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You can reach the person managing the list at beginners-ow...@haskell.org When replying, please edit your Subject line so it is more specific than "Re: Contents of Beginners digest..." Today's Topics: 1. Re: Doing a DFS from scratch .... (Ut Primum) ---------------------------------------------------------------------- Message: 1 Date: Sat, 24 Oct 2020 16:42:30 +0200 From: Ut Primum <utpri...@gmail.com> To: The Haskell-Beginners Mailing List - Discussion of primarily beginner-level topics related to Haskell <beginners@haskell.org> Subject: Re: [Haskell-beginners] Doing a DFS from scratch .... Message-ID: <canjdmkl-8dj6k+nmymbpzqcsvktywhr6d5velk5zb6tru2p...@mail.gmail.com> Content-Type: text/plain; charset="utf-8" Hi Claudio, this is a code for the DFS search. It assumes the graph is represented, as in your code, as an edge list. And, as in your code, the output is a path from a start_node to a finale_node (if it exists). import Data.Maybe dfs_search :: Int -> Int -> [(Int,Int)]-> [Int] dfs_search sn fn graph = dfs_search_aux sn fn graph [sn] [sn] dfs_search_aux :: Int -> Int -> [(Int,Int)] -> [Int] -> [Int] -> [Int] dfs_search_aux sn fn graph visited path |sn==fn = path |not (isNothing neighbour) = dfs_search_aux x fn graph (x:visited) (path++[x]) |has_length_1 path = error "NO SOLUTION" |otherwise = dfs_search_aux (last new_path) fn graph visited new_path where neighbour=not_visited_neighbour sn visited graph x = fromJust neighbour new_path= init path not_visited_neighbour :: Int -> [Int] -> [(Int,Int)] -> Maybe Int not_visited_neighbour _ _ [] = Nothing not_visited_neighbour x visited ((a,b):xs) | (x == a) && not (elem b visited) = Just b | (x == b) && not (elem a visited) = Just a | otherwise = not_visited_neighbour x visited xs has_length_1 :: [Int]->Bool has_length_1 [] = False has_length_1 (x:xs) = xs==[] Cheers, Ut <http://www.avg.com/email-signature?utm_medium=email&utm_source=link&utm_campaign=sig-email&utm_content=webmail> Mail priva di virus. www.avg.com <http://www.avg.com/email-signature?utm_medium=email&utm_source=link&utm_campaign=sig-email&utm_content=webmail> <#DAB4FAD8-2DD7-40BB-A1B8-4E2AA1F9FDF2> Il giorno mer 21 ott 2020 alle ore 08:52 Ut Primum <utpri...@gmail.com> ha scritto: > Hi Claudio, > I had a look at the code, and it looks that in dfs_search function, where > the comment says that "BACTRACKING is happening HERE" there is something > wrong (it looks neighbours are not all considered, because as you said > next_node returns only one neighbour). > I think that the definition of > dfs_search (x:xs) l_closed > is wrong. I'd start to write it from the beginning, instead of correcting > this code. > > Cheers, > Ut > > > <http://www.avg.com/email-signature?utm_medium=email&utm_source=link&utm_campaign=sig-email&utm_content=webmail> > Mail > priva di virus. www.avg.com > <http://www.avg.com/email-signature?utm_medium=email&utm_source=link&utm_campaign=sig-email&utm_content=webmail> > <#m_1000941696827786327_DAB4FAD8-2DD7-40BB-A1B8-4E2AA1F9FDF2> > > Il giorno mer 21 ott 2020 alle ore 00:37 Claudio Cesar de Sa < > ccs1...@gmail.com> ha scritto: > >> Hi everyone >> >> Initially, I hope that this list is active yet. For some days, >> I had been trying to do a Depth First Search on a graph from scratch >> following something very didactical (clear, elegant - legible, any worry >> about efficiency). >> >> This DFS keeps a boolean list to mark the nodes already visited and >> another with current or valid nodes ( the stack of DFS classic). >> >> The code is naive and well documented and is here: >> >> https://github.com/claudiosa/CCS/blob/master/haskell/dfs_graph.hs >> (excessively commented) >> >> but unfortunately, it is not working. My guess is that the problem >> starts on *new_node* function. I am not sure if in DFS, when a current >> node gets a next neighbour to stack, it gets all the neighbours. >> In this code, it is getting one node per time. >> >> The functions next_node and one_node seem very non-sense. Could someone >> help me? >> >> Thanks in advance >> >> >> >> claudio >> >> **************************************************************** >> ************ >> >> *Whatsapp: +55 47 992451825 <%2B55%2047%2092451825>* >> https://claudiocesar.wordpress.com/ (my links HERE) >> https://github.com/claudiosa >> https://www.udemy.com/picat-uma-linguagem-de-programacao-multiparadigma/ >> **************************************************************** >> ************* >> _______________________________________________ >> Beginners mailing list >> Beginners@haskell.org >> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners >> > -------------- next part -------------- An HTML attachment was scrubbed... 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