Welcome to the Treehouse Community

The Treehouse Community is a meeting place for developers, designers, and programmers of all backgrounds and skill levels to get support. Collaborate here on code errors or bugs that you need feedback on, or asking for an extra set of eyes on your latest project. Join thousands of Treehouse students and alumni in the community today. (Note: Only Treehouse students can comment or ask questions, but non-students are welcome to browse our conversations.)

Looking to learn something new?

Treehouse offers a seven day free trial for new students. Get access to thousands of hours of content and a supportive community. Start your free trial today.

Computer Science Introduction to Data Structures Building a Linked List Inserting a Node

Teacher Russell
Teacher Russell
16,873 Points

Inserting a node

Am I crazy or is that an error on line 80 in the inserting a node video? It should be current.next_node, right?

2 Answers

Balazs Peak
Balazs Peak
46,097 Points

You are right. Nice move. Attention to details is a superpower when it comes to programming. :D

(Note: probably the video has been left accidentally with the error in it. If you look at the provided source code for the solution in the notes 2 steps after that video, you can see that it's already fixed there. Reference: https://teamtreehouse.com/library/introduction-to-data-structures/building-a-linked-list/linked-lists-operations )

Teacher Russell
Teacher Russell
16,873 Points

I've seen a couple of inconsistencies between video and notes, yes. I'm going to say it's a testament to Pasan's teaching ability, that even I can spot errors in someone else's code, in a language I don't know, when I'm not only not looking for them, but assuming what I'm looking at is correct. Go Pasan:) Of course, it COULD be my developing super powers, but I think I just got lucky this time. Hopefully someone can add a note in the teacher's notes? It's already a little tricky learning this stuff in a strange language. It might throw off other newbies. Or maybe they'll just see your little chat here. Thanks for the quick reply earlier.

I'm not sure, but I think there's another error in the insert method of the Linked List.

Let's suppose we don't have any values in the linked list. This means that the __count property would be 0. If we want to add a node at index zero, we should be able to, shouldn't we? But we won't because of the first if statement which will throw an error:

def insert(self, data, index):
        """
        Inserts a new Node containing data at index position
        Insertion takes O(1) time but finding node at insertion point takes
        O(n) time.
        Takes overall O(n) time.
        """

        if index >= self.__count:  # An error will be thrown
            raise IndexError('index out of range')

        if index == 0:  # This should run, but it won't because an error was thrown
            self.add(data)
            return
        if index > 0:
            new = Node(data)
            position = index
            current = self.head

            while position > 1:
                current = current.next_node
                position -= 1

            prev_node = current
            next_node = current.next_node

            prev_node.next_node = new
            new.next_node = next_node

        self.__count += 1

I think if the second if statement was moved to be the first one, the algorithm would be correct.

Aaron Uusitalo
Aaron Uusitalo
4,229 Points

Good catch. That jumped out at me, too. I didn't recall any variable being declared called 'node' (as opposed to Node class), so that's a red flag.

Harald N
Harald N
15,831 Points

Thanks, I'm taking this course, but my main language is Javascript not Python, so wasn't sure if I did something wrong or if the code was wrong.