Haskellでモンテカルロ積分

montecarlo.hs

import System.Random
import Data.List

main :: IO ()
main = do
    xGen <- getStdGen
    yGen <- newStdGen
    let n = 100000
    let xs = take n $ randomRs (0,1) xGen
        ys = take n $ randomRs (0,1) yGen
    print $ calPi n xs ys

calPi :: Int -> [Double] -> [Double] -> Double
calPi n xs ys = 4 * count / (fromIntegral n)
    where count = foldl' (+) 0 $ map cond $ zip xs ys

cond :: (Double,Double) -> Double
cond (x,y) | x**2 + y**2 < 1 = 1
           | otherwise = 0