chap1.hs

-- Find the penultimate element in list l
penultimate l = last (init l)

-- Find the element at index k in list l
-- For example: "findK 2 [0,0,1,0,0,0]" returns 1
findK k l = l !! k

-- Determine if list l is a palindrome
isPalindrome l = l == reverse l

{-
 - Duplicate the elements in list xs, for example "duplicate [1,2,3]" would give the list [1,1,2,2,3,3]
 - Hint: The "concat [l]" function flattens a list of lists into a single list. 
 - (You can see the function definition by typing ":t concat" into the interpreter. Perhaps try this with other variables and functions)
 -
 - For example: concat [[1,2,3],[3,4,5]] returns [1,2,3,3,4,5]
 -}
duplicate xs = concat [[x,x] | x <- xs]

{-
 - Imitate the functinality of zip
 - The function "min x y" returns the lower of values x and y
 - For example "ziplike [1,2,3] ['a', 'b', 'c', 'd']" returns [(1,'a'), (2, 'b'), (3, 'c')]
 -}
ziplike xs ys = [(xs!!i, ys!!i) | i <- [0..(min (length xs) (length ys)) -1]]

-- Split a list l at element k into a tuple: The first part up to and including k, the second part after k
-- For example "splitAtIndex 3 [1,1,1,2,2,2]" returns ([1,1,1],[2,2,2])
splitAtIndex k l = (take k l, drop k l)

-- Drop the element at index k in list l
-- For example "dropK 3 [0,0,0,1,0,0,0]" returns [0,0,0,0,0,0]
dropK k l = take (k-1) l ++ drop k l

-- Extract elements between ith and kth element in list l. Including i, but not k
-- For example, "slice 3 6 [0,0,0,1,2,3,0,0,0]" returns [1,2,3]
slice i k l = [l!!j | j <- [i..k-1]]
slice' i k l = drop i (take k l)

-- Insert element x in list l at index k
-- For example, "insertElem 2 5 [0,0,0,0,0,0]" returns [0,0,0,0,0,2,0]
insertElem x k l = take (k) l ++ [x] ++ drop k l
insertElem' x k l = take k l ++ (x:(drop k l)) -- more efficiant to use 

-- Rotate list l n places left.
-- For example, "rotate 2 [1,2,3,4,5]" gives [3,4,5,1,2]
rotate n l = drop n l ++ take n l