Data commit

Task/Dutch-national-flag-problem/Haskell/dutch-national-flag-problem-1.hs

@@ -0,0 +1,26 @@
+import Data.List (sort)
+import System.Random (randomRIO)
+import System.IO.Unsafe (unsafePerformIO)
+
+data Color = Red | White | Blue deriving (Show, Eq, Ord, Enum)
+
+dutch :: [Color] -> [Color]
+dutch = sort
+
+isDutch :: [Color] -> Bool
+isDutch x = x == dutch x
+
+randomBalls :: Int -> [Color]
+randomBalls 0 = []
+randomBalls n = toEnum (unsafePerformIO (randomRIO (fromEnum Red,
+    fromEnum Blue))) : randomBalls (n - 1)
+
+main :: IO ()
+main = do
+    let a = randomBalls 20
+    case isDutch a of
+        True -> putStrLn $ "The random sequence " ++ show a ++
+            " is already in the order of the Dutch national flag!"
+        False -> do
+            putStrLn $ "The starting random sequence is " ++ show a ++ "\n"
+            putStrLn $ "The ordered sequence is " ++ show (dutch a)

Task/Dutch-national-flag-problem/Haskell/dutch-national-flag-problem-2.hs

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+inorder n = and $ zipWith (<=) n (tail n) -- or use Data.List.Ordered
+
+mk012 :: Int -> Int -> [Int]	-- definitely unordered
+mk012 n = (++[0]).(2:).map (`mod` 3).take n.frr where
+	-- frr = Fast Rubbish Randoms
+	frr = tail . iterate (\n -> n * 7 + 13)
+
+dutch1 n = (filter (==0) n)++(filter (==1) n)++(filter (==2) n)
+
+dutch2 n = a++b++c where
+	(a,b,c) = foldl f ([],[],[]) n -- scan list once; it *may* help
+	f (a,b,c) x = case x of
+		0 -> (0:a, b, c)
+		1 -> (a, x:b, c)
+		2 -> (a, b, x:c)
+
+main = do -- 3 methods, comment/uncomment each for speed comparisons
+--	print $ inorder $ sort s	-- O(n log n)
+--	print $ inorder $ dutch1 s	-- O(n)
+	print $ inorder $ dutch2 s	-- O(n)
+	where s = mk012 10000000 42