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