Posts: 5
Threads: 1
Joined: Mar 2019
Mar-04-2019, 07:17 PM
(This post was last modified: Mar-04-2019, 07:49 PM by Yoriz.)
I have a linked list in python that I am presently implementing. However the scenario of deleting the first node isnt working for me.
Deleting the node at any other point in the list works fine.
I have searched youtube tutorials and other online posts but it seems to have the exact same lines of code that I have.
I have also tried putting print statements in before the lines - self.head = HeadVal.nextval - HeadVal = None
From the output, it does seem to pass the headnode onto the second element of the list and clear the original first item, but when I print the list again the original first node hasnt been deleted. Its almost like the new list structure hasnt been committed to memory if that makes sense?
class Node:
def init(self, dataval):
self.dataval = dataval
self.nextval = None
class LinkedList:
def init(self):
self.headval = None
def listprint(self):
printval = self.headval
while printval is not None:
print (printval.dataval)
printval = printval.nextval
def removeNode(self):
nodeToRemove = input("Enter Node To Remove: ")
HeadVal = self.headval
if (HeadVal is not None):
if (HeadVal.dataval == nodeToRemove):
self.head = HeadVal.nextval
HeadVal = None
return
else:
while (HeadVal is not None):
if HeadVal.dataval == nodeToRemove:
break
prev = HeadVal
HeadVal = HeadVal.nextval
prev.nextval = HeadVal.nextval
HeadVal = None
else:
print("Nothing To Delete")
def MainMenu():
print("1. Create A Linked List")
print("2. Delete A Linked List")
print("3. Check If A Linked List Is Empty")
print("4. Print Out The Values In The List")
print("5. Find A Node In A Linked List")
print("6. Insert A Node In A Linked List")
print("7. Delete A Node In A Linked List")
print("99. Exit")
anotherOption = True
while anotherOption == True:
print("")
selection=int(input("Main Menu --- Enter Choice: "))
if selection==1:
myList = LinkedList()
myList.init()
print("Linked List Created")
elif selection==6:
print("")
print("A. Insert A Node Into The Front Of The Linked List")
print("B. Insert A Node Into The End Of The Linked List")
print("")
entrySelection=input("Enter Insert Node Choice: ")
if entrySelection in ["A", "a"]:
myList.insertAtBegining()
elif entrySelection in ["B", "b"]:
myList.insertAtEnd()
else:
print("")
print("Enter A Valid Selection For Inserting Node")
elif selection==4:
myList.listprint()
elif selection==7:
myList.removeNode()
elif selection==99:
anotherOption = False
print("")
print("Exiting Main Menu")
else:
print("")
print("Enter A Valid Selection On Main Menu")
MainMenu()
Posts: 2,168
Threads: 35
Joined: Sep 2016
I've code tags to your code but the indentation need some attention.
Posts: 5
Threads: 1
Joined: Mar 2019
Hi Yoriz,
Thanks for your reply. Yes I lost some of the indentation when I copied and pasted, but the indentation isnt causing any problems when I run it.
Its the lines 24-27 that I am specifically looking at. Am I missing a line of code that commits the list to its new status?
Posts: 4,220
Threads: 97
Joined: Sep 2016
IF the indentation is correct, that seems right to me. Can you show us the code with the correct indentation, and with the insertion methods? Just the class, not the menu interface.
Posts: 5
Threads: 1
Joined: Mar 2019
Mar-04-2019, 09:02 PM
(This post was last modified: Mar-04-2019, 09:06 PM by Yoriz.)
class Node:
def init(self, dataval):
self.dataval = dataval
self.nextval = None
class LinkedList:
def init(self):
self.headval = None
def listprint(self):
printval = self.headval
while printval is not None:
print (printval.dataval)
printval = printval.nextval
def insertAtBegining(self):
nodeToPutAtStart = input("Enter Node To Put At The Start: ")
startNode = Node()
startNode.init(nodeToPutAtStart)
startNode.nextval = self.headval
self.headval = startNode
print(nodeToPutAtStart, " Has Been Added To The Start Of The Linked List")
def insertAtEnd(self):
nodeToPutOnEnd = input("Enter Node To Put On The End: ")
endNode = Node()
endNode.init(nodeToPutOnEnd)
if self.headval is None:
self.headval = endNode
return
laste = self.headval
while(laste.nextval):
laste = laste.nextval
laste.nextval=endNode
print(nodeToPutOnEnd, " Has Been Added To The End Of The Linked List")
def insertInbetween(self):
middleNodeUserInput = input("Enter Middle Node: ")
nodeToAddUserInput = input("Enter New Note To Add: ")
middleNode = Node()
middleNode.init(middleNodeUserInput)
nodeToAdd = Node()
nodeToAdd.init(nodeToAddUserInput)
nodeToAdd.nextval = middleNode.nextval
middleNode.nextval = nodeToAdd
def removeNode(self):
nodeToRemove = input("Enter Node To Remove: ")
HeadVal = self.headval
if (HeadVal is not None):
if (HeadVal.dataval == nodeToRemove):
return
else:
while (HeadVal is not None):
if HeadVal.dataval == nodeToRemove:
self.head = HeadVal.nextval
HeadVal = None
break
prev = HeadVal
HeadVal = HeadVal.nextval
prev.nextval = HeadVal.nextval
HeadVal = None
else:
print("Nothing To Delete")
def isEmpty(self):
HeadVal = self.headval
total = 0
while (HeadVal is not None):
total+=1
HeadVal = HeadVal.nextval
if(total == 0):
print("List Is Empty")
else:
print("List Is Not Empty")
print(total, " Elements In List")
Posts: 4,220
Threads: 97
Joined: Sep 2016
Please use python and output tags when posting code and results. Yoriz has done it for you twice. Here are instructions for doing it yourself next time.
The problem is that you are setting HeadVal to None when it matches the node to remove. I think you have your code for removing the head of the list mixed up with the code for removing a non-head node.
I would also note that you should be using __init__ instead of init. The former is done automatically on object creation, and would simplify your code (and remove a source of potential errors). I would also look at __repr__, it is your friend when debugging.
Posts: 5
Threads: 1
Joined: Mar 2019
Hi Folks,
Apologies. I accidentally cut the code of how Im deleting the first node.
Thanks ichabod801, I take your point about __init__ but is that likely to be a factor in this instance?
class Node:
def init(self, dataval):
self.dataval = dataval
self.nextval = None
class LinkedList:
def init(self):
self.headval = None
def listprint(self):
printval = self.headval
while printval is not None:
print (printval.dataval)
printval = printval.nextval
def insertAtBegining(self):
nodeToPutAtStart = input("Enter Node To Put At The Start: ")
startNode = Node()
startNode.init(nodeToPutAtStart)
startNode.nextval = self.headval
self.headval = startNode
print(nodeToPutAtStart, " Has Been Added To The Start Of The Linked List")
def insertAtEnd(self):
nodeToPutOnEnd = input("Enter Node To Put On The End: ")
endNode = Node()
endNode.init(nodeToPutOnEnd)
if self.headval is None:
self.headval = endNode
return
laste = self.headval
while(laste.nextval):
laste = laste.nextval
laste.nextval=endNode
print(nodeToPutOnEnd, " Has Been Added To The End Of The Linked List")
def insertInbetween(self):
middleNodeUserInput = input("Enter Middle Node: ")
nodeToAddUserInput = input("Enter New Note To Add: ")
middleNode = Node()
middleNode.init(middleNodeUserInput)
nodeToAdd = Node()
nodeToAdd.init(nodeToAddUserInput)
nodeToAdd.nextval = middleNode.nextval
middleNode.nextval = nodeToAdd
def removeNode(self):
nodeToRemove = input("Enter Node To Remove: ")
HeadVal = self.headval
if (HeadVal is not None):
if (HeadVal.dataval == nodeToRemove):
self.head = HeadVal.nextval
HeadVal = None
return
else:
while (HeadVal is not None):
if HeadVal.dataval == nodeToRemove:
self.head = HeadVal.nextval
HeadVal = None
break
prev = HeadVal
HeadVal = HeadVal.nextval
prev.nextval = HeadVal.nextval
HeadVal = None
else:
print("Nothing To Delete")
def isEmpty(self):
HeadVal = self.headval
total = 0
while (HeadVal is not None):
total+=1
HeadVal = HeadVal.nextval
if(total == 0):
print("List Is Empty")
else:
print("List Is Not Empty")
print(total, " Elements In List")
def MainMenu():
print("1. Create A Linked List")
print("2. Delete A Linked List")
print("3. Check If A Linked List Is Empty")
print("4. Print Out The Values In The List")
print("5. Find A Node In A Linked List")
print("6. Insert A Node In A Linked List")
print("7. Delete A Node In A Linked List")
print("99. Exit")
anotherOption = True
while anotherOption == True:
print("")
selection=int(input("Main Menu --- Enter Choice: "))
if selection==1:
myList = LinkedList()
myList.init()
print("Linked List Created")
elif selection==6:
print("")
print("A. Insert A Node Into The Front Of The Linked List")
print("B. Insert A Node Into The End Of The Linked List")
print("C. Insert A Node In Between Nodes On The Linked List")
print("")
entrySelection=input("Enter Insert Node Choice: ")
if entrySelection in ["A", "a"]:
myList.insertAtBegining()
elif entrySelection in ["B", "b"]:
myList.insertAtEnd()
elif entrySelection in ["C", "c"]:
myList.insertInbetween()
else:
print("")
print("Enter A Valid Selection For Inserting Node")
elif selection==4:
myList.listprint()
elif selection==7:
myList.removeNode()
elif selection==99:
anotherOption = False
print("")
print("Exiting Main Menu")
elif selection==3:
myList.isEmpty()
else:
print("")
print("Enter A Valid Selection On Main Menu")
MainMenu()
Posts: 5
Threads: 1
Joined: Mar 2019
So basically its lines 86 - 89 that aernt working correctly for me.
So when I have my linked list of the following values
50 --- 60 --- 70 --- 80 --- 90
I can delete any of the values apart from the head.
Does my code need another line to commit the deletion of the head to memory? From my testing, it does seem to delete the head node and re assign it to the second node, but when I do a print list, its like it "Rolls Back" to its previous state.
Posts: 4,220
Threads: 97
Joined: Sep 2016
When you delete the head node, you are setting self.head, not self.headval.
(Mar-04-2019, 09:38 PM)ribena1980 Wrote: Thanks ichabod801, I take your point about __init__ but is that likely to be a factor in this instance?
Not a problem in this case, just general programming critique. The other thing I forgot to mention is that you are mixing your interface and your data, which is a good thing to avoid. That is, your list is asking the user questions. That means that you can't create a list without a user. So when you insert a value, the insertFoo methods should take the value as a parameter. The interface should be where the user is asked for a value, and the interface should pass the value to the method.
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