Snorre Jakobsen wrote:

Hello

I'm sorry if this is answered in the documentation, but I didn't find it. I want a function which returns a normally distributed number.

#include <boost/random.hpp> #include <time.h>

using namespace boost;

double SampleNormal (double mean, double sigma) { mt19937 rng; // select random number generator rng.seed((unsigned int) time(0)); // seconds since 1970 // select desired probability distribution normal_distribution<double> norm_dist(mean, sigma); // bind random number generator to distribution, forming a function variate_generator<mt19937&, normal_distribution<double> > normal_sampler(rng, norm_dist); // sample from the distribution return normal_sampler(); }

This (from http://surl.se/tvk) works very well, except if it's invoked (with same parameters) more often than once a second. If so, the generator gets equal seeds and returns equal values. How can I construct my program so the seeding happens only once?

Hi, Snorre. Funny! I wrote the very piece of code below this evening! Make your function a functor, so you initialise the random number generator only once, in the class' construnctor, but can then use it mulitple times. Regards, Angus #include <boost/random.hpp> #include <time.h> class SampleNormal { typedef boost::mt19937 rng_t; typedef boost::normal_distribution<double> dist_t; typedef boost::variate_generator<rng_t&, dist_t> generator_t; rng_t rng_; dist_t dist_; generator_t dice_; public: SampleNormal(double mean, double sigma) : rng_(time(0)) , dist_(mean, sigma) , dice_(rng_, dist_) {} double operator()() { return dice_(); } }; #include <iostream> int main() { SampleNormal normal_sampler(3, 0.5); for (int i = 0; i != 10; ++i) std::cout << normal_sampler() << '\n'; std::flush(std::cout); return 0; }