fact2 is O(log n) while fact is O(n). fact2 is partitioning the problem.
On Wed, Dec 30, 2009 at 08:57, Artyom Kazak <[email protected]> wrote: > > Why fact2 is quicker than fact?! > > fact2 :: Integer -> Integer > fact2 x = f x y > where > f n e | n < 2 = 1 > | e == 0 = n * (n - 1) > | e > 0 = (f n (e `div` 2)) * (f (n - (e * 2)) (e `div` 2)) > y = 2 ^ (truncate (log (fromInteger x) / log 2)) > > fact :: Integer -> Integer > fact 1 = 1 > fact n = n * fact (n - 1) > > I tried to write tail-recursive fact, fact as "product [1..n]" - fact2 is > quicker! > > > fact2 1000000 == fact 1000000 - I tested. > > _______________________________________________ > Haskell-Cafe mailing list > [email protected] > http://www.haskell.org/mailman/listinfo/haskell-cafe > > -- Rafael Gustavo da Cunha Pereira Pinto
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