gwern0 at gmail.com
Sat Sep 6 05:42:50 EDT 2008
On 2008.08.26 22:52:57 +0100, Adrian Hey <ahey at iee.org> scribbled 0.5K characters:
> Gwern Branwen wrote:
>> FWIW, when I tested out a number of nub/uniq definitions, the map head version performed the worst, much worse than the O(n^2) one everyone is complaining about. It's a neat definition, but not performant.
> When did you try this? IIRC correctly even the old sort was O(n^2), but
> someone had the sense to replace it a while ago.
> With ghci now on my machine..
> length $ map head $ group $ sort [1..100000]
> finishes "instantly", but..
> length $ nub [1..100000]
> takes about 90 seconds.
> Adrian Hey
Sure, but suppose the list isn't nicely ascending? Suppose it's even repeated? Try out
Prelude Data.List> length $ map head $ group $ sort $ take 10000000 $ repeat 
Prelude> length $ nub $ take 10000000 $ repeat 
Then the situation is the opposite, instant versus a long time.
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