IdelHamza IdelHamza - 16 days ago 4
Python Question

How can I access to adjacency lists inside the Vertex object?

I built a graph in Python, based on Vertex and graph classes, but in

isCycleUtil()
method, when I try to access Vertex object by doing
for i in self.vert_dict[v]
, I get a TypeError: Vertex object not iterable.

Can you help me to fix It and have a functional isCycleUtil method?

class Vertex:
def __init__(self, node):
self.id = node
self.adjacent = {}

def __str__(self):
return str(self.id) + ' adjacent: ' + str([x.id for x in self.adjacent])

def add_neighbor(self, neighbor, weight=0):
self.adjacent[neighbor] = weight

def get_connections(self):
return self.adjacent.keys()

def get_id(self):
return self.id

def get_weight(self, neighbor):
return self.adjacent[neighbor]

def set_weight(self, neighbor, newvalue):
self.adjacent[neighbor] = newvalue


and the graph class:

class Graph:
def __init__(self):
self.vert_dict = {}
self.num_vertices = 0

def __iter__(self):
return iter(self.vert_dict.values())

def add_vertex(self, node):
self.num_vertices += 1
new_vertex = Vertex(node)
self.vert_dict[node] = new_vertex
return new_vertex

def get_vertex(self, n):
if n in self.vert_dict:
return self.vert_dict[n]
else:
return None

def add_edge(self, frm, to, cost = 0):
if frm not in self.vert_dict:
self.add_vertex(frm)
if to not in self.vert_dict:
self.add_vertex(to)

self.vert_dict[frm].add_neighbor(self.vert_dict[to], cost)

def get_vertices(self):
return self.vert_dict.keys()

def isCyclicUtil(self,v,visited,parent):

#Mark the current node as visited
if not visited[v]:
visited[v]= True
parent[v] = True
#Recur for all the vertices adjacent to this vertex
for i in self.vert_dict[v]:
# If the node is not visited then recurse on it
if (not visited[i] and self.isCyclicUtil(i,visited,parent)):
return True
elif (not parent[i]):
return True
parent[v] = False
return False


#Returns true if the graph contains a cycle, else false.
def isCyclic(self):
# Mark all the vertices as not visited
visited = {}
parent = {}
for i in self.vert_dict:
visited[i] = False
parent[i] = False
# Call the recursive helper function to detect cycle in different
#DFS trees
for i in self.vert_dict:
if(self.isCyclicUtil(i,visited,parent)):
return True
return False

Answer

To avoid this error you need to replace:

 for i in self.vert_dict[v]

with

 for i in self.vert_dict[v].get_connections()

and use:

visited[v.get_id()] 

instead of

visited[v] 

to avoid KeyError

Your graph is directed so correct algoritm is descibed in this topic:

How to detect if a directed graph is cyclic?

You need to change implementation of isCyclicuntil method:

def isCyclicUtil(self, v, visited, parent):

    # Mark the current node as visited
    if not visited[v]:
        visited[v] = True
        # Recur for all the vertices adjacent to this vertex
        for i in self.vert_dict[v].get_connections():
            # If the node is not visited then recurse on it
            if visited[i.get_id()]:
                return True
            elif self.isCyclicUtil(i.get_id(), visited, parent):
                return True
    return False