I have created a doubly linked list.
in the list, there is a function that is const, and is not supposed to modify the object. but it is modifyind I dont know why.
#include<iostream>
using namespace std;
class Node {
public:
int data;
Node* prev;
Node* next;
Node(int val) :data(val), next(NULL), prev(NULL) {}
};
class List {
Node* head;
Node* tail;
public:
List() :head(NULL), tail(NULL) {}
void addToTail(int val) {
Node* temp = new Node(val);
if (head == NULL) {
head = temp;
tail = temp;
}
else {
tail = head;
while (tail->next != NULL) {
tail = tail->next;
}
tail->next = temp;
temp->prev = tail;
tail = temp;
}
}
int search(int val) const
{
if (head->data == val)
head->data = 12;
return head->data;
}
};
int main()
{
List l;
l.addToTail(1);
l.addToTail(2);
l.addToTail(3);
l.addToTail(4);
l.addToTail(5);
int c = l.search(1);
//c = 102;
cout << c;
}
Now I have tried to use const before the return type, but obviously it doesnt matter. it does not affect the result;
in the search(int val) function, I am sending a value to check if 'head->data' is equal to the 'val', it should not modify the 'head->data = 12' because the function is const. but it is doing this.
The const qualifier for the member function only tells the compiler that you won't modify this object.
And you don't do that: You modify head->data which is another object.
It would be a different issue if you tried to reassign the variables head or tail.
I'm practicing implementing a Template Class Linked List with the Node struct within the implementation of the Linked List Class. In the createNode() member function, when I initialize a pointer variable to a node struct, I get the compiler error: "Uninitialized local variable 'newNode' used"
I've found if I change the initialization to:
Node* newNode = new Node();
That it works just fine. I'm a bit confused as to why this matters if I can initialize the basic data types like int as:
int* intPtr;
Why can't I do the same with structs?? My code is below:
#include <iostream>
#include <string>
template<class T>
class LinkedList
{
private:
struct Node
{
T data;
Node* next;
};
Node* head;
Node* tail;
int size;
public:
LinkedList() : head{ nullptr }, tail{ nullptr }, size{ 0 }
{
}
Node* createNode(T data)
{
Node* newNode;
newNode->data = data;
newNode->next = nullptr;
return newNode;
}
void display()
{
Node* currentNode = head;
while (currentNode)
{
std::cout << currentNode->data << std::endl;
currentNode = currentNode->next;
}
}
void push(T data)
{
Node* newNode = createNode(data);
if (size == 0)
{
head = newNode;
tail = newNode;
}
else
{
tail->next = newNode;
tail = newNode;
}
++size;
}
};
int main()
{
LinkedList<int> list;
list.push(5);
list.push(3);
list.push(6);
list.display();
std::cin.clear();
std::cin.ignore(32767, '\n');
std::cin.get();
return 0;
}
You said:
I'm a bit confused as to why this matters if I can initialize the basic data types like int as:
int* intPtr;
That's a wrong conclusion. If you use:
int* intPtr;
*intPtr = 10;
you'll probably see the same warning/error from the compiler. The correct way to use intPtr will be to make sure it is initialized to point to a valid object before anything is assigned to it using *intPtr.
int* intPtr = new int;
*intPtr = 10;
This is similar to using
Node* newNode = new Node();
newNode->data = data;
newNode->next = nullptr;
in your code.
I'm trying to construct an adjacency list. The code I wrote is as follows.
struct Node
{
int dest;
struct Node* next;
};
struct list
{
struct list *head;
};
The class is defined as:
class Graph
{
private:
int vertix;
list *arr;
public:
Graph(int v)
{
vertix = v;
arr = new list [vertix];
for(int i=0;i<vertix;i++)
{
arr[i].head=NULL;
}
}
Node* getNewNode(int destination)
{
Node* newNode = new Node;
newNode->dest = destination;
newNode->next = NULL;
return newNode;
}
The errors are in these functions:
void addEdge(int src, int dest)
{
Node* newNode = getNewNode(dest);
newNode->next = arr[src].head;
arr[src].head = newNode;
newNode = getNewNode(src);
newNode->next = arr[dest].head;
arr[dest].head = newNode;
}
void print()
{
cout<<"Adjacency list of vertix: "<<endl;
for(int i = 0; i< vertix; i++)
{
Node *ptr = arr[i].head;
cout<< i << "-->";
while(ptr)
{
cout<< "-->"<<ptr->dest;
ptr=ptr->next;
}
cout<<endl;
}
}
};
The error messages I get are:
[Error] cannot convert 'list*' to 'Node*' in assignment
[Error] cannot convert 'list*' to 'Node*' in initialization
I'm not sure if it is just a typo, but instead of
struct list
{
struct list *head;
};
you should have
struct list
{
Node *head;
};
since the head of a list is a node, not another list. This causes the error in this line:
Node *ptr = arr[i].head;
because you are trying to assign the head of the list (which in your current code is a list*) to a Node*
I am about to create a linked that can insert and display until now:
struct Node {
int x;
Node *next;
};
This is my initialisation function which only will be called for the first Node:
void initNode(struct Node *head, int n){
head->x = n;
head->next = NULL;
}
To add the Node, and I think the reason why my linked list isn't working correct is in this function:
void addNode(struct Node *head, int n){
struct Node *NewNode = new Node;
NewNode-> x = n;
NewNode -> next = head;
head = NewNode;
}
My main function:
int _tmain(int argc, _TCHAR* argv[])
{
struct Node *head = new Node;
initNode(head, 5);
addNode(head, 10);
addNode(head, 20);
return 0;
}
Let me run the program as I think it works. First I initialise the head Node as a Node like this:
head = [ 5 | NULL ]
Then I add a new node with n = 10 and pass head as my argument.
NewNode = [ x | next ] where next points at head. And then I change the place where head is pointing to NewNode, since NewNode is the first Node in LinkedList now.
Why isn't this working? I would appreciate any hints that could make me move in the right direction. I think LinkedList is a bit hard to understand.
When I'm printing this, it only returns 5:
This is the most simple example I can think of in this case and is not tested. Please consider that this uses some bad practices and does not go the way you normally would go with C++ (initialize lists, separation of declaration and definition, and so on). But that are topics I can't cover here.
#include <iostream>
using namespace std;
class LinkedList{
// Struct inside the class LinkedList
// This is one node which is not needed by the caller. It is just
// for internal work.
struct Node {
int x;
Node *next;
};
// public member
public:
// constructor
LinkedList(){
head = NULL; // set head to NULL
}
// destructor
~LinkedList(){
Node *next = head;
while(next) { // iterate over all elements
Node *deleteMe = next;
next = next->next; // save pointer to the next element
delete deleteMe; // delete the current entry
}
}
// This prepends a new value at the beginning of the list
void addValue(int val){
Node *n = new Node(); // create new Node
n->x = val; // set value
n->next = head; // make the node point to the next node.
// If the list is empty, this is NULL, so the end of the list --> OK
head = n; // last but not least, make the head point at the new node.
}
// returns the first element in the list and deletes the Node.
// caution, no error-checking here!
int popValue(){
Node *n = head;
int ret = n->x;
head = head->next;
delete n;
return ret;
}
// private member
private:
Node *head; // this is the private member variable. It is just a pointer to the first Node
};
int main() {
LinkedList list;
list.addValue(5);
list.addValue(10);
list.addValue(20);
cout << list.popValue() << endl;
cout << list.popValue() << endl;
cout << list.popValue() << endl;
// because there is no error checking in popValue(), the following
// is undefined behavior. Probably the program will crash, because
// there are no more values in the list.
// cout << list.popValue() << endl;
return 0;
}
I would strongly suggest you to read a little bit about C++ and Object oriented programming. A good starting point could be this: http://www.galileocomputing.de/1278?GPP=opoo
EDIT: added a pop function and some output. As you can see the program pushes 3 values 5, 10, 20 and afterwards pops them. The order is reversed afterwards because this list works in stack mode (LIFO, Last in First out)
You should take reference of a head pointer. Otherwise the pointer modification is not visible outside of the function.
void addNode(struct Node *&head, int n){
struct Node *NewNode = new Node;
NewNode-> x = n;
NewNode -> next = head;
head = NewNode;
}
I'll join the fray. It's been too long since I've written C. Besides, there's no complete examples here anyway. The OP's code is basically C, so I went ahead and made it work with GCC.
The problems were covered before; the next pointer wasn't being advanced. That was the crux of the issue.
I also took the opportunity to make a suggested edit; instead of having two funcitons to malloc, I put it in initNode() and then used initNode() to malloc both (malloc is "the C new" if you will). I changed initNode() to return a pointer.
#include <stdlib.h>
#include <stdio.h>
// required to be declared before self-referential definition
struct Node;
struct Node {
int x;
struct Node *next;
};
struct Node* initNode( int n){
struct Node *head = malloc(sizeof(struct Node));
head->x = n;
head->next = NULL;
return head;
}
void addNode(struct Node **head, int n){
struct Node *NewNode = initNode( n );
NewNode -> next = *head;
*head = NewNode;
}
int main(int argc, char* argv[])
{
struct Node* head = initNode(5);
addNode(&head,10);
addNode(&head,20);
struct Node* cur = head;
do {
printf("Node # %p : %i\n",(void*)cur, cur->x );
} while ( ( cur = cur->next ) != NULL );
}
compilation: gcc -o ll ll.c
output:
Node # 0x9e0050 : 20
Node # 0x9e0030 : 10
Node # 0x9e0010 : 5
Below is a sample linkedlist
#include <string>
#include <iostream>
using namespace std;
template<class T>
class Node
{
public:
Node();
Node(const T& item, Node<T>* ptrnext = NULL);
T value;
Node<T> * next;
};
template<class T>
Node<T>::Node()
{
value = NULL;
next = NULL;
}
template<class T>
Node<T>::Node(const T& item, Node<T>* ptrnext = NULL)
{
this->value = item;
this->next = ptrnext;
}
template<class T>
class LinkedListClass
{
private:
Node<T> * Front;
Node<T> * Rear;
int Count;
public:
LinkedListClass();
~LinkedListClass();
void InsertFront(const T Item);
void InsertRear(const T Item);
void PrintList();
};
template<class T>
LinkedListClass<T>::LinkedListClass()
{
Front = NULL;
Rear = NULL;
}
template<class T>
void LinkedListClass<T>::InsertFront(const T Item)
{
if (Front == NULL)
{
Front = new Node<T>();
Front->value = Item;
Front->next = NULL;
Rear = new Node<T>();
Rear = Front;
}
else
{
Node<T> * newNode = new Node<T>();
newNode->value = Item;
newNode->next = Front;
Front = newNode;
}
}
template<class T>
void LinkedListClass<T>::InsertRear(const T Item)
{
if (Rear == NULL)
{
Rear = new Node<T>();
Rear->value = Item;
Rear->next = NULL;
Front = new Node<T>();
Front = Rear;
}
else
{
Node<T> * newNode = new Node<T>();
newNode->value = Item;
Rear->next = newNode;
Rear = newNode;
}
}
template<class T>
void LinkedListClass<T>::PrintList()
{
Node<T> * temp = Front;
while (temp->next != NULL)
{
cout << " " << temp->value << "";
if (temp != NULL)
{
temp = (temp->next);
}
else
{
break;
}
}
}
int main()
{
LinkedListClass<int> * LList = new LinkedListClass<int>();
LList->InsertFront(40);
LList->InsertFront(30);
LList->InsertFront(20);
LList->InsertFront(10);
LList->InsertRear(50);
LList->InsertRear(60);
LList->InsertRear(70);
LList->PrintList();
}
Both functions are wrong. First of all function initNode has a confusing name. It should be named as for example initList and should not do the task of addNode. That is, it should not add a value to the list.
In fact, there is not any sense in function initNode, because the initialization of the list can be done when the head is defined:
Node *head = nullptr;
or
Node *head = NULL;
So you can exclude function initNode from your design of the list.
Also in your code there is no need to specify the elaborated type name for the structure Node that is to specify keyword struct before name Node.
Function addNode shall change the original value of head. In your function realization you change only the copy of head passed as argument to the function.
The function could look as:
void addNode(Node **head, int n)
{
Node *NewNode = new Node {n, *head};
*head = NewNode;
}
Or if your compiler does not support the new syntax of initialization then you could write
void addNode(Node **head, int n)
{
Node *NewNode = new Node;
NewNode->x = n;
NewNode->next = *head;
*head = NewNode;
}
Or instead of using a pointer to pointer you could use a reference to pointer to Node. For example,
void addNode(Node * &head, int n)
{
Node *NewNode = new Node {n, head};
head = NewNode;
}
Or you could return an updated head from the function:
Node * addNode(Node *head, int n)
{
Node *NewNode = new Node {n, head};
head = NewNode;
return head;
}
And in main write:
head = addNode(head, 5);
The addNode function needs to be able to change head. As it's written now simply changes the local variable head (a parameter).
Changing the code to
void addNode(struct Node *& head, int n){
...
}
would solve this problem because now the head parameter is passed by reference and the called function can mutate it.
head is defined inside the main as follows.
struct Node *head = new Node;
But you are changing the head in addNode() and initNode() functions only. The changes are not reflected back on the main.
Make the declaration of the head as global and do not pass it to functions.
The functions should be as follows.
void initNode(int n){
head->x = n;
head->next = NULL;
}
void addNode(int n){
struct Node *NewNode = new Node;
NewNode-> x = n;
NewNode->next = head;
head = NewNode;
}
I think that, to make sure the indeep linkage of each node in the list, the addNode method must be like this:
void addNode(struct node *head, int n) {
if (head->Next == NULL) {
struct node *NewNode = new node;
NewNode->value = n;
NewNode->Next = NULL;
head->Next = NewNode;
}
else
addNode(head->Next, n);
}
Use:
#include<iostream>
using namespace std;
struct Node
{
int num;
Node *next;
};
Node *head = NULL;
Node *tail = NULL;
void AddnodeAtbeggining(){
Node *temp = new Node;
cout << "Enter the item";
cin >> temp->num;
temp->next = NULL;
if (head == NULL)
{
head = temp;
tail = temp;
}
else
{
temp->next = head;
head = temp;
}
}
void addnodeAtend()
{
Node *temp = new Node;
cout << "Enter the item";
cin >> temp->num;
temp->next = NULL;
if (head == NULL){
head = temp;
tail = temp;
}
else{
tail->next = temp;
tail = temp;
}
}
void displayNode()
{
cout << "\nDisplay Function\n";
Node *temp = head;
for(Node *temp = head; temp != NULL; temp = temp->next)
cout << temp->num << ",";
}
void deleteNode ()
{
for (Node *temp = head; temp != NULL; temp = temp->next)
delete head;
}
int main ()
{
AddnodeAtbeggining();
addnodeAtend();
displayNode();
deleteNode();
displayNode();
}
In a code there is a mistake:
void deleteNode ()
{
for (Node * temp = head; temp! = NULL; temp = temp-> next)
delete head;
}
It is necessary so:
for (; head != NULL; )
{
Node *temp = head;
head = temp->next;
delete temp;
}
Here is my implementation.
#include <iostream>
using namespace std;
template< class T>
struct node{
T m_data;
node* m_next_node;
node(T t_data, node* t_node) :
m_data(t_data), m_next_node(t_node){}
~node(){
std::cout << "Address :" << this << " Destroyed" << std::endl;
}
};
template<class T>
class linked_list {
public:
node<T>* m_list;
linked_list(): m_list(nullptr){}
void add_node(T t_data) {
node<T>* _new_node = new node<T>(t_data, nullptr);
_new_node->m_next_node = m_list;
m_list = _new_node;
}
void populate_nodes(node<T>* t_node) {
if (t_node != nullptr) {
std::cout << "Data =" << t_node->m_data
<< ", Address =" << t_node->m_next_node
<< std::endl;
populate_nodes(t_node->m_next_node);
}
}
void delete_nodes(node<T>* t_node) {
if (t_node != nullptr) {
delete_nodes(t_node->m_next_node);
}
delete(t_node);
}
};
int main()
{
linked_list<float>* _ll = new linked_list<float>();
_ll->add_node(1.3);
_ll->add_node(5.5);
_ll->add_node(10.1);
_ll->add_node(123);
_ll->add_node(4.5);
_ll->add_node(23.6);
_ll->add_node(2);
_ll->populate_nodes(_ll->m_list);
_ll->delete_nodes(_ll->m_list);
delete(_ll);
return 0;
}
link list by using node class and linked list class
this is just an example not the complete functionality of linklist, append function and printing a linklist is explained in the code
code :
#include<iostream>
using namespace std;
Node class
class Node{
public:
int data;
Node* next=NULL;
Node(int data)
{
this->data=data;
}
};
link list class named as ll
class ll{
public:
Node* head;
ll(Node* node)
{
this->head=node;
}
void append(int data)
{
Node* temp=this->head;
while(temp->next!=NULL)
{
temp=temp->next;
}
Node* newnode= new Node(data);
// newnode->data=data;
temp->next=newnode;
}
void print_list()
{ cout<<endl<<"printing entire link list"<<endl;
Node* temp= this->head;
while(temp->next!=NULL)
{
cout<<temp->data<<endl;
temp=temp->next;
}
cout<<temp->data<<endl;;
}
};
main function
int main()
{
cout<<"hello this is an example of link list in cpp using classes"<<endl;
ll list1(new Node(1));
list1.append(2);
list1.append(3);
list1.print_list();
}
thanks ❤❤❤
screenshot https://i.stack.imgur.com/C2D9y.jpg
i created a linked list : struct Node and List Class and i used it outside with my main method,
#include "Lists.cpp"
#include <cstdlib>
using namespace std;
int main(){
Lists l = new Lists(1);
l.add(2);
l.add(3);
l.add(4);
l.add(5);
system("pause");
return 0;
}
but it produces an error that says "invalid conversion from List* to int". Is my using of outside class right? Im a little confused how I will solve this.
#include <cstdlib>
#include <iostream>
using namespace std;
struct Node{
int data;
Node *next;
Node(int i){
data = i;
next = NULL;
}
};
class List{
Node *head;
public:
List(int i){
head = new Node(i);
}
void addToHead(int i){
Node *temp = new Node(i);
temp->next = head;
head = temp;
}
void add(int i){
Node *currNode = head;
while(currNode!= NULL){
if(currNode->next == NULL){
currNode->next = new Node(i);
break;
}
else{
currNode = currNode-> next;
}
}
}
void deleteNode(int i){
Node *currNode = head;
Node *prevNode = NULL;
while(currNode!= NULL){
if(currNode->data == i) {
if(prevNode== NULL){
head = head->next;
}
else{
prevNode->next = currNode->next;
}
}
prevNode = currNode;
currNode = currNode -> next;
}
}
void insert(int position, Node *n){
Node *currNode= head;
Node *prevNode = NULL;
for(int counter = 0; counter>= position && currNode!= NULL; counter++){
if(counter==position){
Node *temp = currNode;
n->next = currNode;
prevNode->next= n;
}
prevNode = currNode;
currNode = currNode-> next;
}
}
void traverse(Node *node){
if(node!=NULL){
cout<< node-> data <<endl;
traverse(node->next);
}
}
};
Lists l = new Lists(1);
should be:
Lists *l = new Lists(1);
new provides a pointer.
The reason you get that specific error is that the line would be valid if the conversion chain Lists * -> int -> Lists were valid. The second is valid here because of the constructor but the first is not.
At which line do you get the mentionned error ? At first glance, Lists l = new Lists(1); is already wrong : Lists* l = new Lists(1); would be correct.
But this error does not correspond to the one you mention. Also note that you're defining List and trying to use Lists.
Is this the code you're trying to compile ?
Use List *l = new List(1); or std::shared_ptr<List> l = make_shared<List>( 1 ); if you want a dynamically-allocated List.
Or List l(1); if you want a List with automatic storage duration.
Be careful with the names, in the class definition you use List, and in the main function you use Lists.