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
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Il giorno mer 21 ott 2020 alle ore 08:52 Ut Primum
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
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Il giorno mer 21 ott 2020 alle ore 00:37 Claudio Cesar de Sa < ccs1664@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
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