Adding Haskell examples (#17)

* Adding binary search example for Haskell

* Adding selection sort example in Haskell

* Adding Haskell examples for chapter 3

* Adding examples for chapter 4

* Adding examples for chapter 5

* Adding git ignore

* Add Haskell example for BFS

.gitignore

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+.DS_Store

01_introduction_to_algorithms/Haskell/01_binarysearch.hs

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+import Data.Array
+
+binarysearch :: Integer -> Array Integer Integer -> Integer -> Integer -> Maybe Integer
+binarysearch x arr low high
+  | low > high = Nothing
+  | mid > x = binarysearch x arr low (high - 1)
+  | mid < x = binarysearch x arr (low + 1) high
+  | otherwise = Just mid
+  where
+    mid = arr ! ((low + high) `div` 2)
+
+
+find :: Integer -> Array Integer Integer -> Maybe Integer
+find x arr = binarysearch x arr low high
+  where p = bounds arr
+        low = fst p
+        high = snd p
+
+-- Usage
+-- let arr = array (0, 4) [(i, i*i) | i <- [0..4]]
+-- find 4 arr

02_selection_sort/Haskell/01_selectionsort.hs

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+
+helper :: (Integer, [Integer]) -> Integer -> (Integer, [Integer])
+helper (min, xs) e
+  | min <= e = (min, e : xs)
+  | otherwise = (e, min : xs)
+
+minandrest :: [Integer] -> (Integer, [Integer])
+minandrest (x:xs) = foldl helper (x, []) xs
+
+selectionsort :: [Integer] -> [Integer]
+selectionsort [] = []
+selectionsort xs = min : selectionsort rest
+  where (min, rest) = minandrest xs
+
+-- Usage
+-- selectionsort [1,4,2,3,5,6,0,7,8,9,1,9,0]

03_recursion/Haskell/01_countdown.hs

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+countdown :: Integer -> IO()
+countdown n
+  | n < 0 = return ()
+  | otherwise = do
+      putStrLn (show n)
+      countdown (n-1)
+
+main = do
+  countdown 5

03_recursion/Haskell/02_greet.hs

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+
+greet2 name = putStrLn ("how are you, " ++ name ++ "?")
+
+bye = putStrLn "ok bye!"
+
+greet name = do
+  putStrLn ("hello " ++ name)
+  greet2 name
+  putStrLn "getting ready to say bye..."
+  bye
+
+main = greet "adit"

03_recursion/Haskell/03_factorial.hs

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+
+fact n
+  | n <= 1 = 1
+  | otherwise = n * (fact (n-1))
+
+main = (putStrLn . show . fact) 5

04_quicksort/Haskell/01_02_loop_sum.hs

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+
+sumarr [] = 0
+sumarr (x: xs) = x + sumarr xs
+
+main = putStrLn (show (sumarr [1,2,3,4]))

04_quicksort/Haskell/03_recursive_count.hs

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+
+countarr [] = 0
+countarr (x:xs) = 1 + countarr xs
+
+main = (putStrLn . show . countarr) [0, 1, 2, 3, 4, 5]

04_quicksort/Haskell/04_recursive_max.hs

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+
+maxarr (x: []) = x
+maxarr (x:xs) = if x > maxofrest then x else maxofrest
+  where maxofrest = maxarr xs
+
+main = (putStrLn . show . maxarr) [1, 5, 10, 25, 16, 1]

04_quicksort/Haskell/05_quicksort.hs

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+import Data.List
+
+quicksort [] = []
+quicksort (x: []) = [x]
+quicksort (x:xs) = (quicksort lessthan) ++ [x] ++ (quicksort greaterthan)
+  where (lessthan, greaterthan) = partition (<= x) xs
+
+main = (putStrLn . show . quicksort) [0,9,4,5,6,3,1,0,1]

05_hash_tables/Haskell/01_price_of_groceries.hs

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+import qualified Data.HashMap.Strict as Map
+
+prices = Map.fromList [("Apple", 0.67), ("Milk", 1.49), ("Avocado", 1.55)]
+
+main = putStrLn (show (Map.lookup "Apple" prices))

05_hash_tables/Haskell/02_check_voter.hs

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+import qualified Data.HashMap.Strict as Map
+
+all_voters = Map.fromList[("tom", True), ("mike", True)]
+
+check_voter name voters_map = case Map.lookup name voters_map of
+  Just _ -> ("kick them out!", voters_map)
+  otherwise -> ("let them vote!", Map.insert name True voters_map)
+
+log_vote name voters_map = do
+  putStrLn str
+  return new_voters_map
+  where (str, new_voters_map) = check_voter name voters_map
+
+main = do
+  try_mike <- log_vote "mike" all_voters
+  try_john <- log_vote "john" try_mike
+  return try_john

06_breadth-first_search/Haskell/01_bfs.hs

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+import qualified Data.HashMap.Strict as Map
+import Control.Applicative
+
+graph = Map.fromList [
+  ("you", ["alice", "bob", "claire"]),
+  ("bob", ["anuj", "peggy"]),
+  ("alice", ["peggy"]),
+  ("claire", ["mangoes", "jonny"]),
+  ("anuj", []),
+  ("peggy", []),
+  ("mangoes", []),
+  ("jonny", [])
+  ]
+
+connections name = Map.lookup name graph
+
+bfs tosearch searched = case tosearch of
+  Just (x: xs)
+    | x == "mangoes" -> Just x
+    | x `elem` searched -> bfs (Just xs) searched
+    | otherwise -> bfs ((++) <$> Just xs <*> (connections x)) (x : searched)
+  _ -> Nothing
+
+main = do
+  putStrLn (show (bfs (Just ["you"]) []))