Re: [Haskell-cafe] [Newbie] Problem with Data.Map (or something else ?)

chaddai.fouche:

2008/3/31, Bruno Carnazzi :

...

Dears Haskellers,

As an Haskell newbie, I'm learning Haskell by trying to resolve Euler Project problems (http://projecteuler.net/ ). I'm hanging on problem 14 (Collatz problem).

I've written the following program... Which does not end in a reasonable time :( My algorithm seems ok to me but I see that memory consumption is gigantic... Is this a memory problem with Data.Map ? Or an infinite loop ? (Where ?) In a more general way, how can I troubleshoot these kind of problem ?

Others have pointed potential source of memory leaks, but I must say that using Data.Map for the cache in the first place appear to me as a very bad idea... Data.Map by nature take much more place than necessary. You have an integer index, why not use an array instead ?

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import Data.Array import Data.List import Data.Ord

syrs n = a where a = listArray (1,n) $ 0:[ syr n x | x <- [2..n]] syr n x = if x' <= n then a ! x' else 1 + syr n x' where x' = if even x then x `div` 2 else 3 * x + 1

main = print $ maximumBy (comparing snd) $ assocs $ syrs 1000000

This solution takes 2 seconds (on my machine) to resolve the problem.

On the other hand, now that I have read your solution, I see that using Map was the least of the problem... All those Map.map, while retaining the original Map... Your solution is too clever (twisted) for its own good, I suggest you aim for simplicity next time.

and if its got Int indices, Data.IntMap is always a better option than Data.Map and usually outperforms the HashTable type, while being pure.