Hideki Kato wrote:
Gunnar Farnebäck: <[EMAIL PROTECTED]>:
Hideki Kato wrote:
I didn't against you, Álvaro, rather I just made a caution for
programmers who will use your pseudo code as is. :)
First, I prefer SFMT (SIMD-oriented Fast Mersenne Twister) rather
than integer pseudo random number generators in practice where the
quality of play-out is important. Modern processors execute floating
operations as fast as interger ones and
picked = mt_rand() * (double) num_candidates;
is the simplest and safe.
Please note that for uniformity purists this method has exactly the
same problem as good_quality_int_rand() % num_candidates.
Mt_rand() has very good uniform distributions in [0..1)
while
good_quality_int_rand() % num_candidates
doesn't disribute uniformly when num_candidates is not a power of 2,
assuming good_quality_int_rand() ranges [0..2^32 or so) due to modulo
operations. They are not the same, aren't they?
Well, there's nothing magic about floating point numbers. Even a very
good uniform distribution in some interval is implemented by
distributing N discrete values over the interval as uniformly as
possible. When those N values by some mapping procedure are transformed
into a smaller range M, some of those will get at least one more hit
than some others, unless M divides N. It doesn't matter whether the
mapping procedure is an integer modulo operation or a floating point
multiplication + rounding.
/Gunnar
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