Hi, I created some code from scratch - probably "ugly" beginners style - so I'm keen to get tips how to make it more pretty and faster Cheers Phil import Data.List -- Input Data xi :: [Double] xi = [0 .. 10] yi :: [Double] yi = [2, 3, 5, 6, 7, 8, 9, 10 , 9, 8, 7] x = 11 :: Double -- Functions limIndex xi idx | idx < 0 = 0 | idx > (length xi)-2 = (length xi)-2 | otherwise = idx getIndex xi x = limIndex xi (maybe (length xi) id (findIndex (>x) xi)-1) getPnts xi yi idx = [xi !! idx, xi !! (idx+1), yi !! idx, yi !! (idx+1)] interp xi yi x = let pts = getPnts xi yi (getIndex xi x) in (pts!!3-pts!!2)/(pts!!1-pts!!0)*(x-pts!!0)+pts!!2 -- Calc y = interp xi yi x main = do -- Output Data print (y) -- View this message in context: http://haskell.1045720.n5.nabble.com/HMatrix-Vector-Matrix-interpolation-ala... Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.