Daniel Fischer wrote:
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.
I'm just reading about it, but didn't find anything except the Haddock
documentation... This may need some time to work out :)
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.
This was my first attempt at using `seq` for forcing strict evaluation,
and I seemingly did it wrong ;)
Thanks for all the quick answers!
Yours,
Daniel
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