Immutable array update creates a new array for each call to update (//),
thus if you need frequent update, it is recommended to use MArray (such as
ST or IO variant) instead.
i.e: the loop can be rewrite as:
update_ u x = readArray u x >>= \old -> writeArray u x (1+old)
array100 = runSTUArray $ do
stu <- newArray (0, 99) 0 :: ST s (STUArray s Int Int)
mapM_ (update_ stu) (concat . replicate 5000 $ [0..99])
return stu
- baojun
On Tue, Apr 26, 2016 at 4:53 PM Chul-Woong Yang
Hi, all
When I fold a list to update Data.Array, memory usage is very high. Consider following source, which counts occurence of each element in a list (1):
import Data.Array import Data.List import Control.DeepSeq go :: [Int] -> [Int] go = elems . foldl' update (array (0,99) [(i,0) | i <- [0..99]]) where update acc x = acc // [(x, acc ! x + 1)] main = putStrLn . unwords . map show . go . concat . replicate 5000 $ [0..99]
Above program uses about 350MB at heap. Memory usage is same if I try to force strictness in array update with `seq` (2) :
where update acc x = let v = acc ! x + 1 a' = acc // [(x,v `seq` v)] in a' `seq` a'
However, when I use `deepseq`, memory usage is gone (around 35Kbyte) (3):
where update acc x = let v = acc ! x + 1 a' = acc // [(x,v `seq` v)] in a' `deepseq` a'
What's the missing part in (2)? At (2), evaluation of updated array a' is forced and the value of array cell is also evaluated forcefully with `seq`.
Any help would be appreciated deeply. -- Regards, Chul-Woong Yang _______________________________________________ Beginners mailing list Beginners@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners