On 12/06/15 09:15, Michael Titke wrote:
In my understanding the pseudo random number generator is deterministic.
That means for the same input seed /random/ will always return the same
value. This is why one usually has to set a new state for each call of
random.
No, quite on the contrary.
Before going on, please note that I never looked at the specific random
generator used by Racket. I assume it's one of the popular random number
generator algorithms. For what follows, it doesn't even matter which one.
All pseudorandom generators produce a sequence of numbers. The intention
is that the sequence should have as little detectable correlation as
possible. The algorithms differ in how they define this goal exactly,
and how well they reach it.
The principle behind all these generators is a chaotic dynamic process
that transforms the current state of the generator (some finite number
of bits) into a new state, which is used for the next call. The returned
value is some function of the state. Setting a new seed changes to state
to some function of the seed value.
Seeding is provided for two use cases: 1) Reproducibility, mainly for
software testing. If you set the seed to some constant at the start of
your program, it will behave exactly the same way whenever it is run. 2)
"Real" randomization, by setting the seed from some unpredictable
source, such as the system time. In both cases, the idea is to set the
seed once at the beginning of a program run.
The statistical properties of the pseudo-random sequence hold only for
an unperturbed sequence, i.e. without changing the seed. If you set the
seed every time you ask for a random number, your random sequence
becomes simply some complicated function of your input seed, whose
statistical properties are hard to predict and most probably not what
you want.
Konrad.
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