Eugene Kirpichov wrote:
You need to use STUArray. The Data.Array
completely-immutable-and-boxed arrays are ok only for tasks where you
build an array once and don't modify it, and where you need array
elements to be lazy.
The // operation creates a copy of the whole array with one element
modified. It should probably be called
"turtleSlowCopyWholeArrayAndModifySingleElement".
Thanks for the hint!
However, I still don't see why this increments memory usage so much...
Shouldn't even in this case the garbage collector remove the old, no
longer needed versions of the array? My main point of concern is not
that the program may run slowly, but that it uses that much memory.
But of course for the real program this hint is surely very useful!
I'll give it a try...
Thanks,
Daniel
2009/2/26 Daniel Kraft <[email protected]>:
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.
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)
main :: IO ()
main =
do
arr <- return $ procOne 0 emptyArray
print $ arr ! 42
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