CODEWITHSUNDEEP
pythonvariableslinked-list
FibroMyAlgebra
FibroMyAlgebra

Reputation: 55

How to directly reference an object through a variable in Python

Basically, I'm implementing a LinkedList class and then implementing various methods to use it. Below is the code for the LinkedList class(and its dependent Node class)

# A simple linked list implementation

class Node:
# attributes:
#   data (can be anything)
#   next (another Node)

    def __init__(self, data):
        self.data = data
        self.next = None


class LinkedList:
# attributes:
#   head (a Node)
# ****************
# methods:
#   insert
#   find
#   delete

    def __init__(self):
        self.head = None

    def __str__(self):
        output = []
        current_node = self.head
        while current_node:
            output.append(str(current_node.data))
            current_node = current_node.next
        return(", ".join(output))


    # Creates a Node with the given data and inserts into the front of the list.
    def insert(self, data):
        new_node = Node(data)
        new_node.next = self.head
        self.head = new_node

    # Finds the first node with the given data. Returns None if there is no such node.
    def find(self, data):
        current_node = self.head
        while(current_node):
            if (current_node.data == data):
                return current_node
            current_node = current_node.next
        return None # We reached the end of the list without finding anything.

    # Deletes given node. Can be used in conjunction with find.
    def delete(self, deleted_node):
        if deleted_node == self.head:
            self.head = self.head.next
            return
        current_node = self.head
        while(current_node.next != deleted_node):
            current_node = current_node.next
        current_node.next = deleted_node.next

Then, I'm trying to implement a rotate(my_list, N) function, which will, well, rotate my_list by N. The following is my code:

import linkedlists as ll
from ErrorHandler import sanitize
import random, math, time, copy

def length(my_list):
    sanitize(my_list, ll.LinkedList)
    if my_list.head == None: return 0
    count = 1 #First item! Ah, ah, ah
    current_node = my_list.head
    while current_node.next != None:
        current_node = current_node.next
        count += 1 #One more item! Ah, ah, ah
    return count

def get_Nth(my_list, N):
    sanitize(my_list, ll.LinkedList)
    if my_list.head == None: return None
    current_node = my_list.head
    count = 0
    while current_node.next != None:
        if count == N:
            return current_node
        count +=1
        current_node = current_node.next
    return current_node

def rotate(my_list, N):
    sanitize(my_list, ll.LinkedList)
    if my_list.head == None: return None
    N = N % length(my_list)
    lent = length(my_list)
    get_Nth(my_list, lent-1).next = my_list.head
    my_list.head = get_Nth(my_list, lent-N)
    get_Nth(my_list, lent-N-1).next = None

However, calling rotate() on a LinkedList containing the numbers from 0 to 9 in ascending order returns 8,9,0,1,2,3,4,5. Why? I'm pretty sure it has to do with the third- and second-to-last lines, because when assigning get_Nth(my_list, lent-1).next to my_list.head, it only points to my_list.head, instead of the Node object my_list.head points to at the time.

How can I fix this?

Upvotes: 0

Views: 102

Answers (1)

Josie Thompson
Josie Thompson

Reputation: 5818

Your error is right here: get_Nth(my_list, lent-N-1).next = None

I'm assuming you called rotate(my_list, 2) so at this point, the list looks like this [8, 9, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 0, 1, ...]. So when you call get_Nth(my_list, lent-N-1), lent-N-1 is 7, and well, the element at index 7 is actually 5.

You should just use lent-1.

Upvotes: 1

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