Re: [Boost-users] Random permutations using boost::random_number_generator and std::random_shuffle.

Subject: Re: [Boost-users] Random permutations using boost::random_number_generator and std::random_shuffle.
From: Steven Watanabe (watanabesj_at_[hidden])
Date: 2011-01-21 17:16:10

AMDG

On 1/21/2011 1:57 PM, Matthew Gwynne wrote:
> The problem we now have is that we don't know how the result of
> mt19937 is being cast down into the number range we are asking for. As
> far as I am aware, MT19937 only defines how to get a 32 bit integer,
> not how to then scale that to say the range 1..10. Is it specified
> clearly how boost::random_number_generator or
> boost::uniform_distribution does this?
>

boost::random_number_generator is implemented using
boost::uniform_int and no the algorithm isn't specified.
It's considered an implementation detail.  If you
really want to know the details, the source is in
boost/random/uniform_int.hpp.  I don't expect the
implementation to change again, but no guarantees.

>>
>>> However, we have two questions (open problems):
>>>
>>> 1) How does boost::variate_generator interact with mt19937, and the
>>> distribution? Is this documented somewhere?
>>>
>>> In the attached code, it seems we can construct
>>> boost::uniform_distribution with any maximum value, and still the
>>> number generator returns values outside of that range? Is this really
>>> right?
>>>
>>
>> Yes.  You don't need to use uniform_int with
>> random_number_generator.  You can plug mt19937
>> directly into random_number_generator.
>
> Thanks! Is the behaviour exactly the same?

It isn't going to be exactly the same. If you use
mt19937 directly, then random_number_generator
will transform the output of the generator to the
desired range.  If you use uniform_int, the
generator's output will first be transformed
to the range that you specified for uniform_int,
then again by random_number_generator.  The
result will still have the correct distribution,
but the exact values may be different.

> It seems so, but the
> documentation seems to suggest using boost::uniform_distribution.
>

Where in the documentation?

In Christ,
Steven Watanabe