CODEWITHSUNDEEP
pythonlinked-listdeque
Shawn Li
Shawn Li

Reputation: 1

Why can't I get the value of the node in a linkdeque in Python?

class _DoublyLinkedBase:
    class _Node:
        __slots__ ="_element", "_prev", "_next"

        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, e, predecessor, successor):
        newest = self._Node(e, predecessor, successor)
        predecessor._next = newest
        successor._prev = newest
        self._size += 1
        return newest

    def _delete_node(self, node):
        predecessor = node._prev
        successor = node._next
        predecessor._next = successor
        successor._prev = predecessor
        self._size -= 1
        element = node._element
        node._prev = node._next = node._element = None
        return element

class LinkedDeque(_DoublyLinkedBase):
    def first(self):
        if self.is_empty():
            raise Empty("Deque is empty")
        return self._header._next._element


    def last(self):
        if self.is_empty():
            raise Empty("Deque is empty")
        return self._trailer._prev._element

    def insert_first(self, e):
        self._insert_between(e, self._header, self._header._next)

    def insert_last(self, e):
        self._insert_between(e, self._trailer._prev, self._trailer)

    def delete_first(self):
        if self.is_empty():
            raise Empty("Deque is empty")
        return self._delete_node(self._header._next)

    def delete_last(self):
        if self.is_empty():
            raise Empty("Deque is empty")
        return self._delete._node(self._trailer._prev)

The problem is: when I tried this in python:

a = LinkedDeque()    ## create a linked deque
b = _DoublyLinkedBase._Node(5, None, None)   ## create a node
a.insert_first(b)    ##put b in the first place
a.first()       ##check the element

Out[42]: <_DoublyLinkedBase._Node instance at 0x117660830>

why is the output not 5 ?

Upvotes: 0

Views: 115

Answers (1)

Adriano
Adriano

Reputation: 1733

Your insert_first method expects an element to be passed in, not an object of type _Node (it will wrap the element in a _Node on its own). So replace your initialized b with:

b = 5  # Initialize an element.

Upvotes: 1

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