Data commit

Task/Monty-Hall-problem/Haskell/monty-hall-problem-1.hs

@@ -0,0 +1,34 @@
+import System.Random (StdGen, getStdGen, randomR)
+
+trials :: Int
+trials = 10000
+
+data Door = Car | Goat deriving Eq
+
+play :: Bool -> StdGen -> (Door, StdGen)
+play switch g = (prize, new_g)
+  where (n, new_g) = randomR (0, 2) g
+        d1 = [Car, Goat, Goat] !! n
+        prize = case switch of
+            False -> d1
+            True  -> case d1 of
+                Car  -> Goat
+                Goat -> Car
+
+cars :: Int -> Bool -> StdGen -> (Int, StdGen)
+cars n switch g = f n (0, g)
+  where f 0 (cs, g) = (cs, g)
+        f n (cs, g) = f (n - 1) (cs + result, new_g)
+          where result = case prize of Car -> 1; Goat -> 0
+                (prize, new_g) = play switch g
+
+main = do
+    g <- getStdGen
+    let (switch, g2) = cars trials True g
+        (stay, _) = cars trials False g2
+    putStrLn $ msg "switch" switch
+    putStrLn $ msg "stay" stay
+  where msg strat n = "The " ++ strat ++ " strategy succeeds " ++
+            percent n ++ "% of the time."
+        percent n = show $ round $
+            100 * (fromIntegral n) / (fromIntegral trials)

Task/Monty-Hall-problem/Haskell/monty-hall-problem-2.hs

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+import Control.Monad.State
+
+play :: Bool -> State StdGen Door
+play switch = do
+    i <- rand
+    let d1 = [Car, Goat, Goat] !! i
+    return $ case switch of
+        False -> d1
+        True  -> case d1 of
+            Car  -> Goat
+            Goat -> Car
+  where rand = do
+            g <- get
+            let (v, new_g) = randomR (0, 2) g
+            put new_g
+            return v
+
+cars :: Int -> Bool -> StdGen -> (Int, StdGen)
+cars n switch g = (numcars, new_g)
+  where numcars = length $ filter (== Car) prize_list
+        (prize_list, new_g) = runState (replicateM n (play switch)) g

Task/Monty-Hall-problem/Haskell/monty-hall-problem-3.hs

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+The switch strategy succeeds 67% of the time.
+The stay strategy succeeds 34% of the time.