[Haskell-cafe] Incremental array updates
ross at soi.city.ac.uk
Thu Feb 26 11:11:50 EST 2009
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
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
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.
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