Re: Q: confusion about random generators

[Sven Thommesen]

At 03:42 PM 7/31/01 +0200, you wrote:
> Hi there,
>
> I have been checking the mailing list archive and found some mails about
> seeds and random generation. I found one  dated 1997  which about the
> "danger" of using the UniformInteger and the other random number
> distributions. Obviously I have done my program without this valuable
> knowledge  and now I'm panicking!! I'm working with the version 2.1.1 of
> swarm and have now a big confusion about how to generate then random
> numbers and about the problem of declaring many generators in an app.
> I'm trying now to use the the java Random class, but as java doesn't
> allow global variables I'm struggling to find out how to use the same
> random generator in all my classes and give the seed as an argument at
> running the program. So my questions are these two:
> 1. What is the right way of doing this?
> 2. how can I change my seed to run different simulations?
>
>
> thanks a lot for any help,
>
> Isis

Isis,

since you don't tell us which 1997 mail you are talking about, it is not 
clear what "danger" you are talking about, or why you should be panicking.

There is at the moment no reason why you should not use the built-in 
default generator in Swarm; it is high quality and reasonably fast. (Paul 
Johnson even went through and speeded things up recently.) The default 
generator is globally accessible from within your program as 
'Globals.env.randomGenerator'.

It seems from your post that you have not read the documentation, and that 
is always a good place to start. The Swarm User Guide and its appendices 
has an extensive discussion on how to use the random generators and the 
distribution objects. (I'd give you a url, but it seems the swarm.org 
server is down at the moment.)  You can choose to use the default generator 
for all your random numbers (it has a long enough period for that), or you 
may create several different generators; there are many to choose from. The 
documentation describes the factors to consider when you make your choice.

Note that after the documentation was written, Marcus has added the command 
line switch "-S=seedvalue" to allow you to set the seed for the default 
random generator when your simulation starts up.

If after reading the docs you have more specific questions, I'll be happy 
to help.

Cheers,
Sven Thommesen


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