Did set 2b and part of set 3a from the mooc haskell exercises

1 files changed, 135 insertions, 1 deletions

diff --git a/Haskell/mooc-exercises.hs b/Haskell/mooc-exercises.hs
index 879ec8f..77e49d9 100644
--- a/Haskell/mooc-exercises.hs
+++ b/Haskell/mooc-exercises.hs
@@ -1,6 +1,8 @@
 -- 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
@@ -80,6 +82,7 @@ ilog3 n = 1 + ilog3 (div n 3)
 -- 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
 
@@ -135,7 +138,7 @@ safeIndex xs i = if 0 > i || i >= length xs then Nothing
 
 -- Ex. 10
 
-eitherDiv :: Int -> Int -> Either String Int -- a value can be the left one or the right one (specified when given)
+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)
 
@@ -145,3 +148,134 @@ 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
+