Re: tlug: Get your Bumpers ready!

["Stephen J. Turnbull" <turnbull@example.com>]

>>>>> "Alan" == Alan B Stone <stoneab@example.com> writes:

    Alan> I will say this since working with random numbers/generation
    Alan> is something I work with.  If you place a range (limit) like
    Alan> from this to that, then excluding fractions and decimals you

What does this mean in the context of finite precision floats?  Most
fractions and decimals are not representable.  In fact, none of the
relevant ones are in the case of most pseudo-random number generators
since MaxRand is normally relatively prime to the base for the FP
representation.

    Alan> do not have a random number generator.

Ah....  Knuth suggests "this +(floor(((that-this)*rand())/MaxRand))";
do you know something he didn't know in 1981?  It is not quite
uniform, but if (that - this) isn't too big, nobody in TLUG will
notice.

And AFAICT, if MaxRand == a*(that - this) + b, then

int draw ()
{
    int temp = rand() - b;	/* assume MaxRand < MaxInt */
    return temp < 0 ? draw() : this + temp % a;
}

ought to do the trick.  If it doesn't, rand() is unsatisfactory for
Monte Carlo studies in any case.  Statisticians do exactly that kind
of conditioning operation on pseudo-random variables all the time in
constructing tables for statistics whose distributions cannot be
solved in closed form.

And it is an algorithm (despite looking like a potential infinite
recursion), since pseudo-random numbers must cycle, and "good" ones
are usually required to hit all values.  Could be a mite slow on
really bad choices of seed, but running time is bounded....

>>>>> "Craig" == Craig Oda <craig@example.com> writes:

    Craig> for the salt, the system clock?  When I did it with Java, I
    Craig> was getting random numbers clustered in certain areas.  (I

I really don't think so ... Sun isn't that dumb.  Anyway, for many
versions of libc all the random number generators were in fact
variations on a theme.  Even if not, I'm sure that both Java and Perl
use a system-provided function at some level; "rolling your own"
general-purpose random number generator is a very very dangerous thing 
to do.  Knuth devotes 170 (one hundred and seventy) pages of _The Art
of Computer Programming_ to the topic.  170 very very dense pages....

    Craig> Ahh, to use hitech or lotech that is the question.  Which
    Craig> is more random?

Ask Schroedinger's Cat.
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