Question

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。

Answer 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。

Answer 2

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()。

Python认为对象实例是列表对象

2 个答案: