haskell 1

lesson1.hs

-- exercise 1

toDigitsRev n = if (n <= 0)
    then []
    else (n `mod` 10) : (toDigitsRev (n `div` 10))

toDigits = reverse . toDigitsRev

-- exercise 2

doubleEveryOther = reverse . doubleHelperOdd . reverse

doubleHelperOdd [] = []
doubleHelperOdd (x:xs) = x : (doubleHelperEven xs)

doubleHelperEven [] = []
doubleHelperEven (x:xs) = (2 * x) : (doubleHelperOdd xs)

-- exercise 3

sumDigitsX [] = 0
sumDigitsX (x:xs) =
  let sumFirst = if (x>10)
                 then (1 + (x `mod` 10))
                 else (x `mod` 10)
  in
  sumFirst + (sumDigitsX xs)

sumDigitsY ns = sum (foldl (++) [] (map toDigits ns))
sumDigitsZ ns = sum (concatMap toDigits ns)
sumDigits     = sum . (concatMap toDigits)

-- exercise 4

validate digits =
   remainder == 0
   where digitSum  = (sumDigits . doubleEveryOther . toDigits) digits
         remainder = digitSum `mod` 10

-- exercise 5

type Peg = String
type Move = (Peg, Peg)
hanoi :: Integer -> Peg -> Peg -> Peg -> [Move]

hanoi 0 a b c = []
--hanoi 1 a b c = [(a,b)]
hanoi n a b c = (hanoi (n - 1) a c b) ++
                [(a,b)] ++
                (hanoi (n - 1) c b a)