[fpc-pascal] Re: Why is Random(255) some 529x slower compared to Delphi 7?

[Dimitri Smits]

----- "Virgo Pärna" <virgo.parna at mail.ee> schreef:

> On Fri, 09 Dec 2011 09:19:53 +0100, Florian Klaempfl
> <florian at freepascal.org> wrote:
> >
> > Oops, mails crossed. The assignment to randseed is indeed the
> problem.
> > Why is it done? Bad random generator of delphi :)?
> >
> 
>     I don't know, how bad Delphis random generator is, but I once
> years ago did make a mistake
> of  calling randomize before random. It was a school assignment for
> using Turbo Pascal graphics.
> So I made program, that was supposed to fill screen with random dots
> (colour was also set with 
> random). Anyway, when I called randomize before random, then as a
> result dots didn't fill the 
> screen, but were mostly along the diagonal of the screen. So I'd
> guess, that setting randseed 
> multiple times could be actually bad for randomness of results. Or
> maybe modern algorithms are
> better at situations like this?
> 

Randomize() is supposed to be called only once to seed the generator with an initial value. 

If you made it something like so:

begin
  for i := 0 to 1000 do
  begin
    randomize();
    pixel[random(screenwidth),random(screenheight)]:= clSomeColor;
  end;
end;

then you seed with a timestamp before randoming and your distribution does not change much with a lcg. Not the fault of the algorithm itself, just your mistake of doing the randomize in the loop itself (like you mentioned yourself).


kind regards,
Dimitri Smits