Initializing random seeds

[Daniel Luna]

On Wed, 8 Feb 2006, Corrado Santoro (with some cutting by me) wrote:
> I'm trying to use the functions of the random module to generate random 
> numbers in different processes. But I obtain the same random number 
> sequence for each process.

> Number generated from proc <0.128.0> is 9
> Number generated from proc <0.129.0> is 9
> Number generated from proc <0.130.0> is 9
> Number generated from proc <0.131.0> is 9

> Do you have an hint to avoid this problem?

Yes. Patience is the key. Try to generate just one more number and they 
should be different.

With a few more numbers generated we will see only the first numbers
generated are the same. If we look at the seeds we see that the last 
numbers differs in your test:

> Seed of <0.128.0> is {1139,388083,514683}
> Seed of <0.129.0> is {1139,388083,514695}
> Seed of <0.130.0> is {1139,388083,514701}
> Seed of <0.131.0> is {1139,388083,514707}

This is not enough to make any difference in the first generated number 
(for small N). From random.erl:

uniform() = ... + A3/30323
uniform(N) = trunc(uniform() * N) + 1.

So for a small difference in A3 you will have the same random number the 
first time around. Given time though (just once more should be enough), 
these numbers will separate[*].

/Luna
[*] But not that much unfortunately. They will give less "random" numbers 
than 4 processer with differing A1s and A2s. But probably enough for most 
applications.
-- 
Daniel Luna                           | Top reasons that I have a beard:
luna@REDACTED                     |  a) Laziness.
http://www.update.uu.se/~luna/        |  b) I can.
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