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Subject: |
Re: Random number generators. advice? |
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Wed, 21 Feb 2001 12:57:43 -0600 |
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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