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

[Chaddaï Fouché]

2008/3/31, Bruno Carnazzi <bcarnazzi at gmail.com>:
>    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 ?

> 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.

-- 
Jedaï