OpenEuphoria

RE: 32-bit random numbers

• Posted by "Ricardo M. Forno" <rforno at uyuyuy.com> Jul 07, 2004
• 521 views

The resulting distributions are good, but there is a problem.
As rand(power(2, 30) -1) can generate only 2^30 - 1 different numbers, no
matter what you do with these numbers, you will get possibly only 2^30 - 1
different numbers between 1 and 2^32, and so you will never get one of the
remaining 2^32 - 2^30 numbers, or 3/4ths of the total possibilities.
Regards.
----- Original Message -----
From: Juergen Luethje <j.lue at gmx.de>
To: <EUforum at topica.com>
Sent: Tuesday, July 06, 2004 7:47 PM
Subject: Re: 32-bit random numbers


>
>
> Hi Igor, you wrote:
>
> > Hi again Jurgen,
> >
> > Me wrote:
>
> <snip>
>
> >> Get one trick more:
> >>
> >> To get the 32 bit rnd flat distribution
> >> you can do the following steps:
> >>
> >> a. Get rnd number in [0,1] range from the EU standard rand()
> >> b. Get rnd number in needed range from [0,1] range.
> >>
> >> It may be done as:
> >>
> >> }}}
<eucode>
> >> atom k
> >> k = 1 / 1073741823 -- the biggest EU integer
> >> atom rnd_0_1
> >>
> >> function rand_0_N(atom N)
> >>   rnd_0_1 = k * (rand(1073741823)-1)
> >>   return floor(N * rnd_0_1)
> >> end function
> >>
> >> for i=1 to 1000 do
> >>     ? 1 + rand_0_N(#FFFFFFFF) -- this distribution
> >>                               -- is in [1,#FFFFFFFF] range
> >> end for
> >> </eucode>
{{{

>
> Cool! Why didn't I think of that? 
>
> > Yes, it seems to be:
> >
> > for i=1 to 1000 do
> >     ? 1 + rand_0_N(#FFFFFFFF - 1)
> >  -- this distribution
> >  -- is in [1,#FFFFFFFF] range
> > end for   ----    
>
> It seems to me, that your *first* version was correct, wasn't it?
> For testing, I used N=6:
>
> }}}
<eucode>
> atom k
> k = 1 / 1073741823 -- the biggest EU integer
> atom rnd_0_1
>
> function rand_0_N(atom N)
>   rnd_0_1 = k * (rand(1073741823)-1)
>   return floor(N * rnd_0_1)
> end function
>
> atom N, x, min, max
>
> N = 6
> min = 1 + rand_0_N(N)
> max = min
> for i=1 to 1000 do
>    x = 1 + rand_0_N(N)  -- this distribution is in [1,6] range
>    if x < min then
>       min = x
>    elsif x > max then
>       max = x
>    end if
> end for
> printf(1, "range [%d,%d]", {min, max})
> </eucode>
{{{

>
> Thanks, Igor!
>
> Regards,
>    Juergen
>
>
>
>