Question

我在Python中构建了一个基于Vertex和图形类的图形，但是在isCycleUtil()方法中，当我尝试通过for i in self.vert_dict[v]访问Vertex对象时，我得到一个TypeError：Vertex对象不可迭代。

你能帮助我修复它并拥有一个功能正常的isCycleUtil方法吗？

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

和图表类：

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 1

要避免此错误，您需要替换：

 for i in self.vert_dict[v]

与

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

并使用：

visited[v.get_id()]

而不是

visited[v]

避免KeyError

您的图表是针对性的，因此在本主题中描述了正确的算法：

How to detect if a directed graph is cyclic?

您需要更改isCyclicuntil方法的实现：

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

如何访问Vertex对象内的邻接列表？

1 个答案: