=a;main(c,V)char**V;{char*v=c>0?1[V]:V;if(c)for(;(c=*v)&&93^ c;p+=!(62^c)-!(60^c),*p+=!(43^c)-!(45^c),44^c||read(0,p,1),46^c||putchar(*p) ,91^c||(v=*p?main(-1,v+1),v-1:main(0,v)),++v);else for(;c+=!(91^*v)-!(93^*v) ;++v);return v;} /* [email]drpizza (AT) battleaxe (DOT) net[/email] brainf*** program as argv[1] */ Back to top Howard Hinnant Guest Posted: Mon Nov 03, 2003 8:54 pm    Post subject: Re: random_shuffle seed In article <TT_ob.101487$KB.71834 (AT) fe04 (DOT) atl2.webusenet.com>, "DrPizza" <drpizza (AT) battleaxe (DOT) net> wrote: Quote: "Peter Ammon" <peter_ammon (AT) rocketmail (DOT) com> wrote in message news:bnpfl8$mqb$1 (AT) news (DOT) apple.com... I'm calling random_shuffle without passing in a RandomNumberGenerator and getting the same shuffle every time I restart my program. Apparently I need to seed the internal random number generator. But how? I can't find any way to do that, and if there is none, then how is the internal random number generator useful? With difficulty. The Standard provides no function to do this, and random_shuffle is forbidden from using std::rand(). C++ "inherits" rand() from C (see [lib.c.math] -- rand isn't defined in the C++ Standard at all, instead a reference is given to the C Standard), and C states that "The implementation shall behave as if no library function calls the rand function". This constraint thus applies to C++, which thus prevents random_shuffle from using rand(). A resolution is proposed. http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/lwg-active.html#395 And just fyi, the resolution was looked upon favoribly at last week's standards meeting. If I remember correctly, its status was moved to "ready". But don't take that to the bank until the next revision of the issues list comes out. -Howard Back to top Pete Becker Guest Posted: Thu Nov 06, 2003 1:38 am    Post subject: Re: random_shuffle seed DrPizza wrote: Quote: "Peter Ammon" <peter_ammon (AT) rocketmail (DOT) com> wrote in message news:bnpfl8$mqb$1 (AT) news (DOT) apple.com... I'm calling random_shuffle without passing in a RandomNumberGenerator and getting the same shuffle every time I restart my program. Apparently I need to seed the internal random number generator. But how? I can't find any way to do that, and if there is none, then how is the internal random number generator useful? With difficulty. The Standard provides no function to do this, and random_shuffle is forbidden from using std::rand(). C++ "inherits" rand() from C (see [lib.c.math] -- rand isn't defined in the C++ Standard at all, instead a reference is given to the C Standard), and C states that "The implementation shall behave as if no library function calls the rand function". This constraint thus applies to C++, which thus prevents random_shuffle from using rand(). A resolution is proposed. http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/lwg-active.html#395 Yup, we added redundant language to make it clear that statements in the C standard about the C library are not constraints on the C++ library. Shouldn't need to be said... -- Pete Becker Dinkumware, Ltd. (http://www.dinkumware.com) Back to top DrPizza Guest Posted: Thu Nov 06, 2003 2:07 am    Post subject: Re: random_shuffle seed "Pete Becker" <petebecker (AT) acm (DOT) org> wrote Quote: DrPizza wrote: "Peter Ammon" <peter_ammon (AT) rocketmail (DOT) com> wrote in message news:bnpfl8$mqb$1 (AT) news (DOT) apple.com... I'm calling random_shuffle without passing in a RandomNumberGenerator and getting the same shuffle every time I restart my program. Apparently I need to seed the internal random number generator. But how? I can't find any way to do that, and if there is none, then how is the internal random number generator useful? With difficulty. The Standard provides no function to do this, and random_shuffle is forbidden from using std::rand(). C++ "inherits" rand() from C (see [lib.c.math] -- rand isn't defined in the C++ Standard at all, instead a reference is given to the C Standard), and C states that "The implementation shall behave as if no library function calls the rand function". This constraint thus applies to C++, which thus prevents random_shuffle from using rand(). A resolution is proposed. http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/lwg-active.html#395 Yup, we added redundant language to make it clear that statements in the C standard about the C library are not constraints on the C++ library. Shouldn't need to be said... Why not? You wouldn't want to permit your C++ library to call strtok() internally, would you? In general it seems quite desirable to carry that constraint over from C; rand() is probably the only exception. -- Now Playing: Accuface - Theme From Accuface char a[99999],*p=a;main(c,V)char**V;{char*v=c>0?1[V]:V;if(c)for(;(c=*v)&&93^ c;p+=!(62^c)-!(60^c),*p+=!(43^c)-!(45^c),44^c||read(0,p,1),46^c||putchar(*p) ,91^c||(v=*p?main(-1,v+1),v-1:main(0,v)),++v);else for(;c+=!(91^*v)-!(93^*v) ;++v);return v;} /* [email]drpizza (AT) battleaxe (DOT) net[/email] brainf*** program as argv[1] */ Back to top Pete Becker Guest Posted: Thu Nov 06, 2003 2:23 am    Post subject: Re: random_shuffle seed DrPizza wrote: Quote: You wouldn't want to permit your C++ library to call strtok() internally, would you? Sure. Why not? Quote: In general it seems quite desirable to carry that constraint over from C; rand() is probably the only exception. "Seems quite desirable" is not the same as "required by the standard." -- Pete Becker Dinkumware, Ltd. (http://www.dinkumware.com) Back to top DrPizza Guest Posted: Thu Nov 06, 2003 1:19 pm    Post subject: Re: random_shuffle seed "Pete Becker" <petebecker (AT) acm (DOT) org> wrote Quote: DrPizza wrote: You wouldn't want to permit your C++ library to call strtok() internally, would you? Sure. Why not? Because it means you can't use strtok() yourself? Quote: In general it seems quite desirable to carry that constraint over from C; rand() is probably the only exception. "Seems quite desirable" is not the same as "required by the standard." But it (surely) is required by the standard because the standard just defers to the C standard for the description of these functions, and one aspect of the C standard pertinent to these functions is that the implementation shall not call them. -- char a[99999],*p=a;main(c,V)char**V;{char*v=c>0?1[V]:V;if(c)for(;(c=*v)&&93^ c;p+=!(62^c)-!(60^c),*p+=!(43^c)-!(45^c),44^c||read(0,p,1),46^c||putchar(*p) ,91^c||(v=*p?main(-1,v+1),v-1:main(0,v)),++v);else for(;c+=!(91^*v)-!(93^*v) ;++v);return v;} /* [email]drpizza (AT) battleaxe (DOT) net[/email] brainf*** program as argv[1] */ Back to top tom_usenet Guest Posted: Thu Nov 06, 2003 1:52 pm    Post subject: Re: random_shuffle seed On Thu, 6 Nov 2003 13:19:32 -0000, "DrPizza" <drpizza (AT) anti-flash (DOT) co.uk> wrote: Quote: In general it seems quite desirable to carry that constraint over from C; rand() is probably the only exception. "Seems quite desirable" is not the same as "required by the standard." But it (surely) is required by the standard because the standard just defers to the C standard for the description of these functions, and one aspect of the C standard pertinent to these functions is that the implementation shall not call them. The C standard only says that the implementation of the C library won't call them. Taking the original quote: "The implementation shall behave as if no library function calls the rand function" The "library" referred to is the C library, not the C++ one. Just because C++ includes the (slightly modified) C library doesn't mean that it *is* the C library. Tom Back to top Pete Becker Guest Posted: Thu Nov 06, 2003 2:50 pm    Post subject: Re: random_shuffle seed DrPizza wrote: Quote: "Pete Becker" <petebecker (AT) acm (DOT) org> wrote in message news:3FA9B097.746AD23B (AT) acm (DOT) org... DrPizza wrote: You wouldn't want to permit your C++ library to call strtok() internally, would you? Sure. Why not? Because it means you can't use strtok() yourself? Non sequitur. Quote: In general it seems quite desirable to carry that constraint over from C; rand() is probably the only exception. "Seems quite desirable" is not the same as "required by the standard." But it (surely) is required by the standard because the standard just defers to the C standard for the description of these functions, and one aspect of the C standard pertinent to these functions is that the implementation shall not call them. The C++ standard does not change that constraint. No C function calls rand. -- Pete Becker Dinkumware, Ltd. (http://www.dinkumware.com) Back to top DrPizza Guest Posted: Fri Nov 07, 2003 2:24 am    Post subject: Re: random_shuffle seed "tom_usenet" <tom_usenet (AT) hotmail (DOT) com> wrote Quote: The C standard only says that the implementation of the C library won't call them. Well, no, it doesn't, it simply says that the implementation shall behave as if no library function calls rand. It doesn't specify which library. Quote: Taking the original quote: "The implementation shall behave as if no library function calls the rand function" The "library" referred to is the C library, not the C++ one. Just because C++ includes the (slightly modified) C library doesn't mean that it *is* the C library. Upon incorporation into C++, "the library" refers to the C++ library. The C++ library is the only library there is, so it's the only one it could refer to. -- Now Playing: Daft Punk - Around The World char a[99999],*p=a;main(c,V)char**V;{char*v=c>0?1[V]:V;if(c)for(;(c=*v)&&93^ c;p+=!(62^c)-!(60^c),*p+=!(43^c)-!(45^c),44^c||read(0,p,1),46^c||putchar(*p) ,91^c||(v=*p?main(-1,v+1),v-1:main(0,v)),++v);else for(;c+=!(91^*v)-!(93^*v) ;++v);return v;} /* [email]drpizza (AT) battleaxe (DOT) net[/email] brainf*** program as argv[1] */ Back to top Display posts from previous: All Posts1 Day7 Days2 Weeks1 Month3 Months6 Months1 Year Oldest FirstNewest First         C++Talk.NET Forum Index -> C++ language (comp.lang.c++) All times are GMT Goto page 1, 2  Next Page 1 of 2   Jump to: Select a forum C/C++----------------C LanguageC Language (Moderated)C++ Language (Moderated)C++ language (comp.lang.c++)C++ language, library and standards Non-English C/C++ Newsgroups----------------C++ (German)C++ (French)  You cannot post new topics in this forum You cannot reply to topics in this forum You cannot edit your posts in this forum You cannot delete your posts in this forum You cannot vote in polls in this forum Powered by phpBB © 2001, 2006 phpBB Group SEO toolkit © 2004-2006 webmedic.