Getting average of calculated list haskell [duplicate]

问题

This question already has answers here:

Outputting the contents of a list of a custom data type (2 answers)

Closed 5 years ago.

I have

avgRatingsForDirector :: String -> [Film] -> [Int]
avgRatingsForDirector _ [] = []
avgRatingsForDirector requestedDirector ((Film _ director _ ((_, rating):ratings)):restOfFilms)
    | requestedDirector == director = [rating] ++ avgRatingsForDirector requestedDirector restOfFilms
    | otherwise = avgRatingsForDirector requestedDirector restOfFilms

This outputs a list of numbers, I want to have the result be the average of these numbers. is is possible to use the foldr function on the result somehow?

回答1:

You can keep your function as is and post-process its result with a function to calculate an average of any list of numbers.

One way to code it so it does only one traversal of the list is

{-# LANGUAGE BangPatterns #-}
import Data.List

avg :: (Integral a, Fractional b) => [a] -> b
avg xs = g $ foldl' c (0,0) xs
 where
   c (!a,!n) x = (a+x,n+1)
   g (a,n) = fromIntegral a / fromIntegral n

Bang patterns make the calculation efficient.

see also: The Most Fuun You Can Have: Beautiful Folding

来源：https://stackoverflow.com/questions/21997609/getting-average-of-calculated-list-haskell