Am Donnerstag, 26. Februar 2009 14:53 schrieb Daniel Kraft:
Hi,
I seem to be a little stuck with incremental array updates... I've got an array of "few" (some thousand) integers, but have to do a calculation where I incrementally build up the array. For sake of simplicity, for now I don't use a State-Monad but simply pass the array as state down the recursive calls.
Unfortunatelly, I seem to experience problems with lazyness; when I run my program, memory usage goes up to horrific values! The simple program below reproduces this; when run on my machine, it uses up about 300 MB of real and 300 MB of virtual memory, and the system starts swapping like mad! I compiled with ghc --make -O3, ghc version 6.8.3 if that matters.
As Eugene already said, use STUArray (STArray if you need some laziness). It's much much faster.
BTW, I tried to make the array update strict using the `seq` as below, but with no effect... What am I doing wrong here?
Many thanks, Daniel
import Data.Array;
arraySize = 1000 limit = 100000
type MyArray = Array Int Int
emptyArray :: MyArray emptyArray = array (0, arraySize - 1) [ (i, 0) | i <- [0 .. arraySize - 1] ]
procOne :: Int -> MyArray -> MyArray procOne a cnt
| a > limit = cnt | otherwise =
let ind = a `mod` arraySize oldcnt = cnt ! ind newarr = cnt // [(ind, oldcnt + 1)] in procOne (a + 1) (newarr `seq` newarr)
Note that x `seq` x is exactly equivalent to just writing x, it will be evaluated if and only if x is required to be evaluated by something else. Try let ind = a `mod` arraySize oldcnt = cnt ! ind newarr = cnt // [(ind,oldcnt+1)] newcnt = newarr ! ind in newcnt `seq` procOne (a+1) newarr to force newarr to be evaluated, so the old array is eligible for garbage collection.
main :: IO () main = do arr <- return $ procOne 0 emptyArray print $ arr ! 42
Cheers, Daniel