Re[2]: [Haskell-cafe] Mapping over multiple values of a list at once?

Hello Sebastiaan, Thursday, August 27, 2009, 3:49:48 PM, you wrote: you also need to replace (\d -> sum d `div` 3) with (`div` 3) . sum in order to keep Haskell spirit :)

Or, when the list is infinite, turn it into a some neat but cryptic State computation:

avgs = (:) <$> ((\d -> sum d `div` 3) <$> StateT (pure . splitAt 3)) <*> avgs

test = evalState avgs [1,2..]

-- Sebastiaan Visser

On Aug 27, 2009, at 11:19 AM, Eugene Kirpichov wrote:

...

How about this one? Should be pretty efficient.

let mavg n xs = let (sum -> seed,rest) = splitAt n xs in map (%n) . scanl (\a (p,n) -> a+n-p) seed $ xs `zip` rest

2009/8/27 Patai Gergely :

...

...

For example, starting from

[4,3,2,6,7]

you need to find the averages of

4,3,2 and 3,2,6 and 2,6,7

resulting in

[3,4,5]

What is the most elegant way to do that? It's probably less elegant than tails, but very likely more efficient to keep track of running sums instead of summing the sublists over and over again.

import Data.Ratio

nsums n xs = map (% n) $ scanl (+) (sum (take n xs)) $ zipWith (-) (drop n xs) xs

Gergely

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