On 2003-09-17 at 10:31EDT Gordon James Miller wrote: Something I think is more café than language:

Hello all.

I'd be interested in getting some feedback on how to do this linear interpolation function a little more cleanly. The normal way that this is taught is to find the set of indices in the x list that bracket the input value, then use the slope between these points to calculate the y value for the input value.

I had a version working at one point using !! to access particular elements but I wasn't sure which one was the better solution.

!! can be expensive.

linterp :: [Double] -> [Double] -> Double -> Double linterp (x1:x2:xs) (y1:y2:ys) x | x <= x2 || xs == [] = linterpPair x1 x2 y1 y2 x | otherwise = linterp (x2:xs) (y2:ys) x where linterpPair x1 x2 y1 y2 x = x1 + (x - x1) * (y2 - y1) / (x2 - x1)

It seems to me that you have too much going on in one function. It would be better to break it into the part that finds the point where the interpolation is supposed to happen and the function that does the interpolation. You sort-of do this, but don't go far enough for my taste. You don't say whether the xs are supposed to be in increasing order. Also it's not obvious that your version does the right thing in the case where x < head xs, and linterpPair doesn't look right to me. Something like: linterp :: [Double] -> [Double] -> Double -> Double linterp xs ys x = linterpPair (neighbours (zip xs ys)) where linterpPair ((x1,y1),(x2,y2)) = y1 + (x - x1) * (y2 - y1) / (x2 - x1) neighbours all@(p@(x1,y1):rest) | x >= x1 = head (dropWhile ((<x). fst . snd) (all `zip` rest)) would be more to my taste, though it would be better to handle the errors rather than leave them to the runtime system. I think I would also define the function as linterp :: [(Double, Double)] -> Double -> Double since that seems more natural as the data are connected that way, and it would eliminate a zip. -- Jón Fairbairn Jon.Fairbairn@cl.cam.ac.uk