Subject: Re: about pseudo-random numbers (NBY)
From: [email-masked]

20 Jan 1998 02:57:29 -0800
Message-ID: <[email-masked]>
References: <Pine.LNX.3.96.980113114251.1703C-100000@pc66>
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To: Valter EOR Santos <[email-masked]>,
        GNU-Win32 Mailing List <[email-masked]>
Cc: GNU-Win32 Mailing List <[email-masked]>

On 13 Jan 98 at 11:50,  Valter EOR Santos <[email-masked]> wrote:

>   Hi!
>   I am a newby in this mailing list...
>   I am generate a program code that need random number generation...
>   My question is if there is any way to make a limit in the random
> generation. In the Turbo Borland C++ i have the unit random(int limit)
> that produces numbers between 0 and limit. In the gnu, or other linux
> compiler i only have the rand() or srand() units that produces numbers
> between 0 and a pre-defined random_limit variable MAX_RAND (or something
> like that). How i can generate code for resolving my problem???
> 

Hello Valter! 

#include <stdlib.h> /* MAX_RAND, srand(), rand() */

double random (const int limit)
{
	return (((double)rand () / RAND_MAX) * limit);
}

The first part of the return expression -- ((double)rand () / RAND_MAX) --
generates a pseudo-random number, converts it to a double value, then divides
it by the maximum of pseudo-random function rand().  This yields a floating
point number between 0 and 1,0 , with lots of fractional values in between. 
If rand() wasn't converted to a double, the division would yield only 0 or 1.

Next, the result of the division is multiplied by limit to yield the numbers
you're looking for, something between zero and limit.  These numbers will be
fractionary.  If you really need them integer, all you gotta do is change the
random() function header to

	int random (const int limit)

The function srand() sets a seed, or a starting value for the sequence of 
pseudo-random numbers.  If you run your program twice with the same seed, you 
end up with exactly the same number sequence.  That's what the "pseudo" means.
The rand() function, and so our newly defined random(), create numbers that 
appear to be random.  

In our case, the statistical properties of the rand() function won't be
changed, since in mathematical terms what we're doing is a simple
multiplication by (limit / RAND_MAX).

People usually set it to a huge number that changes quickly.  Yes, you got it:
the time in seconds.  In the Cygwin environment, you'd better use the
GetTickCount() function.

So, somewhere in your program you can have this:

#include <stdlib.h>  /* MAX_RAND, srand(), rand() */
#include <windows.h> /* GetTickCount() */

....
	
srand (GetTickCount ());  /* set seed */

....

val = random (limit); /* get a (pseudo) random number */

....

HTH

Regards,
++Hilton

P.S.: If you still need information, write privately to me: i can send you 
references to papers in professional magazines that can help you.
-------------------
Hilton Fernandes
email: [email-masked]
www:   http://www.lsi.usp.br/~hilton.html (inactive)
M. Sc. Student of Parallel Distributed Applications
at Escola Politecnica (Polytechic School)
University of S. Paulo - Brazil
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