# Boost :

From: Cromwell Enage (sponage_at_[hidden])
Date: 2004-06-22 17:50:06

--- Oliver Kullmann <O.Kullmann_at_[hidden]> wrote:
> Hi,

Hello!

> one of my students tried some time in vain
> to use the random number generator from
> the boost library, but he couldn't understand
> the documentation. I must say, the documentation
> is not exactly "user-friendly".

Did he try playing around with the "very quick start"
example? It should at least have gotten him started.
See below for a more thorough explanation.

> Here's the problem:
>
> Given integers min and max, we want to have a
> pseudo-random-number generator for random
> numbers from min, min + 1, ..., max (uniform
> distribution). The generator should take a seed,
> and then producing a sequence of pseudo-random
> numbers (and we should be able to find out about
> the period of the generator).
>
> I couldn't think of anything more basic for a
> library on random numbers, and so I would expect
> an example of how to get such a thing in the
> documentation. (I mean, a real example, complete
> with code, explanations, and how to use it.)
> But there's no such a thing, which I find rather
> strange.

Assuming min and max are plain-vanilla integers, the
following instructions:

typedef boost::uniform_int<> UniformDist;
UniformDist dist(min,max);

produce an object named dist that holds your uniform
distribution. The following instructions:

typedef boost::mt19937 BaseRNG;
BaseRNG rng;

produce a "base pseudo-random number generator", which
produces numbers along the *entire* range of integers
(-2^31 to +2^31 or something like that).
boost::mt19937 is one of many such generators provided
by the library.

The following instruction puts it all together and
gives you the random integer generator you want:

boost::variate_generator<BaseRNG&,UniformDist>
your_generator(rng, dist);

Note the ampersand; your program will fail silently if
you omit it.

The base random number generator (rng) holds the seed
function; simply call

rng.seed(42L);

or whatever seed you want to provide.

Finally, assuming you want to store your sequence in a
vector<int>:

vector<int> sample;
for (int c = 0; c < desired_size; c++)
{
sample.push_back(your_generator());
}

> We have the concept of a "Uniform Random Number
> Generator",
> which looks exactly what we want, but, alas, it
> seems to
> be more of a fictitious concept, since no models
> seem to
> be provided?!?!

Actually, they do exist. In the main page is a
section called "Library Organization"; though the
concepts don't exactly map to the subsections, you
should be able to find what you're looking for now
that I've (hopefully) made things clearer.

[snip other complaints]

> My student didn't get to this point, although he is
> rather good. But there are too many unknowns in
> the "equation" to solve (starting with the concept
> of a concept, ...), so the job becomes overwhelming.
> Now it doesn't look too complicated for me to write
> a nice explanation for the above problem in a
> tutorial style, explaining on the way all the
> concepts and concepts of concepts in the library,
> and that's what I wanted to kindly request with my
> e-mail. (And also the rest of the documentation
> could need a sentence here and there, as mentioned
> in the above case study.)

We'll see what Jens Maurer has to say on that. In the
meantime, tell your student not to give up!

Cromwell Enage

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