Re: [Haskell-cafe] How to use arrays efficiently?

16 May 2008


      Thanks again. Here is my solution in case that somebody else runs into
similar problem. This isn't very elegant and I would be interested to know
if somebody has a better solution. Surely many people have encountered this
kind of problem where you want to force evaluation of some expression at
given point of program flow. Here evaluation of sum must be forced for every
update of array element or otherwise memory consumption is proportional to
the number of samples (100 000) instead of being proportional to the size of
the array (10).

---
import Data.Array.IO
import Test.QuickCheck
import System.Random

uniformSampler :: Gen Double
uniformSampler = choose (0,1)

withSeed sampler seed = generate 1 (mkStdGen seed) sampler

ac = 10
sc = 100000

triplet = do
    i <- uniformSampler
    s <- uniformSampler
    t <- uniformSampler
    return (round $ i * fromIntegral ac, (s,t))

sampling = sequence $ repeat triplet
samples = take sc $ withSeed sampling 1

showElems xs = foldr1 (++) [show x ++ "\n" | x <- xs]

main = do
    a1 <- newArray (0,ac) 0 :: IO (IOUArray Int Double)
    a2 <- newArray (0,ac) 0 :: IO (IOUArray Int Double)
    let addtoElem i s t = do
            s' <- readArray a1 i
            writeArray a1 i (s'+s)
            t' <- readArray a2 i
            writeArray a2 i (t'+t)
        writes = [addtoElem i s t | (i,(s,t)) <- samples]
    sequence writes
    ss <- getElems a1
    ts <- getElems a2
    putStrLn $ showElems (zip ss ts)
---

Regards,
Lauri


On Fri, May 16, 2008 at 2:52 PM, Abhay Parvate 
wrote:
...
As far as I know, you can't. It needs machine representable types, such as
Int, Double, Char, etc. But making a tuple of three UArray Int Double may
help.
2008/5/16 Lauri Oksanen :
...
Thanks for help. I did some tests with UArray and it does the trick.
The problem remaining is, how to implement UArray Int (Double, Double,
Double)?
UArray source code is far too cryptic for me.
Regards,
Lauri
On Fri, May 16, 2008 at 11:37 AM, Bulat Ziganshin <
bulat.ziganshin@gmail.com> wrote:
...
Hello Lauri,
Friday, May 16, 2008, 12:19:29 PM, you wrote:
...
pixelArray :: Array Int Color
it's boxed array which means that its elements are stored as thunks
computed only when you actually use them. try UArray instead:
http://haskell.org/haskellwiki/Modern_array_libraries
--
Best regards,
 Bulat                            mailto:Bulat.Ziganshin@gmail.com
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