[Haskell-cafe] Programming Chalenges: The 3n+1 problem

[Dmitri O.Kondratiev]

Thanks, everybody!
Your feedback is a great food for my mind (as Lewis Carroll once wrote :)
When asking "how to implement cache in Haskell" I was hopping that there
exists some solution without using Data.Array, more "functional" approach,
if I may say so  ...
I must be wrong, though (need more time to fully comprehend solution that
Steven described in this thread ).

On Thu, Apr 14, 2011 at 3:22 PM, Ryan Ingram <ryani.spam at gmail.com> wrote:

> So if we were to emulate your Java solution, we'd do
>
> import Data.Array
>
> cacheSize :: Int
> cacheSize = 65536
>
> table :: Array Int Integer
> table = listArray (1,cacheSize) (1 : map go [2..cacheSize]) where
>     go n
>         | even n = 1 + lookup (n `div` 2)
>         | otherwise = 1 + lookup (3 * n + 1)
>
> lookup :: Integer -> Integer
> lookup n
>     | n < cacheSize = table ! (fromInteger n)
>     | even n = 1 + lookup (n `div` 2)
>     | otherwise = 1 + lookup (3 * n + 1)
>
> The rest of the code is just some simple i/o.
>
> The table is filled up lazily as you request values from it.
>
>
> --
All the best,
Dmitri O. Kondratiev

"This is what keeps me going: discovery"
dokondr at gmail.com
http://sites.google.com/site/dokondr/welcome
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