Re: Seeding the pseudorandom number generator

[chase321 <cvbrady suddenlink net>]

Hal Ashburner-2 wrote:
> chase321 wrote:
> > I have been using Gnumeric for some time in the statistics classes I
> > teach. 
> > This past semester, I did my first online section, and had them submit
> > labs
> > using Gnumeric.  For one lab, I had them use the "randbetween" function
> > to
> > generate random numbers.  9 of the 17 labs I received had identical sets
> > of
> > random numbers.  This makes me more than a little suspicious, but I have
> > also seen students get the same sequence of numbers in the classroom.  I
> > know enough about pseudorandom number generators to know that if two
> > people
> > start at the same point in the sequence, they're going to get the same 
> > set
> > of numbers.  So I have two questions.
> > 1) How is the pseudorandom number generator initialized (seeded)?
> > 2) Is there any way to seed it manually?  I can find no function for this
> > mentioned in the manual.
> >
> > Thanks for any help.
>
> Hi Chase,
> Looking at the code in plugins/fn-random/functions.c and src/mathfunc.c
> it appears rand_01 is the relevant function. By setting the environment
> variable GNUMERIC_PRNG_SEED
> (on unix with a bourne compatible shell with something like
> $ export GNUMERIC_PRNG_SEED=some_random_string
> ) will cause rand_01 to use a pseudo random number generator using the
> specified seed. Otherwise it should get its random numbers from
> /dev/urandom
>
> My understanding of this is that if GNUMERIC_PRNG_SEED is set then each
> time you open a sheet containing randbetween you will get the same
> value, whereas if it isn't then you won't.
>
> Are you using windows by any chance?
>
> Kind regards,
> Hal
>
>
> I'm using Linux, but most if not all of my students are using Windows. 
> What I actually had them do is generate the random numbers and then copy
> and paste "as value" so the numbers would stay fixed for the rest of the
> activity.  So I should be getting the actual numbers that they got.  Of
> course, if one of them did the lab and then passed the finished product
> around to the others, that would certainly explain it, but I don't want to
> claim that if I'm not sure. 
> Would the Windows installer set the environmental variable?  I can check
> tomorrow to see if it's set on my Windows machine at the office.
> Incidentally, grades are already in, so I won't be taking this to the dean
> at this point, but  I would like to know what's going on.
> Thanks for the quick reply!
> --Chase
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