Double checking my understanding of Random Generator Docs

[Paul E. Johnson]

Dear Sven:
I'm copying this to the list, since some other people might want
to correct me.

I want to be sure I don't make a mistake when I put this in the FAQ.
There are so many options in random number generating it is hard 
to give a simple answer.

Question: How do I get random numbers from a particular distribution?

Answer:  You create an object, say "aNormalDist." Then, when
you need a random number, you get it by telling aNormalDist that you want
one with "[aNormalDist getDoubleSample]". 

       Pick the distribution you want (there are many, like "Normal,"
"Uniform" etc listed in docs).  Suppose I want Normal random numbers
with mean 0 and variance 1.  (Note you need to include random.h in your
file to make this work).

        There are many possible random number generators, that's where
all the confusing stuff kicks in.  If you don't want to be an expert,
use one of these two approaches.


      1. Let Swarm do the work for you!

 Create the distribution object with this code:

aNormalDist = [NormalDist createWithDefaults: [self getZone] setMean: 0
           setVariance: 1] ;

The "createWithDefaults" method creates aNormalDist and it also creates
a generator from which that distribution object can get random numbers.
You never specify which random generator and you don't need to!
Which random generator is chosen is determined inside the Swarm libraries.
Each distribution you create causes the creation of a new random number
generator.

     2. Force the usage of the default generator, MT19937.
This random number generator is, practically speaking, an optimum mix of
randomness and computer speed.  A generator called "randomGenerator" is
created when you use the Swarm random library, and you are free to create
distributions that use it as the supplier of random numbers. This is
available globally, in any file where you want to create a
distribution.  This creates the normal distribution object:
  aNormalDist = [NormalDist create: [self getZone] 
            setGenerator: randomGenerator  setMean: 0
           setVariance: 1] ;
If you create many distribution objects in many files, then they will all
be drawing random numbers from the default "randomGenerator."  Some people
in the mailing list think (emphatically) that is the best way to go.

     Every time you run your program, the random number generators
will start at the same "seed" and you will get the same results.  If
you want to have the starting points chosen at random, do it by
adding --varyseed on the command line when you type in the name of
your program (i.e., ./heatbugs --varyseed).

     If you need more control, you can get it. Choose a random number
generator from Sven's incredible list and initialize it yourself and
access it instead of "randomGenerator".  You can control the random number
seed explicitly if you want. You can also make the generators give back
information about themselves.  Check the docs


Paul E. Johnson                       email: address@hidden
Dept. of Political Science            http://lark.cc.ukans.edu/~pauljohn
University of Kansas                  Office: (913) 864-9086
Lawrence, Kansas 66045                FAX: (913) 864-5700


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