Dear Alexander,
On Wed, Jul 23, 2014 at 5:40 PM, Alexander Konovalov < al...@mcs.st-andrews.ac.uk> wrote: > Dear Ha, > > On 23 Jul 2014, at 22:19, Ha T. Lam <hat...@gmail.com> wrote: > > > Dear GAP forum, > > > > I'm trying to generate random 2x2 matrices with entries from GF(947). > > Normally, I would just use RandomMat(2,2,GF(947)), but I want to get > > different results every time I start GAP. I found that RandomSource( r, > dev > > ) from the io package ( > > http://www.gap-system.org/Manuals/pkg/io/doc/chap6.html) allows > randomness > > from /dev/random, but I don't know how to use it as the random source for > > RandomMat. Any advice? > > This is doable - first you have to create a real random source: > > gap> rs:=RandomSource(IsRealRandomSource,"random"); > <a real random source> > > Then you may create a list of all elements of GF(947) and pick up > elements from it using "Random": > > > gap> gf:=AsList(GF(947));; > gap> Random(rs,gf); > Z(947)^42 > gap> Random(rs,gf); > Z(947)^817 > > Now you may take the code for RandomMat from the library and adjust it to > say ReallyRandomMat which will use the approach above instead of calling > Random( GF(947) ); > > There is a warning, however. If it may be enough for you to use dev/urandom > instead of dev/random, then it is advised to go for the 'urandom' option. > I remember once we were very puzzled by the slow performance of GAP on the > server where dev/random was used to generate some random strings. It was > not possible to reproduce the slowdown at all while using GAP > interactively. > Then we figured out what happened: dev/random needs some noise in the > entropy pool, and it may block when the pool is empty to wait for some more > events to happen. There was a plenty of events when one was sitting in > front > of the keyboard; however, that was not the case on the server when GAP was > running as a daemon. So please be aware of this feature. > > Hope this helps, > Alexander > > > Thank you very much for the quick reply and the warning. I was trying to use 'random' instead of 'urandom' and was indeed puzzled by the speed decrease. In my code, I used List(GF(947)) instead of AsList(GF(947)). Do you know if this makes a difference? The manual only tells me that AsList gives roughly the same access time to elements. I am also worried about working with a larger prime, would this method create an in-memory list of all the elements? Is there anyway to do a sort of "lazy evaluation"? Thank you, Ha _______________________________________________ Forum mailing list Forum@mail.gap-system.org http://mail.gap-system.org/mailman/listinfo/forum
