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

Bruno Carnazzi bcarnazzi at gmail.com
Mon Mar 31 14:00:09 EDT 2008

```I've done this modification with no more success :

import qualified Data.List as List
import qualified Data.Map as Map

f :: Integer -> Integer
f n | even n = n `div` 2
| otherwise = 3 * n + 1

chain m n =
let chain' cn cm | Map.member cn m = Map.map (+ (m Map.! cn)) cm
| otherwise = chain' (f cn) \$! Map.insert cn 1
(Map.map (+1) cm)
in chain' n Map.empty

chains n = List.foldl' (\m i -> Map.union m (chain m i))
(Map.singleton 1 1) [2..n]

maxCollatz c1@(_,l1) c2@(_,l2) | l1 < l2 = c2
| otherwise = c1

maxChain = List.foldl' maxCollatz (0,0) . Map.toList . chains

main =
let n = 1000000
in putStrLn \$ show \$ maxChain n

Best regards,

Bruno.

2008/3/31, Ketil Malde <ketil at malde.org>:
> "Bruno Carnazzi" <bcarnazzi at gmail.com> writes:
>
>  > The program ends for values up to 400000 :
>
>
> Wild guess here - I know nothing about the problem, and haven't
>  examined your program in detail - but could it be that you default to
>  Int, and that it wraps silently at some power of two, thereby making
>  your algorithm wrap around?  Try to stick some 'Integer' type
>  annotations in there, and see if it helps.
>
>  -k
>
>  (cetera censeo...)
>
> --
>  If I haven't seen further, it is by standing in the footprints of giants
>
```