Graph implementation using python adding an edge

Question

The teststring represents the structure given in euler project problem 18. Trying to solve it using graph theory. The graph structure I am trying to create is as below. Every number given in the string is a node in the graph.

    3->(7,4)
    7->(2,4)
    4->(4,6)
    2->(8,5)

    mytesttring='''
        3
        7 4
        2 4 6
        8 5 9 3'''

The node class object accepts a tuple as position and value. In the above string mytesttring the first number 3 the position is (0,0) and value is 3 , similarily 7 position is (1,0) and value is 7

class node(object):
    def __init__(self,position,value):
        '''
        position : gives the position of the node wrt to the test string as a tuple
        value    : gives the value of the node
        '''
        self.value=value
        self.position=position

    def getPosition(self):
        return self.position

    def getvalue(self):
        return self.value

    def getNodeHash(self):
        return hash(str(self.position)+str(self.value))

    def __str__(self):
        return 'P:'+str(self.position)+' V:'+str(self.value)

The edge class is as below accepts a source node and a destination node.

class edge(object):
    def __init__(self,src,dest):
        '''src and dest are nodes'''
        self.src = src
        self.dest = dest

    def getSource(self):
        return self.src

    def getDestination(self):
        return self.dest
    #return the destination nodes value as the weight
    def getWeight(self):
        return self.dest.getvalue()

    def __str__(self):
        return (self.src.getPosition(),)+'->'+(self.dest.getPosition(),)

The directed graph class is as below,it stores the edges in the form of a python dictionary, where key is the source node and value is a list of destination nodes. While adding an edge to the graph through the method addEdge , it checks if the node is present as a key in the edges dictionary. if the source of destination node is not present as a key in the edges dictionary, then 'Node not in graph' error is thrown.

class Diagraph(object):
    '''the edges is a dict mapping node to a list of its destination'''
    def __init__(self):
        self.edges = {}

    def addNode(self,node):
        if node in self.edges:
            raise ValueError('Duplicate node')
        else:
            self.edges[node]=[]

    def addEdge(self,edge):
        src = edge.getSource()
        dest = edge.getDestination()
        if not (src in self.edges and dest in self.edges):
            raise ValueError('Node not in graph')
        self.edges[src].append(dest)

    def getChildrenof(self,node):
        return self.edges[node]

    def hasNode(self,node):
        return node in self.edges

Converting the input string into a list of lists containing the individual numbers.

testlist2=[ list(map(int,elements.split())) for elements in mytesttring.strip().split("\n")]
print(testlist2)
out: [[3], [7, 4], [2, 4, 6], [8, 5, 9, 3]]

I am able to add the nodes to the Diagraph object in the below way.

y=Diagraph()
for i in range(len(testlist2)):
    for j in range(len(testlist2)):
        if i<=j:
            y.addNode(node((j,i),testlist2[j][i]))

The 10 nodes are present in the edges dictionary. But when I try to add the edges using the method addEdge ,its raising the 'Node not in graph'. Can anyone suggest the proper way of adding the edges. The keys to the dictionary is an object of class node.

score 0 · Accepted Answer · answered Jun 04 '18 at 07:17

The code I had for adding the edge was failing with 'Node not in graph' as I was creating the nodes again node((j,i),testlist2[j][i]) and trying to add it to the edge. The point I forgot was the it creates a new node object even though the inputs are the same.

Each node is present as the key of the dictionary edges. We could convert the keys of the dictionary edges into a list listofkeys=list(y.edges.keys()) and then based on the getPosition method of the node class create a diagraph structure or we could capture the nodes created in a list and then make use of this list emplist to create the edge. I modified the code to add the nodes such that it's captured inside a list. I can make use of this list emplist to create the edge and the diagraph.

emplist=[]
for i in range(len(testlist2)):
    for j in range(len(testlist2)):
        if i<=j:
            mynode=node((j,i),testlist2[j][i])
            emplist.append(mynode)
            y.addNode(mynode)

code to add all the edges.

for node in emplist:
    # iterate through all the nodes again and form a logic add the edges
    for allnodes in emplist:
        if node.getPosition()[0]==allnodes.getPosition()[0]-1 and node.getPosition()[1]==allnodes.getPosition()[1]-1:
            y.addEdge(edge(node,allnodes))
        if node.getPosition()[0]==allnodes.getPosition()[0]-1 and node.getPosition()[1]==allnodes.getPosition()[1]:
            y.addEdge(edge(node,allnodes))

Graph implementation using python adding an edge

1 Answers1