On Thu, Feb 26, 2009 at 02:53:42PM +0100, Daniel Kraft wrote:
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 noted above, updating an array one element at a time is the problem. But before writing an imperative program with STUArray, you might try building a whole new Array, specifying a new value for each element, using array or accumArray. These are often enough. In your simple example: procOne :: Int -> MyArray -> MyArray procOne a cnt | a >= limit = cnt | otherwise = procOne (a + arraySize) $! array (0, arraySize - 1) [ (i, cnt!i + 1) | i <- [0 .. arraySize - 1] ] If elements of the new array depend on previously computed elements of the same array, you can define the array recursively.