[boost] Re: random.hpp - suggestion for utility function

From: Vladimir Prus (ghost_at_[hidden])
Date: 2005-05-24 01:28:37

christopher diggins wrote:

> AFAICT the shortest way to emulate the std::rand() function using
> boost/random.hpp would be as follows:
> 
>   unsigned int longhand_boost_rand() {
>     boost::mt19937 rng;
>     boost::uniform_int<> dist(0, RAND_MAX);
>     boost::variate_generator
>     <
>       boost::mt19937,
>       boost::uniform_int<>
>     > generator(rng, dist);
>     return generator();
>   }
> 
> (apologies if I made any mistakes)

You can make it a bit shorter, I believe:

   boost::mt19937 e;
   return boost::uniform_int<>(0, 100)(e);

I find 'variate_generator' to be only usefull when you want to get
functional object to pass to some other function. If you want to
directly produce random number, it's not needed.

> 
> I was wondering if there would be any reason not to provide a utility
> function which made it easier to write random number generators. For
> instance, the following:
> 
>   template<class Dist_T, class Rng_T>
>   typename boost::variate_generator<Rng_T, Dist_T>::result_type
>   generate_rand(Dist_T dist, Rng_T rng) {
>     return boost::variate_generator<Rng_T, Dist_T>(rng, dist)();
>   }
> 
> would allow us to rewrite our rand function as follows.
> 
>   unsigned int shorthand_boost_rand() {
>     return generate_rand(boost::uniform_int<>(0, RAND_MAX),
> boost::mt19937());
>   }

Did you try the above code? I have some suspicions that you might copy  the
mt19937 instance, and IIRC it's not that small.

On a related note, I've run 

   boost::mt19937 e;
   return boost::uniform_int<>(0, 100)(e);

under profiler, and the price for constructing the mt19937 object and
getting the int is roughly the same. This means that creating mt19937
object for each random number will give 2x running time increase. And as
mentioned by Michael, the numbers won't be very randon either.

- Volodya