Re: [boost] uniform_real rather alarmingly non-random?

From: Manfred Doudar (manfred.doudar_at_[hidden])
Date: 2006-03-18 02:28:30

Hi John,

John Maddock wrote:
> I seriously hope I'm doing something wrong here, but the last 20 bits in a 
> double returned from uniform_real appear to always be zero.  Is this by 
> design?  I really hope not!
> 
> Here's the test code:
> 
> template <class T>
> void test_random(T)
> {
>   boost::mt19937 rnd;
>   boost::uniform_real<T> ur_a(0.5, 1);
>   boost::variate_generator<std::tr1::mt19937, std::tr1::uniform_real<T> > 
> gen(rnd, ur_a);
>   for(int j = 0; j < INT_MAX-2; ++j)
>   {
>     T v = gen();
>     // get the digits of T:
>     unsigned long long i = (unsigned long long)std::floor(std::ldexp(v, 
> std::numeric_limits<T>::digits));
>     i &= 0xfffff;  // last 20 bits
>     std::cout << std::hex << i << std::endl;
>   }
> }
> 
> Try calling with a double as an argument and it'll output nothing but 
> zero's.
> 

If memory serves, I've hit on this before - but I don't think you'll 
like answer - you're going to have to push the variate_generator gen (, 
and its dependants rnd, and ur_a) into the global namespace.

Cheers,
-- 
18.03.2006
Manfred Doudar - Research Engineer
National ICT Australia (NICTA)
Research School of Information Sciences and Engineering (RSISE)
The Australian National University - Canberra, ACT 0200 AUSTRALIA