Haskell - Max number in a list

Question

I want to find the maximum integral value in a list of integers. The following is my code -

maximum :: [Int] -> Int
maximum [x] = x
maximum (x:xs) =
 | (maximum xs) > x = maximum xs
 | otherwise = x

I do NOT want to use the in-built function max. So, I have NOT used : maximum (x:xs) = max x (maximum xs)

Why is the code not executing ?

Answer 1

You should remove the = before the wards block. Now, for making your function properly:

You can fold the list:

maximum' :: Ord a => [a] -> a
maximum' = foldr1 (\x y ->if x >= y then x else y)

For the recursive version (no double checking):

maximum'' :: Ord a => [a] -> a
maximum'' [x]       = x
maximum'' (x:x':xs) = maximum' ((if x >= x' then x else x'):xs)

If you want wards:

maximum'' :: Ord a => [a] -> a
maximum'' [x]       = x
maximum'' (x:x':xs) | x >= x'   = maximum' (x:xs)
maximum'' (x:x':xs) | otherwise = maximum' (x':xs)

Here you have a live example

Answer 2

You have an extra = before the first |.

maximum (x:xs) | (maximum xs) > x = maximum xs
               | otherwise        = x

Note that you compute maximum xs twice which will probably make your code run very slowly.

Answer 3

Firstly there is a syntax error, you need to remove the = after maximum (x:xs).

Secondly, the function maximum conflicts with Main.maximum, I suggest you rename it, for example:

maximum' :: [Int] -> Int
maximum' [x] = x
maximum' (x:xs)
 | (maximum' xs) > x = maximum' xs
 | otherwise = x

Answer 4

Recursion

Using where Clause

maxrec :: (Ord a) => [a] -> a
maxrec [] = error "maximum of empty list"
maxrec [x] = x
maxrec (x:xs)
  | x > maxTail = x
  | otherwise = maxTail
  where maxTail = maxrec xs