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Context: Create a circular, doubly linked list from a file (input.txt), containing a list of names. The number of names is unknown.
Prompt the user for the number of nodes to delete and then delete accordingly from the list.
Assumption: The number inputted by the user will not exceed the actual number of nodes in the list.
Question: My delete node function is not working, as when I go to display what is in the node that was supposedly deleted, the data is still there. Any Help?
My code: http://www.cplusplus.com/forum/general/210015/
(I am linking to this other website because it was easier to format there.)
My delete node function is not working, as when I go to display what is in the node that was supposedly deleted, the data is still there.
Accessing a destroyed (deleted) object has undefined behaviour.
You cannot expect how the program behaves when the behaviour is undefined. Therefore your expectation that "data shouldn't be there" was ill-founded. A correct expectation is that data might, or might not be there, and that the program might or might not crash and that daemons might or might not fly out of the users nose.
Your code for void deleteNode(struct node *&head) has a lot of bugs.
you don't check if head is nullptr
you don't check if current->next and/or current->prev are nullptr
accessing nullptr pointers is undefined behaviour, that may mean a crash or unexpected results or whatever.
Try using following delete function to delete the node.
Note: assuming delete starts from the head.
void deleteNode(struct node *&head,int deleteCount)
{
if(head != NULL){
struct node *current=head; //copy of head.
struct node *temp,*headPrev,*headNext;
headPrev = head->prev;
headNext = head->next;
int count = 0;
while(count++ < deleteCount)
{
//add code to free current node from me
temp = current; //get rid of this node
currrent = current->next; //skip all the nodes you want delete
}
current->prev = headPrev;
headPrev->next = current;
head = current;
}
}
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I define my Node as a struct and trying to get it. But when I run my code compiler says that "head", "tail" was not declared. Am I define it wrongly?
Tried to use template but it didn't work;
You are having head and tail declared inside class List as it's private member fields. But you are accessing them from methods outside the class. The compiler is throwing errors because of that.
In the insertFront function, you have coded
head = node
node->next = head;
This is wrong. head is the only access you have to the remaining linked list. By doing head = node, you lose access to the rest of the list. and doing node->next = head after that essentially points the node to itself. It should be done in reverse.
node->next = head;
head = node;
Another mistake in the insertBack function. prev is not declared in struct Node. The right way to insert at tail is
tail->next = node;
tail = node;
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I am trying to create a singly linked list by inserting nodes at end, and despite having no errors I am unable to print my linked list. Please help me debug my code.
I tried online compiler on codechef and it shows SIGSEGV Runtime error. What is this supposed to mean?
struct node
{
int data;
struct node *next;
};
void insert(struct node *root,int data)
{
struct node *temp=new(struct node);
if(root==NULL)
{
temp->data=data;
temp->next=NULL;
}
root->next=temp;
temp->data=data;
temp->next=NULL;
}
void print(struct node *root)
{
struct node *temp;
temp=root;
while(temp!=NULL)
{
cout<<temp->data;
temp=temp->next;
}
}
int main()
{
struct node *root=NULL;
insert(root,1);
insert(root,2);
insert(root,3);
insert(root,4);
print(root);
return 0;
}
Please help me debug my code.
OK lets try a dry run.
Imagine your list is empty and you are inserting your first item. So root equals NULL and we call insert.
1) first thing
struct node *temp=new(struct node);
You allocate a new node, and set temp equal to it, so far so good.
2) next thing
if(root==NULL)
this is true as explained in the preamble, so we enter the if statement
3) next thing
temp->data=data;
temp->next=NULL;
these statements in the if body get executed and initialise the newly allocated object. It's not clear why you only want to do this when root == NULL, I would think you would want to initialise the newly allocated node always. But anyway, so far no errors.
4) next thing
root->next=temp;
Now here's the error. Ask yourself, what is the value of root at this point? When we started it was NULL, has anything changed it since? The answer of course is no, so you are dereferencing a NULL pointer. That explains the error.
You need to be able to look at the code you've written and see it for what it really does. The ability to dry run your code like I did above is a very valuable skill to have.
Unfortunately your code really is not very close to being correct. So I think the best thing would be to look at some working code and see how it operates and then start again.
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Here is the snippet of code that confuses me :
T* pop(){
if(head == 0)
return 0;
T* result = head->data;
Link* oldHead = head;
head = head->next;
delete oldHead;
return result;
}
The result pointer is maked to point to the "data field" of head's object. After that, head is deleted. Does not result have to point to null or a blank space since the data field is deleted together with head ?
Does not result have to point to null or a blank space since the data field is deleted together with head?
When you execute
delete oldHead;
the memory for oldHead is deallocated. However, unless you also delete data in the destructor of Link, data continues to be a valid pointer. Hence, it's ok to return result and continue to use the return value in the calling function.
If you have
Link::~Link() {}
there is no problem.
However, if you have
Link::~Link() { delete data; }
there is a problem.
I am pretty new to pointers, one of my practice problems is to add a node to a linked-list recursively. This code works but I am wondering if this is the correct way to go about this problem.
void addNode(node* head){
if (head->next != NULL)
addNode(head->next);
if (head->next == NULL) {
node* newNode = new node;
newNode->next = NULL;
head->next = newNode;
}
}
I am suspicious about two things in your code.
What the data type node is and whether it includes some data fields that you need to somehow initialize. Currently a default constructor will be called, so all nodes in the linked list, added using your function, will be the same. That may or may not be OK. (Depending on what your prof had in mind. This is a homework question after all).
Another issue that is not quite clear, is that you assume the linked list is always non-empty. If head is null your function will not work. I doubt that this is OK.
EDITED:
using c++ to code.
void circularList::deleteNode(int x)
{
node *current;
node *temp;
current = this->start;
while(current->next != this->start)
{
if(current->next->value == x)
{
temp = current->next;
current->next = current->next->next;
delete current->next;
}
else{
current = current->next;
}
}
}
Added the else i'm sorry i kinda forgot to copy that part of the code and yes it is for learning purposes. I'm new to coding with c++ and probably come off as a noob sorry about that.
Also as for this line of code
this->start->value == x
im not sure what you mean by it or where you think it goes, yes there are nodes in the linked list and assume that it will always have at lease 1 node all the time.
Think about this two lines:
current->next = current->next->next;
delete current->next;
Try to determine what you are actually deleting (no its not current->next;, at least not the one you want to delete).
You never move to the next node in your while loop. After your if, you should have:
else
current = current->next;
Also, you might want to consider returning from the function after you've found the node (unless you suspect that two nodes have the same value).
In addition to Justin's and Let_Me-Be's answers, consider what you might need to take care of when
this->start->value == x
If you don't handle that right, you'll lose your whole list (or crash trying to get to it)...
Do you have only a singularly linked list?
Have you considered the STL, perhaps a deque?
Or, if you must have a single linked list then why not take something that it 90% STL (;-) and look at the Boost libraries?
You do seem to be reinventing the wheel here. Why not take some existing - and tested - code and use that?
Is "deleting a node from linked list" the issue or "deleting a node from a circular linked list" the issue?
The difference from non-circular to circular is the line:
while(current->next != this->start)
If the list were non circular it would have been
while(current->next != NULL )
Apart from that, there are issues with the code snippet, which others have already pointed out.
Another thing: Do you check that you have at least one valid node in your list before you call deleteNode?
I ask this as your function does not check for a NULL-value of the start element.
The solution would be to add the line
if(start == NULL) return
as first line of your method to avoid this.