>>>>> "glen" == glen herrmannsfeldt <gah@[EMAIL PROTECTED]
> writes:
glen> Andor wrote:
glen> (snip)
>> In the second edition of NR, they say that all LC PRNG are about
>> equally non-random, and the quick and dirty solution above is just
as
>> good (or bad) as the others. As I wrote, in the current edition
(3),
>> they discourage the use of LC PRNGs all together.
>>> A general Linear congruential RNG has the form X_{n+1} = (a*X_n +
c)
>>> mod m.
>>> For the ANSI C RNG, m=2^32, a=1103515245, c=12345, and bits 30..16
are
>>> returned.
>> It makes sense only to return the higher bits. It can be shown that
>> each bit in a sequence of LC PRNs has a period of at most 2^(bit
>> position+1), eg. the LSB has a period of at most 2. I learned that
the
>> hard way :-).
glen> This is true if m is a power of two. I believe it isn't
necessarily
glen> true if m is not a power of two. It is very common, though, for
m
glen> to be a power of two. Otherwise, if I need random low bits I
divide
glen> by a prime number sort of near sqrt(m) before using it.
Do you take the remainder as the random number? If so, this has a
bias, so the result isn't quite uniform. May or may not matter.
Ray
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