Question

Possible Duplicate:
Haskell – Selectively Adding Lists

I have a data type:

data Film = Film String String Int [Rating]
     deriving (Show,Ord,Eq, Read)

A sample film:

("Ridley Scott","Alien",1979,[("Mark",5),("Zoe",3)])

How would I go about getting the average director rating across all films in a list of [Film]?

So if Ridley Scott had 2 films which both individually had an average score of 5 then he would score an overall average of 5.

I only have the code to work out a single Film

filmRating :: [(String,Int)] -> Float
filmRating ratings = average (map snd ratings) 

Answer 1

Not a real answer, just some newbie notes.

You can use type aliases for Director, Title and all others types.

type Director = String
type Rate = Double
type Rater = String
type Ratings = [(Rater, Rate)]
type Title = String
type Year = Int

data Film = Film Director Director Year Ratings
     deriving (Show,Ord,Eq, Read)

That’s why your film example should looks like:

film = Film "Ridley Scott" "Alien" 1979 [("Mark",5),("Zoe",3)]

If you have a function named filmRating it should be something like :: Film -> Rate:

filmRating :: Film -> Rate
filmRating (Film _ _ _ ratings) = average (map snd ratings)

How would I go about getting the average director rating across all films in a list of [Film]?

You want something like directorsAverageRate :: Director -> [Film] -> Rate.

First of all, you need to filter films by Director value with some :: Film -> Director function. For this filtered list you simply get average rate with average . (map filmRating).

---

UPD:

I don’t understand how to get the overall average after I have the films involved.

Let’s take the definition of average function from the similar question:

average :: (Real a, Fractional b) => [a] -> b
average xs = realToFrac (sum xs) / genericLength xs

Then

> let one_film = Film "Ridley Scott" "Alien" 1979 [("Mark",5),("Zoe",3)]
one_film :: Film

> let another_film = Film "Ridley Scott" "Alien2" 2979 [("Mark",4),("Zoe",2)]
another_film :: Film

> let films = [one_film, another_film]
films :: [Film]

> average . (map filmRating) $ films
3.5
it :: Double