I need to do a subtraction in Haskell

Question

i need to do a subtraction in haskell, and the result add an list, but the new list doesn't give me the real valors

for example: [1,2,3,4,5]

• prom = (1 + 2 + 3 + 4 + 5) / 5 = 3

• subs = (1 - 3) = -2 add to list

• subs = (2 - 3) = -1 add to list

• subs = (3 - 3) = 0 add to list

• subs = (4 - 3) = 1 add to list

• subs = (5 - 3) = 2 add to list

• new list = [-2,-1,0,1,2]

this's the code:

add (x:xs) = x + add xs

prom (x:xs) = add (x:xs) `div` length (x:xs)

subs [] = []
subs (x:xs) = x - prom (x:xs) : subs xs

how i can do that? and why the results is different?

Answer 1

prom (x:xs) will return the average for that list, but if you recurse, you thus consider the tail of the list, and you thus calculate the average on the remaining elements.

This means that your list will contain [1-3, 2-3, 3-4, 4-4, 5-5], so [-2, -1, -1, 0, 0]. You should thus determine the average of the entire list, and subtract that from all elements, so:

subs :: [Int] -> [Int]
subs xs = map (subtract (prom xs)) xs

Your add should also consider the empty list case:

add :: [Int] -> Int
add [] = 0
add (x:xs) = x + add xs

and for prom you can work with an xs variable as pattern to bind with the entire list:

prom :: [Int] -> Int
prom xs = add xs `div` length xs