Calculating the Moving Average of a List

Question

This weekend I decided to try my hand at some Scala and Clojure. I\'m proficient with object oriented programming, and so Scala was easy to pick up as a language, but wante

Answer 1

Using Haskell:

movingAverage :: Int -> [Double] -> [Double]
movingAverage n xs = catMaybes . (fmap avg . take n) . tails $ xs
  where avg list = case (length list == n) -> Just . (/ (fromIntegral n)) . (foldl (+) 0) $ list
                        _                  -> Nothing

The key is the tails function, which maps a list to a list of copies of the original list, with the property that the n-th element of the result is missing the first n-1 elements.

So

[1,2,3,4,5] -> [[1,2,3,4,5], [2,3,4,5], [3,4,5], [4,5], [5], []]

We apply fmap (avg . take n) to the result, which means we take the n-length prefix from the sublist, and compute its avg. If the length of the list we are avg'ing is not n, then we do not compute the average (since it is undefined). In that case, we return Nothing. If it is, we do, and wrap it in "Just". Finally, we run "catMaybes" on the result of fmap (avg . take n), to get rid of the Maybe type.