Hi,
I have the below code which is a doubly linked list , which returns
the Position of the node rather than the node itself.The code is quite
self explanatory.In the below code i call following methods ,
p = L.add_last(8)
print p.element()
the above methods works fine adds the element into the List instance
and also printing the element shows the element in the list.
But when i call the below methods,
q = L.add_after(p,5)
q.element()
throws following error
_______________________________________
in make position
in make position
8
inside validate
8
in make position
Traceback (most recent call last):
File "positional_dlist.py", line 128, in <module>
print q.element()
File "positional_dlist.py", line 45, in element
return self._node.element
AttributeError: 'NoneType' object has no attribute 'element'
____________________________________________________
Complete code below
#!/usr/bin/python
class _DoublelinkedBase:
class _Node:
def __init__(self,element,prev,Next):
self.element = element
self._prev = prev
self._Next = Next
def __init__(self):
self._header = self._Node(None,None,None)
self._trailer = self._Node(None,None,None)
self._header._Next = self._trailer
self._trailer._prev = self._header
self._size = 0
def __len__(self):
return self._size
def is_empty(self):
return self._size == 0
def _insert_between(self,element,predecessor,successor):
newest = self._Node(element,predecessor,successor)
predecessor._Next = newest
successor._prev = newest
self._size += 1
def _delete_node(self,node):
predecessor = node._prev
successor = node._Next
predecessor._Next = successor
successor._prev = predecessor
element = node.element
node.element = node._Next = node._prev = None
self._size -= 1
class PositionalList(_DoublelinkedBase):
class Position:
def __init__(self,container,node):
self._container = container
self._node = node
def element(self):
return self._node.element
def __eq__(self,other):
return type(other) is type(self) and other._node is self._node
def __ne__(self,other):
return not (other == self)
def _validate(self, p):
if not isinstance(p,self.Position):
raise TypeError("p is not of type Position")
if p._container is not self:
raise ValueError("p is not the instance of this list")
if p._node._Next is None:
print p._node
raise ValueError("No longer a valid node")
print "inside validate"
print p.element()
return p._node
def _make_position(self,node):
if node is self._header or node is self._trailer:
print "in make position for none"
return None
else:
print "in make position"
return self.Position(self,node)
def first(self):
return self._make_position(self._header._Next)
def last(self):
return self._make_position(self._trailer._prev)
def before(self,p):
node = self._validate(p)
return self._make_position(node._prev)
def after(self,p):
node = self._validate(p)
return self._make_position(node._Next)
def __iter__(self):
cursor = self.first()
while cursor is not None:
yield cursor.element()
cursor = self.after(cursor)
def _insert_between(self,e,predecessor,successor):
#node =
super(_DoublelinkedBase,self)._insert_between(e,predecessor,successor)
node = _DoublelinkedBase()._insert_between(e,predecessor,successor)
return self._make_position(node)
def add_first(self,e):
return self._insert_between(e,self._header,self._header._Next)
def add_last(self,e):
return self._insert_between(e,self._trailer._prev,self._trailer)
def add_before(self,p,e):
original = self._validate(p)
return self._insert_between(e,original._prev,original)
def add_after(self,p,e):
original = self._validate(p)
return self._insert_between(e,original,original._Next)
def delete_node(self,p):
original = self._validate(p)
return self._delete_node(original)
def replace(self,p,e):
original = self._validate(p)
old_value = original.element
original.element = e
return old_value
L = PositionalList()
p = L.add_last(8)
p = L.first()
print p.element()
q = L.add_after(p,5)
print q.element()
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