C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  RAND GENERATOR IN C++ Goto page 1, 2  Next          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message HUNGER Guest Posted: Fri Aug 15, 2003 5:33 pm    Post subject: RAND GENERATOR IN C++ HI, I need to generate a long random number upto 4 byte width but my compiler is generating 2 byte int type as RAND_MAX i want to genetrate a rand num like 123456789 that long plz help me. thanks for helping [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Bob Bell Guest Posted: Sat Aug 16, 2003 7:56 am    Post subject: Re: RAND GENERATOR IN C++ [email]hunger_axel (AT) yahoo (DOT) com[/email] (HUNGER) wrote in message news:<f99f73d.0308150134.6df47e30 (AT) posting (DOT) google.com>... Quote: HI, I need to generate a long random number upto 4 byte width but my compiler is generating 2 byte int type as RAND_MAX i want to genetrate a rand num like 123456789 that long plz help me. thanks for helping Check out the boost random number library. http://www.boost.org Bob [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Siemel Naran Guest Posted: Sat Aug 16, 2003 8:14 am    Post subject: Re: RAND GENERATOR IN C++ "HUNGER" <hunger_axel (AT) yahoo (DOT) com> wrote in message Quote: I need to generate a long random number upto 4 byte width but my compiler is generating 2 byte int type as RAND_MAX i want to genetrate a rand num like 123456789 that long plz help me. thanks for helping On your system int is 2 bytes and long 4 bytes? In this case you can make two random numbers and put them side by side. long superrand() { int x1 = rand(); int x2 = rand(); long x = x1; x <<= CHAR_BITS * sizeof(int); x |= x2; return x; } But it's not portable. It is possible that long is also 2 bytes, or int and long are 4 bytes, or int is 4 bytes and long is 8 bytes. I think this might work, but could be wrong: template long dosuperrand() { long x = dosuperrand<N-1>(); x <<= CHAR_BITS * sizeof(int); int x2 = rand(); x |= x2; return x; } template <> long dosuperrand<1>() { int x1 = rand(); return x1; } template <> long dosuperrand<2>() { int x1 = rand(); int x2 = rand(); long x = x1; x <<= CHAR_BITS * sizeof(int); x |= x2; return x; } long superrand() { // assuming sizeof(long)%sizeof(int) is zero return dosuperrand } -- +++++++++++ Siemel Naran [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Thorsten Ottosen Guest Posted: Sat Aug 16, 2003 8:29 pm    Post subject: Re: RAND GENERATOR IN C++ "HUNGER" <hunger_axel (AT) yahoo (DOT) com> wrote Quote: HI, I need to generate a long random number upto 4 byte width but my compiler is generating 2 byte int type as RAND_MAX i want to genetrate a rand num like 123456789 that long plz help me. the normal std::rand() is rarely very random. a very good overview can be found in "Numerical Recipes in C++, Second. Edition" cheers Thorsten, AAU [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Pete Becker Guest Posted: Sun Aug 17, 2003 1:38 am    Post subject: Re: RAND GENERATOR IN C++ Thorsten Ottosen wrote: Quote: the normal std::rand() is rarely very random. That is the usual statement. Repetition is not truth, however. Quote: a very good overview can be found in "Numerical Recipes in C++, Second. Edition" It's not a good overview. It's superficial and snide. -- Pete Becker Dinkumware, Ltd. (http://www.dinkumware.com) [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Ben Liddicott Guest Posted: Sun Aug 17, 2003 1:42 am    Post subject: Re: RAND GENERATOR IN C++ Anyone considering using a numerical method to generate random numbers, is, of course, in a state of sin. (Knuth) Or you can use inline long rand_long(){return long(rand()) << 16 & rand();}; -- Cheers, Ben Liddicott "HUNGER" Quote: I need to generate a long random number upto 4 byte width but my compiler is generating 2 byte int type as RAND_MAX i want to genetrate a rand num like 123456789 that long [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Thorsten Ottosen Guest Posted: Sun Aug 17, 2003 11:45 am    Post subject: Re: RAND GENERATOR IN C++ "Pete Becker" <petebecker (AT) acm (DOT) org> wrote Quote: Thorsten Ottosen wrote: the normal std::rand() is rarely very random. That is the usual statement. Repetition is not truth, however. I agree about "repetition is not truth". On which compilers would you consider std.:rand() good?. Quote: a very good overview can be found in "Numerical Recipes in C++, Second. Edition" It's not a good overview. It's superficial and snide. I'm glad to hear that...then I can start unlearning what I've learnt. Could you explain why it's snide? Thanks Thorsten [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Pete Becker Guest Posted: Sun Aug 17, 2003 4:35 pm    Post subject: Re: RAND GENERATOR IN C++ Thorsten Ottosen wrote: Quote: "Pete Becker" <petebecker (AT) acm (DOT) org> wrote in message news:3F3E9C10.A576D02F (AT) acm (DOT) org... Thorsten Ottosen wrote: the normal std::rand() is rarely very random. That is the usual statement. Repetition is not truth, however. I agree about "repetition is not truth". On which compilers would you consider std.:rand() good?. At least every one that uses our library. But that's not the issue. You said they're bad. If you have data to back that up, say so. -- Pete Becker Dinkumware, Ltd. (http://www.dinkumware.com) [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Pete Becker Guest Posted: Sun Aug 17, 2003 4:36 pm    Post subject: Re: RAND GENERATOR IN C++ John Potter wrote: Quote: On 16 Aug 2003 21:42:34 -0400, "Ben Liddicott" [email]ben.liddicott (AT) bencat (DOT) demon.co.uk[/email]> wrote: Or you can use inline long rand_long(){return long(rand()) << 16 & rand();}; You, of course, meant | not &. This produces half as many not as random numbers since it uses two calls to rand. Well, not really. Every generator provides some number of bits, n, between repetitions. If you use those bits 16 at a time you get n/16 values before you hit repetitions from the generator. If you use those same bits 32 at a time instead you get n/32 values, i.e. half as many, before you hit repetitions from the generator. It doesn't matter whether the generator returns 16-bit values or 32-bit values. What matters is the period of the generator. For example: static unsigned next_value; bool valid = false; unsigned long rand32(); // returns 32-bit "random" values unsigned rand16() { if (!valid) { unsigned long val = rand32(); next_value = val >> 16; valid = true; return val & 0xffff; } else { valid = false; return next_value; } } Now, if you generate 32-bit values by calling rand16 twice you're just reassembling the values that rand16 splits apart from calls to rand32. So you get exactly the same 32-bit values either way. Part of assessing the quality of a random number generator is looking at the size of its internal state. The number of bits in the value that it returns is irrelevant. -- Pete Becker Dinkumware, Ltd. (http://www.dinkumware.com) [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Francis Glassborow Guest Posted: Sun Aug 17, 2003 4:38 pm    Post subject: Re: RAND GENERATOR IN C++ In article <3f3f28e7$0$14561$afc38c87 (AT) news (DOT) optusnet.com.au>, Thorsten Ottosen <nesotto (AT) cs (DOT) auc.dk> writes Quote: It's not a good overview. It's superficial and snide. I'm glad to hear that...then I can start unlearning what I've learnt. Could you explain why it's snide? I do not know about snide but I do know that the intellectual property right claims made by the authors of this book make it a very dangerous book for professional programers to be on record as having read http://www.numerical-recipes.com/com/screen-license.html This is without doubt the most draconian license that I have ever read. And the single screen license actually is as expensive as the book (actually much more so if you use Linux) This is one of two books that I recommend that professional programmers should NOT buy because the license agreement makes it prejudicial to your continued employment as a software developer. -- Francis Glassborow ACCU 64 Southfield Rd Oxford OX4 1PA +44(0)1865 246490 All opinions are mine and do not represent those of any organisation [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Siemel Naran Guest Posted: Mon Aug 18, 2003 10:35 am    Post subject: Re: RAND GENERATOR IN C++ "Siemel Naran" <SiemelNaran (AT) REMOVE (DOT) att.net> wrote Quote: But it's not portable. It is possible that long is also 2 bytes, or int and long are 4 bytes, or int is 4 bytes and long is 8 bytes. I think this might work, but could be wrong: long superrand() { // assuming sizeof(long)%sizeof(int) is zero return dosuperrand<sizeof(long)/sizeof(int)>(); } Oops. In the issue of portability I've made a big omission. Sorry! Namely you want a 32 bit random number, and we don't know which type is 32 bit. Sure it's long for you, but it's int for me. So you want something like this #ifdef 16bitinteger /* use proper macro */ typedef long int32; #elif 32bitinteger /* as above */ typedef int int32; #endif The change the signature of function superrand() to int32 rand32(); // assuming sizeof(int32)%sizeof(int) is zero return dosuperrand<sizeof(int32)/sizeof(int)>(); // oops: need to handle special case when sizeof(int32)/sizeof(int) is zero } To get the int32 you can use the following template trick. But it works only if int32 is actually one of the basic types. If not, you have to use std::bitset template <bool cond, class A, class B> struct template_if { }; template <class A, class B> struct template_if<true,A,B> { typedef A result; }; template <class A, class B> struct template_if<false,A,B> { typedef B result; }; //////////////////////////////////////////////////////////////////////////// //// template <class T, class U=void> struct typelist { typedef T car; typedef U cdr; }; template <template struct typelist_find { typedef typename list::car car; typedef typename list::cdr cdr; typedef typename template_if< Predicate car, typename find_typelist<Predicate,cdr>::result Quote: :: result result; }; template <template struct typelist_find<Predicate,void> { typedef void result; }; const int CHAR_BITS=8; template <class T> struct Eq32 { static const bool result=((sizeof(T)*CHAR_BITS)==32); }; typedef typelist<long, typelist > > BasicTypes; typedef find_typelist<Eq32, BasicTypes>::result int32; -- +++++++++++ Siemel Naran [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Siemel Naran Guest Posted: Mon Aug 18, 2003 10:36 am    Post subject: Re: RAND GENERATOR IN C++ "Ben Liddicott" <ben.liddicott (AT) bencat (DOT) demon.co.uk> wrote in message Quote: Or you can use inline long rand_long(){return long(rand()) << 16 & rand();}; Should that be & or | In addition, people may be confused how operators group in some cases, and I'm one of them here . So I suggest using parenthesis inline long rand_long(){return (long(rand()) << 16) & rand();}; -- +++++++++++ Siemel Naran [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Thorsten Ottosen Guest Posted: Mon Aug 18, 2003 11:56 pm    Post subject: Re: RAND GENERATOR IN C++ "Francis Glassborow" <francis (AT) robinton (DOT) demon.co.uk> wrote Quote: In article <3f3f28e7$0$14561$afc38c87 (AT) news (DOT) optusnet.com.au>, Thorsten Ottosen <nesotto (AT) cs (DOT) auc.dk> writes It's not a good overview. It's superficial and snide. I'm glad to hear that...then I can start unlearning what I've learnt. Could you explain why it's snide? I do not know about snide but I do know that the intellectual property right claims made by the authors of this book make it a very dangerous book for professional programers to be on record as having read http://www.numerical-recipes.com/com/screen-license.html this is very enjoyable reading. I have never reda it before, and I most say its amazing, thay can wrute that. Quote: This is without doubt the most draconian license that I have ever read. And the single screen license actually is as expensive as the book (actually much more so if you use Linux) This is one of two books that I recommend that professional programmers should NOT buy because the license agreement makes it prejudicial to your continued employment as a software developer. I woldn't worry that much. I still think the book is good regarding the math topics and that was why I recommended it. On the other hand, even without reading the license, I would never dream about using the source code. It's very old-fashioned and full of low level code...not mmuch C++ in that. Numbers are hardcode in great numbers too, no code is resued in functions, but put inline again etc. best regards Thorsten [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top AngleWyrm Guest Posted: Tue Aug 19, 2003 12:31 am    Post subject: Re: RAND GENERATOR IN C++ "Thorsten Ottosen" <nesotto (AT) cs (DOT) auc.dk> wrote Quote: "Pete Becker" <petebecker (AT) acm (DOT) org> wrote in message news:3F3E9C10.A576D02F (AT) acm (DOT) org... Thorsten Ottosen wrote: the normal std::rand() is rarely very random. That is the usual statement. Repetition is not truth, however. I agree about "repetition is not truth". On which compilers would you consider std.:rand() good?. To clear things up a bit: std::rand() is a Linear Congruential pseudo-random number generator; it uses a formula of the form: output = (lastOutput+constant) modulo anotherConstant. The caution expressed has to do with a couple generally undesirable traits of LC generators. 1). Don't use them in large groups to generate vectors (n-tuples) because they are not sequentially random--the points they generate fall into planes. That is to say, if a vector of numDimensions is created and filled with numDimension random numbers, all points will lie in a numDimension-1 plane. 2). The low-order bits are not random, and this can become a problem during use. Some care in converting a rand() to a range is required. Here is an example of a reasonable method of doing so: // Generate a random integer, given the number of possibilities static bool srandCalled=false; int RandomInt(const int numChoices) // for numChoices<=RAND_MAX; else returns 0 { if( !srandCalled ){ // make sure that srand has been called srand( time(NULL) ); rand(); // burn one for good measure srandCalled = true; } // Divide RAND_MAX into numChoices buckets const int bucketSize = RAND_MAX / numChoices; static int randomInteger; do{ randomInteger = rand() / bucketSize; // Get a random bucket } while ( randomInteger >= numChoices ); // re-roll the remainder bucket return randomInteger; }; [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Pete Becker Guest Posted: Tue Aug 19, 2003 10:26 am    Post subject: Re: RAND GENERATOR IN C++ AngleWyrm wrote: Quote: To clear things up a bit: std::rand() is a Linear Congruential pseudo-random number generator; There is no such requirement. -- Pete Becker Dinkumware, Ltd. (http://www.dinkumware.com) [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. 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