Hi Sudhier, and welcome,
Unfortunately I'm about to step away from the computer, so I can't
really answer your question in detail, but since it's been 2 or 3 days
since you asked your question, and I haven't seen any replies at all, I
thought I'd give a quick reply so you know we aren't ignoring you.
(My inbox is suffering from overload at the moment, so it is possible
that others have answered your question and I haven't noticed. If so, my
apologies for wasting everyone's time.)
Regards,
Steve
On Sat, Sep 19, 2015 at 07:52:07PM +0530, Sudhier Batara wrote:
> 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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