Question

I’m trying to build a tree with data in the nodes as well as in the arcs. What I did so far is build classes Tree, Arc and Node like this:

class Tree():

  def __init__(self):
    self.root = None
    self.nodes = {}
    self.end_arcs = {}

class Arc():

  def __init__(self,start,end,data={}):
    self.start = start
    self.end = end
    self.data = data

class Node():

  def __init__(self,token,data={}):
    self.token = str(token)
    self.state = self.OPEN
    self.data = data

Nodes are numbered increasingly with the end node being labeled as -1. However, when I want to reverse the tree and renumber the nodes, I have to change a lot of things: the keys of my nodes dict, the label of every node and perhaps (or can they point to the nodes in my nodes dict?) also the nodes in my arcs dict. So I’m thinking that my way of implementing this common structure is not a good one. Is there a standard way of solving this problem? Or how would you build a tree structure like this?

Thanks in advance,

Joris

Answer 1

Extending my comment a bit, consider the following

class Tree():
  def __init__(self):
    self.root = None
    self.nodes = {}
    self.end_arcs = {}

class Arc():
  def __init__(self,start,end,data={}):
    self.start = start
    self.end = end
    self.data = data

  def __str__(self):
    return "%s-%s" % (self.start.token, self.end.token)

class Node():
  def __init__(self,token,data={}):
    self.token = str(token)
    #self.state = self.OPEN
    self.data = data


n1 = Node(1)
n2 = Node(2)
n3 = Node(3)
n4 = Node(4)
n5 = Node(5)

t = Tree()
t.root = n1
t.nodes = [n1,n2,n3,n4,n5]

a1 = Arc(n1,n2)
a2 = Arc(n2,n3)
a3 = Arc(n2,n4)
a4 = Arc(n1,n5)

print(a1)  # 1-2
# "Rename" Nodes
n1.token = 101 
n2.token = 102
print(a1)  # 101-102

“Renaming” the nodes is no problem because the arc keeps tracks of the node “by reference”, not by it’s ID. When we look at, for example, the arcs, using references allow us to not care about how the underlying node is changed.