On 6 mrt 2006, at 13:37, Antal wrote:
The Mersenne Twister Free Pascal uses is one of the best PRNGs known
today, it just has to be used the right way. But calling it from
several threads and "randomly" overwriting its state array is
definitely not the right way to use it.
I'm disappointed, because I compiled the following program (on linux):
rnd.pp
begin
Randomize;
writeln(random(394));
end.
and I ran it for 11 times, and the results are rather strange:
310.
198
344
322*
322*
317-
317-
192
336.
382.
339.
(upon noting the dups I had to re-run again twice, getting:)
120
297.
There is nothing strange about this. Randomize initialises randseed
based on the system clock value expressed in seconds. So if you run
it twice in the same second, you will get the same number.
so, I've got two times a repetition, though I only re-ran the program.
(noted with * and - )
Then I made a new series of random generating:
297.
120
310.
382.
336.
192
317
322
344
198
339.
I only have two problems with these random numbers:
Firstly, I can notice a repetition of some "random" numbers (noted
with ".")
Then also 70% of them are 3xx
I learnt about random numbers, that they do not behave by this way.
You are not generating a series of random numbers based on the
Mersenne Twister here, because you only generate one random number
per program run. If you take all these values "together" as
supposedly belonging to a "series of random numbers", then the result
is that you are using a different "random" generator, where the next
"random" number is generated based on the next clock/time value as
opposed to the previously generated random number.
To test it properly, you should change your test program into this:
var
i: longint;
begin
Randomize;
for i := 1 to 20 do
writeln(random(394));
end.
And then you have to compare the random numbers generated in one such
a program run, not between different runs.
Jonas
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