[Haskell-cafe] In-place modification

[Hugh Perkins]

On 7/15/07, Derek Elkins <derek.a.elkins at gmail.com> wrote:
>
> Read http://www.cs.hmc.edu/~oneill/papers/Sieve-JFP.pdf
>


Ok, so switched to using the Data.Map version from this paper, which looks
like a lazy, but real, version of the sieve of Arostothenes.

This does run quite a lot faster, so we're going to run on a sieve of
1000000 to increase the timings a bit (timings on 200000 in C# are a bit
inaccurate...).

Here are the results:

J:\dev\haskell>ghc -O2 -fglasgow-exts -o Prime2.exe Prime2.hs

J:\dev\haskell>prime2
number of primes: 78493
19.547

J:\dev\test\testperf>csc /nologo primecs.cs

J:\dev\test\testperf>primecs
number of primes: 78498
elapsed time: 0,0625

So, only 300 times faster this time ;-)

Here's the Haskell code:

module Main
   where


import IO
import Char
import GHC.Float
import List
import qualified Data.Map as Map
import Control.Monad
import System.Time
import System.Locale

sieve xs = sieve' xs Map.empty
   where
      sieve' [] table = []
      sieve' (x:xs) table =
         case Map.lookup x table of
            Nothing -> ( x : sieve' xs (Map.insert (x*x) [x] table) )
            Just facts -> (sieve' xs (foldl reinsert (Map.delete x table)
facts))
           where
             reinsert table prime = Map.insertWith (++) (x+prime) [prime]
table

calculateNumberOfPrimes :: Int -> Int
calculateNumberOfPrimes max = length (sieve [ 2.. max ])

gettime :: IO ClockTime
gettime = getClockTime

main = do starttime <- gettime
          let numberOfPrimes = (calculateNumberOfPrimes 1000000)
          putStrLn( "number of primes: " ++ show( numberOfPrimes ) )
          endtime <- gettime
          let timediff = diffClockTimes endtime starttime
          let secondsfloat = realToFrac( tdSec timediff ) +
realToFrac(tdPicosec timediff) / 1000000000000
          putStrLn( show(secondsfloat) )
          return ()
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