Re: [Boost-users] Newbie question: Random numbers.

I was just checking over my suggestion and realized that clock() is not a good choice for seeding the generator, especially with my optimization. It returns the amount of time since the process started, which in a static initializer for a global variable, will provide very low entropy. In a few test runs, it even returned 0, which caused an assertion failure in the random number library. An ideal solution would be to use random_device, which is defined in boost/nondet_random.hpp. You could then replace the definition of generator with something like this: //Declare the generator as a global variable for easy reuse random_device nondet_generator; base_generator_type generator(nondet_generator()); If random_device isn't implemented on your platform, then you can look into other ways to seed the generator. On my system (Windows XP running VC8) I used GetTickCount(), which returns the number of milliseconds since Windows started. As a last resort, you could call clock() after the user has entered something, either the normal data entry for your program (preferred), or a special request to hit enter. Does anybody else know of other good sources of entropy to seed the generator? I also found that the loop to seed the generator (initialize() in my version) doesn't really help, since the seed function completely replaces the current value of the generator; it doesn't combine with the previous value.

-----Original Message----- From: Andrew Holden Sent: Friday, May 19, 2006 9:18 AM To: boost-users@lists.boost.org Subject: Re: [Boost-users] Newbie question: Random numbers.

My first thought is that you probably shouldn't reinitialize the generator for every random number you need. I would recommend that you create and seed "generator" once at the start of your program, then reuse it whenever you need a random number.

Perhaps something like the following:

namespace Random { typedef boost::minstd_rand base_generator_type; typedef boost::uniform_int<> distribution_type; typedef boost::variate_generator gen_type;

//Declare the generator as a global variable for easy reuse base_generator_type generator(static_cast<unsigned long>(clock()));

/*************************************************** * initializes the random number generator * **************************************************/ void initialize() { for (int i = 0; i < 30000; ++i) generator.seed(static_cast<unsigned long>(clock())); }

/*************************************************** * generates a random number between a and b * * @param a the starting range * * @param b the integer ending range * * @return N such that a <= N <= b * **************************************************/ template <class T> T rand_int (const T& a, const T& b) { //Check the parameters BOOST_STATIC_ASSERT(boost::is_integral<T>::value); assert (b >= a);

//Set up the desired distribution gen_type ran_gen(generator, distribution_type(a, b));

//Get a random number return ran_gen(); } }

using Random::rand_int;

int main(int argc, char* argv[]) { Random::initialize();

std::cout << rand_int (1,1000) << std::endl << rand_int (1,1000) << std::endl << rand_int (1,1000) << std::endl << rand_int (1,1000) << std::endl;

return 0; } ________________________________________ From: chun ping wang [mailto:cablepuff@gmail.com] Sent: Friday, May 19, 2006 3:55 AM To: boost-users@lists.boost.org Subject: [Boost-users] Newbie question: Random numbers.

I have a simple question, is there a very fast and effecient way to generate random numbers that are fast on the fly. The numbers are really randomize and distorted...

/***************************************************  * generates a random number between a and b       *  * @param a the starting range                     *  * @param b the integer ending range               *  * @return N such that a <= N <= b                 *  **************************************************/ template <class T> T randint(const T& a, const T& b) {   BOOST_STATIC_ASSERT(boost::is_integral<T>::value);   assert (b >= a);   typedef boost::minstd_rand base_generator_type;   typedef boost::uniform_int<> distribution_type;   typedef boost::variate_generator gen_type;   base_generator_type generator(boost::numeric_cast<unsigned long>(clock()));   gen_type ran_gen(generator, distribution_type(a, b));   for (int i = 0; i < 30000; ++i)       generator.seed(boost::numeric_cast<unsigned long>(clock()));   return ran_gen(); }

This works but is super slow... are there effective ways to do it...

Thanks.

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