Ross Paterson wrote:
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:
Well, my main problem was the lazy evaluation... And using STUArray did
also speed it up notably, of course :) I finally managed to get it
working with it ;)
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.
For this example program... yes of course :) I'm trying to do so when
possible, but for my real problem I couldn't figure out a nice way to do
it like that, unfortunatelly. Which does not mean it is impossible of
course, but maybe just I need more experience in functional
programming... :D
Daniel
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