# [Haskell-cafe] How to use arrays efficiently?

Lauri Oksanen lassoken at gmail.com
Fri May 16 08:25:57 EDT 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
writeArray a1 i (s'+s)
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 <abhay.parvate at gmail.com>
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 <lassoken at gmail.com>:
>
>> 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 at 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:
>>>
>>>
>>>
>>> --
>>> Best regards,
>>>  Bulat                            mailto:Bulat.Ziganshin at gmail.com
>>>
>>>
>>
>> _______________________________________________