Re: cellfun and parcellfun

[David Bateman]

Robert Patterson wrote:
> On Mon, 2010-06-14 at 19:16 +0200, David Bateman wrote:
>   
> > Robert Patterson wrote:
> >     
> > > On Fri, 2010-06-11 at 23:28 +0200, David Bateman wrote:
> > >   
> > >
> > > I think I am doing something pretty stupid in trying to reset my RNGs:
> > >
> > > octave:187> rand("state", 123); rande()
> > > ans =  0.042798
> > > octave:188> rand("state", 123); rande()
> > > ans =  0.49064
> > > octave:189> rand("state", 123); rande()
> > > ans =  4.2087
> > >
> > > I was expecting to get the same value each time.  Is the hash used to
> > > initialise the Mersenne Twister non-deterministic?
> > >
> > > Thanks
> > >
> > > Robert
> > >
> > >
> > >   
> > >       
> > The older randlib generators have the same base generator so the above 
> > will work if you replace "state" by "seed". However the cost of that 
> > will be speed and a short length of the pseudo random sequence...
> >
> > The Mersenne Twister hash is deterministic but you have to initialize 
> > each generator separately
> >
> > octave:1> rande("state",123); rande()
> > ans =  0.27640
> > octave:2> rande("state",123); rande()
> > ans =  0.27640
> > octave:3> rande("state",123); rande()
> > ans =  0.27640
> >
> > D.
> >
> >     
>
> This gradually became clear from the source.  However, in
> S 17.4 Special Utility Matrices the manual states:
>
> The new random generators all use a common Mersenne Twister generator,
> and so the state of only one of the generators needs to be reset. The
> old generator function use separate generators.
>
> If I understand correctly, the manual is wrong.
>
>   
This was true, but since early 2008 with the changes made in the long 
discussion

http://octave.1599824.n4.nabble.com/Difference-between-Matlab-and-Octave-in-random-seeding-td1669992.html

the generator now have separate states. The randlib generators also have 
separate seeds, though I'd forgotten about that. Note that the change 
was also documented in 2008

http://octave.1599824.n4.nabble.com/manual-update-for-general-documentation-for-random-generators-td1671067.html

so I suspect the manual you are referring to is out of date

D.