Initial data commit

Task/Statistics-Basic/Haskell/statistics-basic.hs

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+import Data.Foldable (foldl') --'
+import System.Random (randomRs, newStdGen)
+import Control.Monad (zipWithM_)
+import System.Environment (getArgs)
+
+intervals :: [(Double, Double)]
+intervals = map conv [0 .. 9]
+  where
+    xs = [0.0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0]
+    conv s =
+      let [h, l] = take 2 $ drop s xs
+      in (h, l)
+
+count :: [Double] -> [Int]
+count rands = map (\iv -> foldl'' (loop iv) 0 rands) intervals
+  where
+    loop :: (Double, Double) -> Int -> Double -> Int
+    loop (lo, hi) n x
+      | lo <= x && x < hi = n + 1
+      | otherwise = n
+
+-- ^ fuses length and filter within (lo,hi)
+data Pair a b =
+  Pair !a
+       !b
+
+-- accumulate sum and length in one fold
+sumLen :: [Double] -> Pair Double Double
+sumLen = fion2 . foldl'' (\(Pair s l) x -> Pair (s + x) (l + 1)) (Pair 0.0 0)
+  where
+    fion2 :: Pair Double Int -> Pair Double Double
+    fion2 (Pair s l) = Pair s (fromIntegral l)
+
+-- safe division on pairs
+divl :: Pair Double Double -> Double
+divl (Pair _ 0.0) = 0.0
+divl (Pair s l) = s / l
+
+-- sumLen and divl are separate for stddev below
+mean :: [Double] -> Double
+mean = divl . sumLen
+
+stddev :: [Double] -> Double
+stddev xs = sqrt $ foldl'' (\s x -> s + (x - m) ^ 2) 0 xs / l
+  where
+    p@(Pair s l) = sumLen xs
+    m = divl p
+
+main = do
+  nr <- read . head <$> getArgs
+  -- or in code, e.g.  let nr = 1000
+  rands <- take nr . randomRs (0.0, 1.0) <$> newStdGen
+  putStrLn $ "The mean is " ++ show (mean rands) ++ " !"
+  putStrLn $ "The standard deviation is " ++ show (stddev rands) ++ " !"
+  zipWithM_
+    (\iv fq -> putStrLn $ ivstr iv ++ ": " ++ fqstr fq)
+    intervals
+    (count rands)
+  where
+    fqstr i =
+      replicate
+        (if i > 50
+           then div i (div i 50)
+           else i)
+        '*'
+    ivstr (lo, hi) = show lo ++ " - " ++ show hi
+
+-- To avoid Wiki formatting issue
+foldl'' = foldl'