[Numpy-discussion] Numarray: question on RandomArray2.seed(x=0, y=0) system clock default and possible bug

Eric Maryniak e.maryniak at pobox.com
Tue Jul 23 13:03:02 CDT 2002


On Tuesday 23 July 2002 20:54, Todd Miller wrote:
> Eric Maryniak wrote:
> >...
> That should not, however, discourage you from writing a new and improved
> random number package for numarray.

Yes, thank you :-)

> >...
> >3. I wonder what the design philosophy is behind the decision
> >   to use 'mathematically suspect' seeding as default behavior.
>
> Using time for a seed is fairly common.   Since it's an implementation
> detail,  I doubt anyone would object if you can suggest a better default
> seed.

Well, as said, a fixed seed, provided by the class implementation
and therefore 'good', instead of a not-so-random 'random' seed.
And imho it would be better not to (only) use the clock, but a /dev/random
kinda thing.
Personally, I find the RNG setup much more appealing: there the default is:

    standard_generator = CreateGenerator(-1)

where

    seed < 0  ==>  Use the default initial seed value.
    seed = 0  ==>  Set a "random" value for the seed from the system clock.
    seed > 0  ==>  Set seed directly (32 bits only).

And indeed 'void Mixranf(int *s,u32 s48[2])' uses a built-in constant
as initial seed value (actually, two).

>...
> >     If you use default seed()'ing now and re-run your program/model
> >     later with identical parameters, you will get different output.
>
> When you care about this, you need to set the seed to something
> deterministic.

Naturally, but how do I know what a 'good' seed is (or indeed it's type,
range, etc.)? I just would like, as e.g. RNG does, let the number generator
take care of this... (or at least provide the option to)

>...

In the programs I've seen so far, including a lot of ours ahem, usually a
program (simulation) is run multiple times with the same parameters and,
in our case for neural nets, seeded each time with a clock generated seed
and then the different simulations are compared and checked if they are
similar or sensitive to chaotic influences.
But I don't think this is the proper way to do this.
My point is, I guess, that the sequence of these clock-generated seeds
itself is not random, because (as for RandomArray) the generated numbers
are clearly not random.
Better, and reproducible, would be to start the first simulation with a
supplied seed, get the seed and pickle after the first run and use the
pickled seed for run 2 etc. or indeed have a kind of master script
(as you suggest) that manages this.
That way you would start with one seed only and are not re-seeding
for each run. Because if the clock-seeds are not truly random, you
will a much greater change of cycles in your overall sequence of numbers.

Bye-bye, Eric
-- 
Eric Maryniak <e.maryniak at pobox.com>
WWW homepage: http://pobox.com/~e.maryniak/
Mobile phone: +31 6 52047532, or (06) 520 475 32 in NL.

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