Re: question about random generator

Antonio wrote:
> Robert Gamble wrote:
> > Antonio wrote:
> > > pete wrote:
> > > > Does "pseudo-random" mean the same thing as "uniformly distributed" ?
> > >
> > > Completely OT, but anyway... No, pseudo-random means that it looks like
> > > it's random but it really isn't. There is no way to generate trully
> > > random numbers with a computer, everything you do is deterministic, but
> > > you can generate sequences that look like they're random but that
> > > aren't. Hence the term _pseudo_-random.
> >
> > Yes, we know, you didn't say anything that wasn't completely obvious.
> > The C Standard specifies that rand() generates pseudo-random numbers,
> > the questions is whether a conforming implementation could generate a
> > series of normal distributed numbers via the rand() function or if the
> > term pseudo-random implies that the numbers must be generated with a
> > uniform distribution. I was wondering the same thing myself, I think
> > the intention is that the numbers be uniform but that may be debatable.
>
> It may be completely obvious to you, and to many people (including
> myself), but it doesn't seem to be obvious to "pete", since he asked.
> What I was trying to explain is that pseudo-random does not imply
> anything about the distribution of the numbers. You may get
> pseudo-random numbers that look like a uniform distribution, or
> pseudo-random numbers that look like a poisson distribution, or a
> gaussian distribution, or anything you want. In fact if you are able to
> generate pseudo-random numbers with any distribution, you can operate
> with them to obtain any other distribution you want.
>
> On the topic of wether the standard requires the distribution to be
> uniform. Well I don't know the standard by hard, but many people here
> seem to have a copy of it, so it should be a simple matter to check it.
> Every implementation I've seen of C and almost any other language/tool
> generates pseudo-random number using a multiplicative seed, simply
> becase it's a good enough method and requires relatively few
> operations.
>
> And finally, I don't know if an implementation that generated normally
> distributed numbers with the rand() function would conform to the
> standard, but it would be beyond foolish.

You are missing the point. The standard only states that the numbers
generated are "pseudo-random", pete knows this, I know this, anyone who
has a copy of the Standard knows this. He was specifically refering to
this wording in the Standard and asking if this requirement alone
implied a uniform disribution since the Standard does not discuss
distribution requirements.

Robert Gamble

.

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