Andy Gimblett wrote:
Hi all,
Is this a reasonable way to compute the cartesian product of a Set?
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
It's a fairly "obvious" way to do it, but I wondered if there were any
hidden gotchas. I'm particularly concerned by toList (O(n)) fromList
(O(n log n)) - but for other reasons I'd really like to be using Set
rather than List for this (I think).
Many thanks for any thoughts,
-Andy
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).
Of course the order of the generators was key (i before j).
Dan
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