Trouble doing Data Structures and Algorithms with Python

Python
1

So Mosh said in his Data Structures and Algorithms video that you can take the lessons there and apply it to other coding languages and it be pretty straight forward, but right now I’m really having difficulty converting it over to python. Right now I’m at creating Linked Lists from scratch and I’m having trouble creating the ‘addLast()’ functions(method). I don’t know what I’m doing wrong here.

class LinkedList:
    # Nested Node class
    class Node:
        def __init__(self, value, next):
            self.value = value
            self.next = next

    # Linked List class/functions
    def __init__(self):
        self.first = None
        self.last = None

    def addLast(item):
        self.value = item
2

If I have understood correctly, the addLast method should be updating the first and last variables and creating new Node objects as needed. So it should look something like this:

class Node:
  def __init__(self, value):
    self.value = value
    self.next = None

def addLast(value):
  node = Node(value)
  if not first:
    first = node
    last = node
  last.next = node
  last = node
3

Thanks I actually figured it out now, converting the syntax to python makes it come out like this:

    def addLast(self, item):
        node = Node(item)
        if self.first == None:
            self.first = node
            self.last = node
        else:
            self.last.next = node
            self.last = node

Now I’m actually having trouble making the indexOf function because with my current code it gives me this message:

    def indexOf(self, item):
        index = 0
        current = self.first
        while current != None:
            if current.value == item:
                return index
            else:
                current = current.next
                index += 1
        return 'That numbers not in list'

    def contains(item):
        return indexOf(item) != 'That numbers not in list'

My current code keeps giving me this Type Error:

TypeError: LinkedList.contains() takes 1 positional argument but 2 were given

4

First argument for contains should be self.

5

I did change that, but it then gives me this error message:

    def contains(self, item):
        return indexOf(item) != 'That numbers not in list'

line 45, in contains
return indexOf(item) != ‘That numbers not in list’
TypeError: indexOf expected 2 arguments, got 1

6

I believe you need to call it as self.indexOf

1 Like
7

For the record this is a terrible practice: a method that returns a number in some cases and a string in another. You should have it either return a signal value (like -1) or throw an error. Returning -1 on this type of method for not found is fairly standard.

8

Thank you! That worked for me, I’m still new to classes/OOP and I didn’t realize that previous functions can be called in other functions as methods through ’ self.(Function/method) '. Also for raising an error, do you mean like a Value Error?

9

If you want to follow what list does, raise a ValueError.

Your code will probably be simpler though if you just return -1 for not found, but it does mean that callers need to know and handle what -1 means which makes the error route more appealing.

1 Like
10

Ok, thanks for the tip!