Re: Random number generators. advice?

[Sven N. Thommesen]

I should make the following additional comment since you are trying to do 
the same thing in ObjC and in Java:

As you know, in ObjC the generators provide random variates in the form of 
doubles [0.0, 1.0) or unsigned integers [0, max-1]. For 32-bit generators, 
"max" may be as high as 2^32-1. For 31-bit generators, "max" is less than 2^31.

Unfortunately, Sun designers in their wisdom decided to equip java with 
*signed* integers only, in the range [-2^31, +2^31-1]. Consider the fact 
that the underlying random generators in Swarm are written in ObjC. So when 
a Java program calls a 32-bit generator (such as the default one), variates 
that exceed 2^31 will map to a negative number in Java.

(31-bit generators are not affected by this issue.)

If your simulation expects a positive random integer, you might want to 
throw away all negative variates in Java. (And if you want the ObjC and 
Java sims to run the same, you'd want to throw away variates larger than 
2^31 in ObjC as well.) Doing this will of course have unknown effects on 
the quality of the random streams you end up using.

Or, use only the floating point variates from the generators, and map those 
to integers as needed.

Sven



At 10:40 AM 2/21/2001 -0600, you wrote:
> I'm trying to make two "similar" models, one in obj-c and one in java,
> generate the exact same results.  In order to do that, I need the exact
> same random numbers at the start, and I'm having trouble because one
> program makes draws on the random generator more than the other, so the
> numbers for some variables don't match up.
>
> As a result, I need to create three or four different generator objects
> which can be called at just the right time so that the numbers will be
> identical in the two simulations.  At random, I chose to add the
> generator C3MWCgen and a couple others.  On the objc side that's OK, but
> there is no Java Swarm interface for that one.
>
> So then I thought I might as well just ask the experts which generators
> to use, if one needs separate generators to keep random numbers matched
> up.
>
> Advise me please.
> --
> Paul E. Johnson                       email: address@hidden
> Dept. of Political Science            http://lark.cc.ukans.edu/~pauljohn
> University of Kansas                  Office: (785) 864-9086
> Lawrence, Kansas 66045                FAX: (785) 864-5700
>
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                 ==================================
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  to day usage of Swarm.  For list administration needs (esp.
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  with "help" in the body of the message.