Don Stewart wrote:
alex:
<snip>
You can replace most of your loops with Data.List functions, and
simplify the code overall by threading around a lazy list of randoms,
rather than calling into IO all the time:
import System.Random
import System.Environment
import Data.List
import Control.Monad
randMax = 32767
unitRadius = randMax * randMax
countPair :: (Int, Int) -> Int
countPair (x, y) = fromEnum (x*x + y*y < unitRadius)
calculatePi total g = fromIntegral (4*count) / fromIntegral total
where
count = sum
. map countPair
. take total
$ zip (randomRs (0,randMax) a)
I wish I'd known about randomRs a couple of hours ago :-)
(randomRs (0,randMax) b)
(a,b) = split g
main = do
[v] <- getArgs
g <- newStdGen
print $ calculatePi (read v) g
Compiled like so:
$ ghc -O2 A.hs -o A
$ time ./A 100000
3.13548
./A 100000 0.08s user 0.02s system 98% cpu 0.101 total
We get no stack overflow.
Not with -O2, but:
C:\Users\Alex\Documents\HaskellLearning\MonteCarlo>ghc BetterPi.hs
C:\Users\Alex\Documents\HaskellLearning\MonteCarlo>main.exe 1000000
Stack space overflow: current size 8388608 bytes.
Use `+RTS -Ksize' to increase it.
But:
C:\Users\Alex\Documents\HaskellLearning\MonteCarlo>ghc -O2 BetterPi.hs
C:\Users\Alex\Documents\HaskellLearning\MonteCarlo>main.exe 1000000
3.140636
This is a little confusing. Is there a simple explanation for this
behaviour, or is it just a matter of "always use -O2 unless there's a
reason not to?"
Thanks - I really appreciate the explanations.
(P.S. Again - sorry for the duplication. I really need to watch my
mail client more closely :-)
--
Alex
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