99 Haskell Problems

--Problem 1 : Find the last element of a list.
myLast [] = error "No end for empty lists!"
myLast (x:xs) = if xs==[] then x else myLast xs

--Problem 2 : Find the last but one element of a list.
myButLast [] = error "No end for empty lists!"
myButLast (x:xs) = if length xs==1 then x else myButLast xs

--Problem 3 : Find the K'th element of a list. The first element in the list is number 1.
elementAt :: [a]->Int->a
elementAt [] _ = error "No elements in empty lists!" 
elementAt a 1 = head a
elementAt (x:xs) k = if k > length (x:xs) then error "index out of bounds!" else elementAt xs (k-1)

--Problem 4 : Find the number of elements of a list.
myLength [] = 0
myLength [x] = 1
myLength (x:xs) = 1 + myLength xs

--Problem 5 : Reverse a list.
myReverse [] = []
myReverse (x:xs) = myReverse xs ++ [x] 

--Problem 6 : Find out whether a list is a palindrome. A palindrome can be read forward or backward; e.g. (x a m a x).
isPalindrome a 
  | a == myReverse a = True
  |otherwise = False
  
  
 --Problem 8 : (**) Eliminate consecutive duplicates of list elements.
compress [] = []
compress [x] = [x]
compress (x:y:xs) = if (x==y) then compress  (y:xs) else [x] ++ compress (y:xs)

--Problem 10 : Pack consecutive duplicates of list elements into sublists. If a list contains repeated elements they should be placed in separate sublists. 

--Quizz FS17 Asignment 3
pad2 :: Char -> String -> String
pad2 _ "" = ""
pad2 c (x:xs) = if (xs == []) then x:pad2 c xs else x:c:c:pad2 c xs

--Problem 14 : Duplicate the elements of a list.
dupli [] = []
dupli [x] = [x,x]
dupli (x:xs) = [x,x] ++ dupli xs

--Problem 15 : Replicate the elements of a list a given number of times.
repli a 0 = []
repli a 1 = a
repli "" _ = ""
repli (x:xs) k = helper x k ++ repli xs k

helper :: Char -> Int -> [Char]
helper x 0 = []
helper x k = [x]++helper x (k-1)

--Problem 16: Drop every N'th element from a list.

dropEvery list count = helper' list count count
  where helper' [] _ _ = []
        helper' (x:xs) count 1 = helper' xs count count
        helper' (x:xs) count n = x : (helper' xs count (n - 1)) 

--Problem 17 : Split a list into two parts; the length of the first part is given.


--Problem 19 :  Rotate a list N places to the left.
rotate [] _ = []
rotate a 0 = a
rotate (x:xs) n =  rotate (xs ++ [x]) (n-1)