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

2011-12-10 Thread Virgo Pärna 
On Fri, 9 Dec 2011 13:28:49 +0100 (CET), Dimitri Smits smi...@telenet.be 
wrote:

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


I do know that:) I made that mistake long time ago.  What I thought was, 
that constantly 
setting RandSeed is somewhat like constantly calling Randomize. In the example 
code randseed 
was dependant of randomize output - which might or might not be as awful as 
randomize. And 
the graphical output was just really good illustration, why it was bad.

-- 
Virgo Pärna 
virgo.pa...@mail.ee

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

2011-12-09 Thread Virgo Pärna 
On Fri, 09 Dec 2011 09:19:53 +0100, Florian Klaempfl flor...@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?

-- 
Virgo Pärna 
virgo.pa...@mail.ee

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

2011-12-09 Thread Dimitri Smits 

- Virgo Pärna virgo.pa...@mail.ee schreef:

 On Fri, 09 Dec 2011 09:19:53 +0100, Florian Klaempfl
 flor...@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
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Re: [fpc-pascal] Re: Why is Random(255) some 529x slower compared to Delphi 7?

2011-12-09 Thread Marco van de Voort 
In our previous episode, Dimitri Smits said:
 
 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).

(specially since high quality rng's might call OS dependent sources for
initial entropy in that first call)
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