I don't know if anyone actually answered the question you didn't ask, but you can always improve an inaccurate guess when you need to. A limit will always exist, and should be unique (independent of the initial guess), assuming (+) and (*) are well-conditioned. In practice, a single first-order Taylor step should be enough: logBase' :: Double -> Double -> Double logBase' b y = if b == 0.0 then 1.0 else improve x0 where bLogInv = 1.0 / log(b) f x = x + (1.0-b**x/y) * bLogInv -- First step is enough, if we guess smartly improve = f x0 = log(y) * bLogInv -- or use the limit from any initial guess -- improve x = let y = f x in if y == x then y else improve y -- x0 = 0.0 Dan Roberto wrote:
Hi,
There is a mistake is logBase:
$ ghci GHCi, version 6.10.4: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer ... linking ... done. Loading package base ... linking ... done. Prelude> logBase 10 10 1.0 Prelude> logBase 10 100 2.0 Prelude> logBase 10 1000 2.9999999999999996 <--- eeeerrgghhhh! Prelude> logBase 10 10000 4.0
My host is a Debian GNU/Linux 5.0.2 (lenny) with the following GHC packages:
ii ghc6 6.10.4-1 ii ghc6-doc 6.10.4-1 ii libghc6-mtl-dev 1.1.0.2-7+b1 ii libghc6-utf8-string-dev 0.3.5-1+b1 ii libghc6-x11-dev 1.4.5-6 rc libghc6-x11-doc 1.4.2-1 ii libghc6-x11-xft-dev 0.3-3+b3 ii libghc6-xmonad-contrib-dev 0.8.1-3+b3 rc libghc6-xmonad-contrib-doc 0.8-2 ii libghc6-xmonad-dev 0.8.1-5
Regards!
_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe