所以我在Graph上实现BFS来检测所有周期。我通过邻接列表实现了图形。但是当我运行我的代码时，我收到以下错误
Traceback (most recent call last):
File "C:\Python27\Data Structures\Graph\bfstree.py", line 228, in <module>
main()
File "C:\Python27\Data Structures\Graph\bfstree.py", line 223, in main
traverse(g.getVertex(2))
File "C:\Python27\Data Structures\Graph\bfstree.py", line 168, in traverse
while (x.getPred()):
AttributeError: 'list' object has no attribute 'getPred'
因此，当我调用traverse（）函数时会出现问题。 这是我的主要功能
def main():
g = Graph()
for i in range(1,9):
g.addVertex(i)
g.addEdge(1,2)
g.addEdge(1,4)
g.addEdge(1,8)
g.addEdge(2,3)
g.addEdge(2,1)
g.addEdge(3,2)
g.addEdge(3,4)
g.addEdge(3,7)
g.addEdge(3,8)
g.addEdge(4,1)
g.addEdge(4,3)
g.addEdge(4,5)
g.addEdge(5,4)
g.addEdge(5,6)
g.addEdge(5,7)
g.addEdge(6,5)
g.addEdge(6,7)
g.addEdge(7,3)
g.addEdge(7,6)
g.addEdge(7,5)
g.addEdge(8,3)
g.addEdge(8,1)
for v in g:
for w in v.getConnections():
print("(%s,%s)"%(v.getId(),w.getId()))
print("\nDoing BFS...")
bfs_tree(g,g.getVertex(1))
a = g.getVertex(2)
print(type(a))
traverse(g.getVertex(2))
main()
这是遍历函数：
def traverse(y):
x = y
while (x.getPred()):
print(x.getId())
x = x.getPred()
print(x.getId())
以下是图表的邻接列表实现：
class Graph:
def __init__(self):
self.vertList = {} #this is the masterlist
self.numVertices = 0
def addVertex(self,key): #turn something into a Vertex object
self.numVertices = self.numVertices + 1
newVertex = Vertex(key)
self.vertList[key] = newVertex #maps vertex names to vertex objects
return newVertex
def getVertex(self,n):
if n in self.vertList:
return self.vertList[n] #returns the Vertex object
else:
return None
def __contains__(self,n):#tweak the built-in operator 'in'(containment check)
return n in self.vertList
def addEdge(self,f,t,cost = 0):
if f not in self.vertList: #if f is not a node in the graph
nv = self.addVertex(f)
if t not in self.vertList: #if t is not a node in the graph
nv = self.addVertex(t)
self.vertList[f].addNeighbor(self.vertList[t], cost)
def getVertices(self):
return self.vertList.keys()
def __iter__(self): # iterate over Vertex objects over the Graph
return iter(self.vertList.values())
class Vertex:
def __init__(self,key):
self.id = key
self.connectedTo={} #dictionary which contains all the other vertices it is connected to
self.pred = [] #for BFS tree / a list because we are dealing with cycles
self.color = "white" #for BFS tree
def addNeighbor(self,nbr,weight=0):
self.connectedTo[nbr] = weight #nbr is another Vertex object
def __str__(self):
#TODO: lookup how that for loop works
return str(self.id) + "connected to " + str([x.id for x in self.connectedTo])
def getConnections(self):
return self.connectedTo.keys()
def getId(self):
return self.id
def getWeight(self,nbr):
return self.connectedTo[nbr]
def getColor(self):
return self.color
def setColor(self,color):
self.color = color
def setPred(self,node):
self.pred.append(node)
def getPred(self):
if len(self.pred)>1:
return self.pred
elif len(self.pred) == 0:
return self.pred[0]
else:
return self.pred
为什么说g.getVertex（2）是一个列表对象？我很确定它是一个Vertex对象。我甚至打印出main函数中的类型，它说它是一个实例而不是列表对象。
答案 0 :(得分：1)
x = x.getPred()是问题所在。 while循环中的第一次检查很好，但在第一次更新x后会中断，然后重新检查。
正如已实施，getPred正在返回self.pred（唯一一种从self.pred返回值的情况，而不是整个事情被打破;长度是0，你索引，所以它会引发IndexError）。 self.pred是list。
答案 1 :(得分：1)
您将x的结果替换为x.getPred()：
while (x.getPred()):
print(x.getId())
x = x.getPred()
x.getPred()返回self.pred：
def getPred(self):
if len(self.pred)>1:
return self.pred
elif len(self.pred) == 0:
return self.pred[0]
else:
return self.pred
（请注意，对于len(self.pred) == 0，您尝试返回self.pred[0]，这将引发IndexError例外。
self.pred是一个列表：
class Vertex:
def __init__(self,key):
# ...
self.pred = [] #for BFS tree / a list because we are dealing with cycles
因此，您使用x对象替换了list，然后循环并在该列表对象上调用x.getPred()。