Data commit

Task/Cumulative-standard-deviation/Haskell/cumulative-standard-deviation-1.hs

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+{-# LANGUAGE BangPatterns #-}
+
+import Data.List (foldl') -- '
+import Data.STRef
+import Control.Monad.ST
+
+data Pair a b = Pair !a !b
+
+sumLen :: [Double] -> Pair Double Double
+sumLen = fiof2 . foldl' (\(Pair s l) x -> Pair (s+x) (l+1)) (Pair 0.0 0) --'
+  where fiof2 (Pair s l) = Pair s (fromIntegral l)
+
+divl :: Pair Double Double -> Double
+divl (Pair _ 0.0) = 0.0
+divl (Pair s   l) = s / l
+
+sd :: [Double] -> Double
+sd xs = sqrt $ foldl' (\a x -> a+(x-m)^2) 0 xs / l --'
+  where p@(Pair s l) = sumLen xs
+        m = divl p
+
+mkSD :: ST s (Double -> ST s Double)
+mkSD = go <$> newSTRef []
+  where go acc x =
+          modifySTRef acc (x:) >> (sd <$> readSTRef acc)
+
+main = mapM_ print $ runST $
+  mkSD >>= forM [2.0, 4.0, 4.0, 4.0, 5.0, 5.0, 7.0, 9.0]

Task/Cumulative-standard-deviation/Haskell/cumulative-standard-deviation-2.hs

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+import Data.List (mapAccumL)
+
+
+-------------- CUMULATIVE STANDARD DEVIATION -------------
+
+cumulativeStdDevns :: [Float] -> [Float]
+cumulativeStdDevns = snd . mapAccumL go (0, 0) . zip [1.0..]
+  where
+    go (s, q) (i, x) =
+      let _s = s + x
+          _q = q + (x ^ 2)
+      in ((_s, _q), sqrt ((_q / i) - ((_s / i) ^ 2)))
+
+
+
+--------------------------- TEST -------------------------
+main :: IO ()
+main = mapM_ print $ cumulativeStdDevns [2, 4, 4, 4, 5, 5, 7, 9]