无法访问python链表中分配的值

Question

I was given an assignment to delete the elements in a linked list whose sum is 0. I have tried this code. But I get an error like "AttributeError: 'NoneType' object has no attribute 'data'"

import random
class Node :
    def __init__( self, data ) :
        self.data = data
        self.next = None
        self.prev = None
    def givedata(self):
        return self.data    

class LinkedList :
    def __init__( self ) :
        self.head = None        

    def add( self, data ) :
        node = Node( data )
        if self.head == None :  
            self.head = node
        else :
            node.next = self.head
            node.next.prev = node                       
            self.head = node            

    def search( self, k ) :
        p = self.head
        if p != None :
            while p.next != None :
                if ( p.data == k ) :
                    return p                
                p = p.next
            if ( p.data == k ) :
                return p
        return None

    def remove( self, p ) :
        tmp = p.prev
        p.prev.next = p.next
        p.prev = tmp

    def list_print(self):
        node = self.head 
        while node:
            print (node.data,end=" ")
            node = node.next

    def getdata(self):
        self.nodedata=self.head
        return self.nodedata.data

script:

ll=LinkedList()
for i in range(random.randint(2,10)):
    ll.add(random.randint(-50,50))  
ll.list_print()
print('\n')
l1=LinkedList()
while ll:
  while l1:
     if l1.getdata()+ll.getdata()==0:
            print("0\n")#just to know if this works or not
        l1=l1.head.next
   ll=ll.head.next

I tried to access data in the linked list of a particular element. Will you please help me in traversing through linked list

Answer 1

You may choose to omit some but you cannot choose to add further attributes beyond those defined.

This error is simply saying that the attribute data-width cannot be used where you used it.

Answer 2

You are getting this error because

l1=LinkedList()

creates a new linked list named l1 and you have not added any element to it, it is an empty linked list whose head is None.

When you call

l1.getdata()

you see that error

Edit

ll=LinkedList()
for i in range(random.randint(2,10)):
    ll.add(random.randint(-50,50))  
ll.list_print()
print('\n')
ptr1 = ll.head
while ptr1:
  ptr2 = ptr1.next
  while ptr2:
     if ptr1.data+ptr2.data == 0:
            print("0\n")#just to know if this works or not
        ptr2 = ptr2.next
   ptr1 = ptr1.next

This code should do what you want

Answer 3

When you create l1 = LinkedList() , the .head is initialized to None . This causes a problem when you call .getdata() , since that method is trying to call self.head.data , which is equivalent to calling None.data .

You can fix the .getdata method using:

def getdata(self):
    self.nodedata=self.head
    if not self.nodedata:
        return 0
    return self.nodedata.data