Difference between revisions of "Introduction/Direct Translation"
(Error in quickSort) |
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import Data.Array.IArray |
import Data.Array.IArray |
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import Data.Array.MArray |
import Data.Array.MArray |
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− | import System.IO.Unsafe |
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− | qsort :: (IArray a e,Ix i,Enum i,Ord e) => a i e -> a i e |
+ | qsort :: (IArray a e, Ix i, Enum i, Ord e) => a i e -> a i e |
qsort arr = processArray quickSort arr |
qsort arr = processArray quickSort arr |
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− | processArray |
+ | processArray :: (IArray a e, IArray b e, Ix i) |
⚫ | |||
− | :: (IArray a e,IArray b e,Ix i) |
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+ | processArray f (arr :: a i e) = runST $ do |
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⚫ | |||
− | + | arr' <- thaw arr :: ST s (STArray s i e) |
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⚫ | |||
− | arr' <- thaw arr :: ST s (STArray s i e) |
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− | + | unsafeFreeze arr' |
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− | unsafeFreeze arr') |
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quickSort :: (MArray a e m, Ix i, Enum i, Ord e) => a i e -> m () |
quickSort :: (MArray a e m, Ix i, Enum i, Ord e) => a i e -> m () |
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− | quickSort arr = |
+ | quickSort arr = do |
− | + | (lo, hi) <- getBounds arr |
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+ | qsort' lo hi |
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− | | otherwise = do |
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+ | where |
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⚫ | |||
− | + | qsort' lo hi | lo >= hi = return () |
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− | + | | otherwise = do |
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− | + | p <- readArray arr hi |
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− | + | l <- mainLoop p lo hi |
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− | + | swap l hi |
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− | + | qsort' lo (pred l) |
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− | + | qsort' (succ l) hi |
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− | l' <- doTil (\l' b -> l' < h && b <= p) succ l |
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− | h' <- doTil (\h' b -> h' > l' && b >= p) pred h |
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⚫ | |||
⚫ | |||
⚫ | |||
⚫ | |||
⚫ | |||
⚫ | |||
⚫ | |||
− | + | mainLoop p l h | l >= h = return l |
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− | + | | otherwise = do |
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− | + | l' <- doTil (\l' b -> l' < h && b <= p) succ l |
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− | + | h' <- doTil (\h' b -> h' > l' && b >= p) pred h |
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⚫ | |||
⚫ | |||
⚫ | |||
+ | |||
⚫ | |||
⚫ | |||
⚫ | |||
+ | |||
+ | swap xi yi = do |
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⚫ | |||
+ | readArray arr yi >>= writeArray arr xi |
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+ | writeArray arr yi x |
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</haskell> |
</haskell> |
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Revision as of 13:48, 19 May 2008
The quicksort quoted in Introduction isn't the "real" quicksort and doesn't scale for longer lists like the c code does.
http://programming.reddit.com/info/5yutf/comments/
Here are some points to how the "real" quicksort would look in haskell.
Lennart Augustsson has a quicksort entry on his blog which is pure (no unsafe):
http://augustss.blogspot.com/2007/08/quicksort-in-haskell-quicksort-is.html
Another version (uses System.IO.Unsafe), is below.
There is also a "parallel" quicksort at
http://www.macs.hw.ac.uk/~dsg/gph/nofib/
roconnor claims that in haskell the "real" quicksort is really a treesort:
http://programming.reddit.com/info/2h0j2/comments
Unfortunately none of the above "real" quicksorts seems to compile as given, when copy/pasted into ghci. Can someone fix? The "parallel" quicksort gave error "unknown package concurrent" when I ran make in quicksort/gransim.
Has anyone got a functioning "real" quicksort that works on copy/paste?
import Control.Monad (when)
import Control.Monad.ST
import Data.Array.ST
import Data.Array.IArray
import Data.Array.MArray
qsort :: (IArray a e, Ix i, Enum i, Ord e) => a i e -> a i e
qsort arr = processArray quickSort arr
processArray :: (IArray a e, IArray b e, Ix i)
=> (forall s. (STArray s) i e -> ST s ()) -> a i e -> b i e
processArray f (arr :: a i e) = runST $ do
arr' <- thaw arr :: ST s (STArray s i e)
f arr'
unsafeFreeze arr'
quickSort :: (MArray a e m, Ix i, Enum i, Ord e) => a i e -> m ()
quickSort arr = do
(lo, hi) <- getBounds arr
qsort' lo hi
where
qsort' lo hi | lo >= hi = return ()
| otherwise = do
p <- readArray arr hi
l <- mainLoop p lo hi
swap l hi
qsort' lo (pred l)
qsort' (succ l) hi
mainLoop p l h | l >= h = return l
| otherwise = do
l' <- doTil (\l' b -> l' < h && b <= p) succ l
h' <- doTil (\h' b -> h' > l' && b >= p) pred h
when (l' < h') $
swap l' h'
mainLoop p l' h'
doTil p op ix = do
b <- readArray arr ix
if p ix b then doTil p op (op ix) else return ix
swap xi yi = do
x <- readArray arr xi
readArray arr yi >>= writeArray arr xi
writeArray arr yi x
This uses various extensions to make the types ridiculously general, but the actual algorithm (quickSort) is plain Haskell.