path: root/Haskell/mooc-exercises.hs

-- This are my solutions to the exercises from https://github.com/moocfi/haskell-mooc

import Data.List
import Data.Char
import Data.Either

---------
-- Set.01
-- https://github.com/moocfi/haskell-mooc/blob/master/exercises/Set1.hs

-- Ex. 1

one :: Integer -- function declaration: types of arguments and output
one = 1        -- function definition

two :: Integer
two = 2

-- Ex. 2

double :: Integer -> Integer
double x = x * 2

-- Ex. 3

quadruple :: Integer -> Integer
quadruple x = double (double x)

-- Ex. 4 

distance :: Double -> Double -> Double -> Double -> Double
-- exponentiation operator: ^ for integers, ^^ for fractionals and ** for floating point
distance x1 y1 x2 y2 = sqrt (abs(x1 - x2)**2 + abs(y1 - y2)**2)

-- Ex. 5

eeny :: Integer -> String
eeny x = if even x 
         then "eeny" 
         else "meeny"

-- Ex. 6

checkPassword :: String -> String
checkPassword password = if password == "swordfish" || password == "mellon"
                         then "You're in."
                         else "ACCESS DENIED!"

-- Ex. 7

postagePrice :: Int -> Int
postagePrice weight = if weight > 5000 then 6000
                      else if weight > 500 then 300 + weight * 1
                      else 250

-- Ex. 8

isZero :: Int -> Bool
isZero 0 = True -- this is pattern matching
isZero x = False

-- Ex. 9

sumTo :: Int -> Int
sumTo 1 = 1
sumTo n = n + sumTo (n - 1)

-- Ex. 10

power :: Int -> Int -> Int
power n 1 = n
power n k = n * power n (k - 1)

-- Ex. 11

ilog3 :: Int -> Int
ilog3 0 = 0
ilog3 n = 1 + ilog3 (div n 3)

----------
-- Set.02a
-- https://github.com/moocfi/haskell-mooc/blob/master/exercises/Set2a.hs

-- Functions you'll need: head, tail, take, drop, length, null
-- Can only use Data.List

-- Ex. 1

years = [1982, 2004, 2020]

-- Ex. 2

takeFinal :: Int -> [a] -> [a] -- a is the Haskell equivalent of generics (T) in C#
takeFinal n xs
    | n > length xs = xs
    | otherwise = drop (length xs - n) xs

-- Ex. 3

updateAt :: Int -> a -> [a] -> [a]
-- elements before i + x + elemenets after i ; ++ concatenates arrays
updateAt i x xs = take i xs ++ [x] ++ drop (i + 1) xs 

-- Ex. 4

substring :: Int -> Int -> String -> String
substring i j s = drop i (take j s)

-- Ex. 5

isPalindrome :: String -> Bool
isPalindrome s = s == reverse s

-- Ex. 6

palindromify :: String -> String
palindromify s = if isPalindrome s then s -- can easily make it: if s == reverse s then s , but decided this way was more readable
                 else palindromify (take (length s - 2) (drop 1 s))

-- Ex. 7

safeDiv :: Int -> Int -> String
safeDiv x y
    | y == 0 = "Nothing"
    | otherwise = "Just " ++ show (div x y) -- show casts the value to string

-- Ex. 8

greet :: String -> Maybe String -> String -- maybe means the value can (String) or can not be given (Nothing) ; you need to use pattern matching
greet firstName Nothing = "Hello, " ++ firstName ++ "!"
greet firstName (Just lastName) = "Hello, " ++ firstName ++ " " ++ lastName ++ "!"

-- Ex. 9

safeIndex :: [a] -> Int -> Maybe a
safeIndex xs i = if 0 > i || i >= length xs then Nothing
                 else Just (xs !! i)

-- Ex. 10

eitherDiv :: Int -> Int -> Either String Int -- a value type can be the left one or the right one (specified when given)
eitherDiv x 0 = Left (show x ++ "/0")
eitherDiv x y = Right (div x y)

-- Ex. 11

addEithers :: Either String Int -> Either String Int -> Either String Int
addEithers (Right x) (Right y) = Right (x + y)
addEithers (Right x) (Left y)  = Left y
addEithers (Left x)  (Right y) = Left x

----------
-- Set.02b
-- https://github.com/moocfi/haskell-mooc/blob/master/exercises/Set2b.hs

-- Ex. 1

binomial :: Int -> Int -> Int
binomial n 0 = 1
binomial 0 k
    | k > 0 = 0
    | otherwise = (-1)
binomial n k = binomial (n - 1) k + binomial (n - 1) (k - 1)

-- Ex. 2

oddFactorial :: Int -> Int
oddFactorial 1 = 1 -- technically having a pattern match for 3 is more efficient
oddFactorial n
    |even n = oddFactorial (n - 1)
    |otherwise = n * oddFactorial (n - 2)

-- Ex. 3

myGcd :: Int -> Int -> Int
myGcd 0 b = b
myGcd a 0 = a
myGcd a b
    | a > b = myGcd (a - b) b
    | otherwise = myGcd a (b - a)

-- Ex. 4

leftpad :: String -> Int -> String
leftpad s a = if length s >= a then s
              else leftpad (" " ++ s) a

-- Ex. 5

countdown :: Int -> String
countdown n = "Ready! " ++ count n ++ "Liftoff!"

count :: Int -> String  -- recursive helper function
count 0 = ""
count n = show n ++ "... " ++ count (n - 1) 

-- Ex. 6

smallestDivisor :: Int -> Int
smallestDivisor n = smallest n 2 

smallest :: Int -> Int -> Int
smallest n k  = if mod n k == 0 then k
                else smallest n (k + 1)

-- Ex. 7

isPrime :: Int -> Bool
isPrime 1 = False
isPrime n = smallestDivisor n == n

-- Ex. 8

biggestPrimeAtMost :: Int -> Int
biggestPrimeAtMost n = if isPrime n then n
                       else biggestPrimeAtMost (n - 1)

----------
-- Set.03a
-- https://github.com/moocfi/haskell-mooc/blob/master/exercises/Set3a.hs

-- Can only use Data.List, Data.Either, Data.Char

-- Ex. 1

maxBy :: (a -> Int) -> a -> a -> a
maxBy measure a b = if (measure a) > (measure b) then a else b

-- Ex. 2

mapMaybe :: (a -> b) -> Maybe a -> Maybe b
mapMaybe f Nothing = Nothing
mapMaybe f (Just a) = Just (f a)

-- Ex. 3

mapMaybe2 :: (a -> b -> c) -> Maybe a -> Maybe b -> Maybe c
mapMaybe2 f (Just a) Nothing = Nothing
mapMaybe2 f Nothing (Just b) = Nothing
mapMaybe2 f (Just a) (Just b) = Just (f a b) 

-- Ex. 4

palindromeHalfs :: [String] -> [String]
palindromeHalfs xs = map firstHalf (filter palindrome xs)

firstHalf :: String -> String
-- div rounds down to negative infinity, meaning 2.5 will be rounded down to 2 and -2.5 to -3
-- using this feature, we can round up a division of integers by one of them being negative and taking the absolute result
firstHalf s = take (abs (div (length s) (-2))) s 

palindrome :: String -> Bool
palindrome s = s == reverse s

-- Ex. 5

capitalize :: String -> String
capitalize s = unwords (map capitlizeFirst (words s))
    where capitlizeFirst s = [toUpper (head s)] ++ tail s -- helper function, only available in capitalize

-- Ex. 6

powers :: Int -> Int -> [Int]
powers k max = filter (<=max) (map (\x -> k ^ x) [0..max])

-- Ex. 7

while :: (a -> Bool) -> (a -> a) -> a -> a
while check update value = if check value then while check update (update value)
                           else value

-- Ex. 8

whileRight :: (a -> Either b a) -> a -> b
whileRight f x = either (\y -> y) (\y -> whileRight f y) (f x)

step :: Int -> Int -> Either Int Int -- function for testing
step k x = if x < k then Right (2*x) else Left x

-- Ex. 9