[Haskell-cafe] Cartesian product of a Set
overdrigzed at gmail.com
Wed Aug 1 17:37:03 EDT 2007
toList generates a sorted list since it is implemented as toList =
toAscList, but it's probably bad form to rely on that.
On 01/08/07, Andy Gimblett <A.M.Gimblett at swansea.ac.uk> wrote:
> On Wed, Aug 01, 2007 at 01:42:52PM -0700, Dan Weston wrote:
> > Andy Gimblett wrote:
> > >
> > >>cartesian :: Ord a => S.Set a -> S.Set (a,a)
> > >>cartesian x = S.fromList [(i,j) | i <- xs, j <- xs]
> > >> where xs = S.toList x
> > Your list comprehension always generates a sorted list, so changing
> > S.fromList to its "unsafe" version (but guarateed by you)
> > S.fromDistinctAscList should get you back to O(n).
> That'll do it: with that change, my program runs eight times faster;
> according to the profiler, it's gone from spending ~85% of its time in
> cartesian to ~0%. :-)
> I don't see why my list comprehension always generates a sorted list,
> however: toList generates a sorted list? It doesn't claim to in the
> docs. Do I perhaps need to use toAscList instead?
> I happen to have put my data into the Set in order in this example, so
> maybe I just lucked out? Or am I missing something?
> > Of course the order of the generators was key (i before j).
> Lucky me. :-)
> Andy Gimblett
> Computer Science Department
> University of Wales Swansea
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