99-problems/Haskell.org

99 Haskell Problems

• #1 Find the last element of a list.
  • Solution
• #2 Find the last but one element of a list.
  • Solution
• #3 Find the K'th element of a list.
  • Solution
• #4 Find the number of elements of a list.
  • Solution

https://wiki.haskell.org/H-99:_Ninety-Nine_Haskell_Problems

#1 Find the last element of a list.

λ> myLast [1,2,3,4]
4
λ> myLast ['x','y','z']
'z'

Solution

myLast [] = error "Cannot accept an empty list"
myLast [x] = x
myLast (_:xs) = myLast xs

myLast [1..5]

#2 Find the last but one element of a list.

λ> myButLast [1,2,3,4]
3
λ> myButLast ['a'..'z']
'y'

Solution

myButLast x
  | length x < 2 = error "Cannot accept an empty list"
  | otherwise = case x of
                  [x,y] -> x
                  (x:xs) -> myButLast xs

#3 Find the K'th element of a list.

The first element in the list is number 1.

λ> elementAt [1,2,3] 2
2
λ> elementAt "haskell" 5
'e'

Solution

elementAt (x:xs) i = if i == 1 then x else elementAt xs (i - 1)

#4 Find the number of elements of a list.

λ> myLength [123, 456, 789]
3
λ> myLength "Hello, world!"
13

Solution

myLength [] = 0
myLength xs = foldl (\acc _ -> acc + 1) 0 xs
-- or
myLength' [] = 0
myLength' [x] = 1
myLength' (_:xs) = 1 + myLength xs

myLength [1..5]
myLength' [1..5]

5