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I've recently learned how to implement linked list in Python. Can anyone help me to refine my code of implementing methods 'insert()', and 'pop()'.

pop(pos) - remove and return item at position pos.

insert(pos, item) - adds a new item to the list at position pos.

Did I consider all the cases? Thank you in advance!

EDITED: I've add test cases for pop and insert

class Node:
    def __init__(self, data):
        '''create a node'''
        self.data = data
        self.next = None

# LINKED LIST IMPLEMENTATION
class LinkedList:
    def __init__(self):
        self.head = None
        
    def isEmpty(self):
        return self.head == None
    
    def __repr__(self):
        '''ll representation
        O(n)'''
        nodes = []
        cur = self.head
        while cur:
            nodes.append(str(cur.data))
            cur = cur.next
        nodes.append("None")
        return '->'.join(nodes)
        
    
    def add(self, item):
        '''add a new item at the head of ll
        O(1)'''
        # create a new node for item
        newNode = Node(item)
        # set newnode to refer to head
        newNode.next = self.head
        # set newnode to be new head
        self.head = newNode
        
    def size(self):
        '''return #nodes
        O(n)'''
        # traverse ll and count nodes
        cur = self.head
        count = 0
        while cur != None:
            count += 1
            cur = cur.next
        return count
    
    def search(self, key):
        '''search for key in ll'''
        # traverse ll to find key
        # O(n)
        cur = self.head
        while cur:
            if cur.data == key:
                return True
            cur = cur.next
        return False
    
    def remove(self, key):
        '''remove key from ll
        O(n)'''
        # if ll empty, raise error
        if self.head == None:
            raise Exception("Linked list is empty!")
        # if head holds key, set a new head
        if self.head.data == key:
            self.head = self.head.next
            return
        
        # otherwise, traverse ll for key
        prev = None
        cur = self.head
        while cur:
            # key found
            if cur.data == key:
                prev.next = cur.next
                return
            # key not found yet, move prev and cur 1 node ahead
            prev = cur
            cur = cur.next
        # cur is None, key not present
        raise Exception('Key not present in ll!')
        
    def append(self, item):
        '''append an item to the end of ll.
        O(n)'''
        # create a new node for item, by default node points to None
        newNode = Node(item)
        # if ll is empty, set head to be new node
        if self.head == None:
            self.head = newNode
            return
        # otherwise, traverse the whole ll
        cur = self.head
        while cur.next:
            cur = cur.next
        cur.next = newNode
        
    def index(self, key):
        '''return idnex of key in ll
        O(n)'''
        # if ll empty, raise error
        if self.head == None:
            raise Exception("LL is empty!")
        # traverse ll to find key
        pos = 0
        cur = self.head
        while cur:
            if cur.data == key:
                return pos  # found
            # else, move to next node
            cur = cur.next
            pos += 1
        # key not present
        return -1

    def popLastNode(self):
        '''remove and return last item of ll.
        O(n)'''
        # if ll is empty, cant pop
        if self.head == None:
            raise Exception('ll is empty!')
        # only 1 node, set ll to empty
        if self.head.next == None:
            self.head = None
            return
        # otherwise, traverse ll and remove last node
        cur = self.head
        while cur.next.next:
            cur = cur.next
            
        lastVal = cur.next.data
        cur.next = None
        return lastVal
    
    def pop(self, pos=0):
        '''remove and return item at pos
        O(n)'''
        # invalid pos
        if pos < 0 or pos >= self.size():
            raise IndexError('Index out of range!')
        
        # otherwise, traverse ll to pos
        prev = None
        cur = self.head
        idx = 0 # index of cur node
        while idx < pos:
            prev, cur = cur, cur.next
        # pop at the beginning
        if idx == 0:
            val = self.head.data
            self.head = self.head.next
            return val
        val = cur.data
        prev.next = cur.next
        return val
    
            
    def insert(self, item, pos=0):
        '''insert an item at pos.
        invalid pos > error
        pos == 0, change head'''
        # create a new node for item
        newNode = Node(item)
        # pos == 0, set new head (add method)
        if pos == 0:
            newNode.next = self.head
            self.head = newNode
            return
        
        # invalid pos
        if pos < 0 or pos >= self.size():
            raise IndexError('Index out of range!')
              
            
        # otherwise, traverse ll
        prev = None
        cur = self.head # insert between prev and cur
        idx = 0
        while idx < pos:
            prev = cur
            cur = cur.next
            idx += 1
        prev.next = newNode
        newNode.next = cur

# TEST CASES
# pop method
l5 = LinkedList()
# l5.pop(-1) - error, index out of range
# l5.pop(0) - error, index out of range
# l5.pop(5) - error, index out range
l5.add(1)
print(l5.pop()) # 1
l5.add(2)
l5.add(3)
l5.add(4)
print(l5)
# print(l5.pop(4)) - error, idx out of range
print(l5.pop(2))
print(l5)

#%%
# insert
l6 = LinkedList()
# l6.insert(2, 1) - error, index out of range
l6.insert(2)
print(l6)
# l6.insert(3, 2) - error, index out of range
# l6.insert(3, 1) - error, index out of range
l6.insert(3)


l6.insert(4, 1)
print(l6)
l6.insert(5, 1)
print(l6)
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1
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What you requested advice on

LinkedList.insert

You probably want to do your boundary-checking as the first thing, before any manipulation.

This is not very easily readable in my opinion:

        prev = None
        cur = self.head # insert between prev and cur
        idx = 0
        while idx < pos:
            prev = cur
            cur = cur.next
            idx += 1
        prev.next = newNode
        newNode.next = cur

I think it's better if you're more explicit with the variable names, but more importantly I think this would all be more clear with a for-loop.

This would lead to something like:

def insert(self, item, pos=0) -> None:
  """Insert `item` at `pos`.

  :raises IndexError: If position is out of range
  """
  if pos < 0 or pos >= self.size():
    raise IndexError
  elif pos == 0:
    temp = self.head
    self.head = Node(item)
    self.head.next = temp
    return

  previous, current = None, self.head
  for _ in range(pos):
    previous, current = current, current.next
  previous.next = Node(item)
  previous.next.next = current
LinkedList.pop

Here you should be able to do basically the same, so it should look something like:

def pop(self, pos=0) -> Node:
  """Remove and return `item` at `pos`.

  :raises IndexError: If position is out of range
  """
  if pos < 0 or pos >= self.size():
    raise IndexError
  elif pos == 0:
    temp = self.head
    self.head = temp.next
    return temp

  previous, current = None, self.head
  for _ in range(pos):
    previous, current = current, current.next
  previous.next = current.next
  return current

Other things

  • Try to stick with conventions. In python people tend to use (and expect) snake_case for variables and functions, instead of camelCase.

  • next is a builtin; try to avoid naming things the same as builtins (or keywords for that matter).

  • Try to avoid implementing methods like size in python and rely on __len__ instead. Same goes for your boolean search method, which probably just should have been a __contains__.

  • Offer a way to construct a linked list with actual contents, instead of first having to create the list and then subsequently having to add all elements to it.

  • Write tests to figure out if your implementations are correct or not, instead of asking people to do so through inspection of the code. ;)


I'd also recommend not having custom implementations of linked lists in the first place, and instead relying on existing (standard library) data models. But I suspect that this is for school or for self-learning.

--

Finally, for your later added tests, I'd strongly recommend using a framework like pytest or unittest. You could then convert this

l5 = LinkedList()
# l5.pop(-1) - error, index out of range
# l5.pop(0) - error, index out of range
# l5.pop(5) - error, index out range

into this

def test_pop_nonexisting_index_raises_exception():
  lst = LinkedList()
  with pytest.raises(IndexError):
    lst.pop(-1)
    lst.pop(0)
    lst.pop(5)
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5
  • \$\begingroup\$ Thank you for your very detailed answer. Yes I have written some test cases. i'll post right now. I considered the case pos = 0 cuz when pos=0 previous node will still be None after the while loop, hence previous.next will raise an error as None has no next attribute. \$\endgroup\$
    – virus tous
    Jan 5 at 17:36
  • 1
    \$\begingroup\$ Ah, fair enough @virus-tous. I can fix that in my reply - do you want feedback also on your test cases? For future reference, by the way, it's better to make a new post instead of adding to your existing one. \$\endgroup\$
    – ades
    Jan 5 at 20:49
  • \$\begingroup\$ Yes please do. I'm not sure if I consider all the cases. \$\endgroup\$
    – virus tous
    Jan 6 at 17:51
  • 1
    \$\begingroup\$ @virustous I've fixed the bugs and have added some advice on test. I'm not gonna care about coverage/all cases - that would basically require me to rewrite all of your code for you, which isn't what a code review is for. \$\endgroup\$
    – ades
    Jan 7 at 10:40
  • \$\begingroup\$ I got it. Thank you for your time!! \$\endgroup\$
    – virus tous
    Jan 8 at 10:51

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