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".
2009/2/26 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.
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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