Re: [Haskell-beginners] Lions, Wolves and Goats

I tried a set-based solution and it can process ~1600 items in 25 seconds on this i7. Seems really slow compared to the times posted here: http://unriskinsight.blogspot.co.at/2014/06/fast-functional-goats-lions-and-... I'm curious if anyone spots any major flaw. If not, I'll profile it tonight -- I can't afford to spend more time on this at work Tim On Fri, Jun 13, 2014 at 8:54 AM, Elric wrote:

Thank You Bob,

I learnt quite a bit from your solution. I have been restricting myself to Lists so far. I think I will have to start exploring other data structures like Sets in Haskell as well. :)

Thank You, Elric

On 06/08/2014 03:41 PM, Bob Ippolito wrote:

Here's another approach that more closely models what's going on in the C++ version. I defined an ordNub rather than using nub as nub is O(n^2) as it only requires Eq.

https://gist.github.com/etrepum/5bfedc8bbe576f89fe09

import qualified Data.Set as S import Data.List (partition) import System.Environment (getArgs)

data LWG = LWG { _lion, _wolf, _goat :: {-# UNPACK #-} !Int } deriving (Show, Ord, Eq)

lionEatGoat, lionEatWolf, wolfEatGoat :: LWG -> LWG lionEatGoat (LWG l w g) = LWG (l - 1) (w + 1) (g - 1) lionEatWolf (LWG l w g) = LWG (l - 1) (w - 1) (g + 1) wolfEatGoat (LWG l w g) = LWG (l + 1) (w - 1) (g - 1)

stableState :: LWG -> Bool stableState (LWG l w g) = length (filter (==0) [l, w, g]) >= 2

validState :: LWG -> Bool validState (LWG l w g) = all (>=0) [l, w, g]

possibleMeals :: LWG -> [LWG] possibleMeals state = filter validState . map ($ state) $ [lionEatGoat, lionEatWolf, wolfEatGoat]

ordNub :: Ord a => [a] -> [a] ordNub = S.toList . S.fromList

endStates :: [LWG] -> [LWG] endStates states | not (null stable) = stable | not (null unstable) = endStates (concatMap possibleMeals unstable) | otherwise = [] where (stable, unstable) = partition stableState (ordNub states)

main :: IO () main = do [l, w, g] <- map read `fmap` getArgs mapM_ print . endStates $ [LWG l w g]

On Sat, Jun 7, 2014 at 11:33 PM, Francesco Ariis wrote:

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On Sat, Jun 07, 2014 at 08:04:09PM -0400, Elric wrote:

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Hi,

I came across this article: http://unriskinsight.blogspot.co.at/2014/06/fast-functional-goats-lions-and-... a couple of days ago. This compares performance of solving a problem (which I will get to) using the functional constructs alone in languages like C++11 and Java 8. Since, Haskell is my first foray into FP, I thought I should try solving this in Haskell.

Hello Elric, I gave a go at the problem, managed to get a result (23). I attach the .hs file (not my best Haskell, but hopefully clear enough).

The crucial point in my solution lies in this lines:

carnage :: [Forest] -> [Forest] let wodup = nub aa in -- etc. etc.

Which means after every iteration I call |nub| on my list of possible states; nub is a function from |Data.List| and removes duplicate elements from a list.

If I omit that nub call, the program doesn't reach a solution (as it is computationally quite inefficient). I think that's the problem with your versions.

Let me know if this helps

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