Re: [Haskell-beginners] While Condition in Haskell

11 Oct 2010


      Thanks.
Actually, your example looks remarkably compact.
Cheers

Lorenzo


On 10/11/2010 01:46 PM, Thomas Miedema wrote:
...
Hi Lorenzo,
what Patrick said was correct. Here's your program a bit more the
Haskell way. Also see:
http://www.haskell.org/ghc/docs/6.12.2/html/libraries/base-4.2.0.1/Data-List...
import Data.List
main = do
   let (past, future) = splitAt 8 [4,55,66,77,88,99,12,9,77,88,99,12,-99]
       subfutures = tail . inits $ future
   print . takeWhile (`isInfixOf` past) $ subfutures
Regards,
Thomas
On Mon, Oct 11, 2010 at 4:09 AM, Patrick LeBoutillier
mailto:patrick.leboutillier@gmail.com>
wrote:
Lorenzo,
On Sun, Oct 10, 2010 at 4:15 PM, Lorenzo Isella
    mailto:lorenzo.isella@gmail.com> wrote:
     > ...
     > In order to find the length of the longest list in the future
    which has
     > already been seen in the past, I could select the "true" values
    in the
     > output of function
     >
     > iter_find list i
     >
     > but this is a waste of CPU: I simply would like a while condition
    to tell my
     > function to stop checking new sublists as soon as it finds one
    which has not
     > occurred in the past and I would like a counter (a kind of i++)
    telling me
     > how many times the process has been iterated.
I'm still a beginner myself, but here's my take on it. Since Haskell
    is lazy, the results
    will only be generated as they are needed. For example, if you change:
let b = iter_find list i
for
let b = takeWhile id $ iter_find list i
Haskell will stop generating the list as soon as it sees a False result.
    Then you can take the length of b as your answer.
Patrick
> Any suggestion is helpful.
     > Cheers
     >
     > Lorenzo
     >
     > -------------------------------------------------------------------
     >
     > import Data.Ord
     >
     >
     > import Data.List
     >
     > main :: IO ()
     >
     > main = do
     >
     >
     >  let list = [4,55,66,77,88,99,12,9,77,88,99,12,-99]
     >
     >  let i = 9
     >
     >  let b = iter_find list i
     >
     >  putStrLn "b is, "
     >  print b
     >
     >
     > is_sublist sublist list = sublist `isInfixOf` list
     >
     >
     > gen_fut_list list i j = take j $ drop (i-1) list
     >
     > gen_past_list list i = take (i-1) list
     >
     > find_in_list list i j = is_sublist (gen_fut_list list i j)
    (gen_past_list
     > list i)
     >
     > iter_find list i  = map   (find_in_list list i) [1..n]
     >                    where n = (length list) - i +1
     >
     >
     >
     >
     > _______________________________________________
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     > Beginners@haskell.org mailto:Beginners@haskell.org
     > http://www.haskell.org/mailman/listinfo/beginners
     >
--
    =====================
    Patrick LeBoutillier
    Rosemère, Québec, Canada
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