How to find a second largest number in a list in Haskell?

Question

The question is like this:

Write a function that takes an integer list and returns its length and the second largest integer in the list.

I can solve this with two functions but is there any solution that use only one function to do it?

Any help is appreciated! Thanks!

Answer 1

Don't make it complicated.

f xs = (sort xs !! 1, length xs)

If you choose to make it complicated, make it complicated in the right way.

Answer 2

Edited to use @ThomasM.DuBuisson's suggestion

You can solve this the same way that you could finding the max: using a fold. Max can be pretty trivially implemented with

mymaximum :: Ord a => [a] -> a
mymaximum xs = foldl searcher (head xs) xs
    where
        searcher :: Ord a => a -> a -> a
        searcher a b
            | a > b     = a
            | otherwise = b

So we can implement it similarly by just keeping up with the two largest values (notice that we have to "seed" the fold with a tuple as well):

nextmaximum :: Ord a => [a] -> a
nextmaximum xs = fst $ foldl searcher (h, h) xs
    where
        h = head xs
        searcher :: (Ord a) => (a, a) -> a -> (a, a)
        searcher (s, f) x = (min f (max s x), max f x)