Linked list Behaviour - c++

Can Someone please explain the difference in Behaviour ?
#include <iostream>
using namespace std;
struct Node {
int data;
Node* next;
};
// only for the 1st Node
void initNode(Node *head,int n){
head->data = n;
head->next =NULL;
}
void insertFront(Node *head, int n) {
Node *newNode = new Node;
newNode->data = n;
newNode->next = head;
head = newNode;
}
void display(Node *head) {
Node *list = head;
while(list) {
cout << list->data << " ";
list = list->next;
}
cout << endl;
cout << endl;
}
int main()
{
Node *l1 = new Node;
initNode(l1,10);
display(l1);
insertFront(l1,5);
display(l1);
insertFront(l1,6);
display(l1);
insertFront(l1,7);
display(l1);
return 0;
}
Some how the nodes are not linking. The output is :
10
10
10
10
If The program is coded using pointer to a pointer as below then it works fine. what am is missing ?
#include <iostream>
using namespace std;
struct Node {
int data;
Node* next;
};
// only for the 1st Node
void initNode(Node *head,int n){
head->data = n;
head->next =NULL;
}
void insertFront(Node **head, int n) {
Node *newNode = new Node;
newNode->data = n;
newNode->next = *head;
*head = newNode;
}
void display(Node *head) {
Node *list = head;
while(list) {
cout << list->data << " ";
list = list->next;
}
cout << endl;
cout << endl;
}
int main()
{
Node *l1 = new Node;
initNode(l1,10);
display(l1);
insertFront(&l1,5);
display(l1);
insertFront(&l1,6);
display(l1);
insertFront(&l1,7);
display(l1);
return 0;
}
Output correct as expected :
10
5 10
6 5 10
7 6 5 10

In the first case, in function
void insertFront(Node *head, int n) {
Node *newNode = new Node;
newNode->data = n;
newNode->next = head;
head = newNode;
}
head is a copy of the pointer l1 used in the scope of main(). When head is modified, l1 is left unchanged. This is the reason why a pointer to l1 (&l1) is passed to the function in the second case void insertFront(Node **head, int n). Then *head is l1, not just a copy of l1.
First case is an example of passing argument by value, and the second case is passing an argument by reference What's the difference between passing by reference vs. passing by value?
For instance, the following function is basically useless :
void useless(int a){
a=42;
}
If int b=2;useless(b);cout<<b<<endl; is called it will print 2, not 42.
The following function is the right way to go :
void rightwaytogo(int*a){
*a=42;
}
Don't forget to write a function to delete the nodes of your linked list.

Related

Why is this linked list program not giving any output?

So I made these two simple functions regarding linked lists. One adds a node at the front and the other just displays the linked list in a sequence front to end. I'm wondering why this code wouldn't give me any output.
#include <iostream>
using namespace std;
class Node
{
public:
int data;
Node *next;
};
Node *head;
void addFront(Node *head, int item)
{
Node *temp = new Node();
temp->data = item;
temp->next = head;
head = temp;
}
void traverse(Node *head)
{
Node *temp = head;
while(temp!=NULL)
{
cout << temp->data << " ";
temp = temp->next;
}
}
int main()
{
addFront(head, 1);
addFront(head, 2);
addFront(head, 3);
traverse(head);
}
You're operating on a copy of head pointer in addFront(). You have
to pass a pointer to pointer:
void addFront(Node **head, int item)
The entire code could look like this:
#include <iostream>
using namespace std;
class Node
{
public:
int data;
Node *next;
};
Node *head;
void addFront(Node **head, int item)
{
Node *temp = new Node();
temp->data = item;
temp->next = *head;
*head = temp;
}
void traverse(Node *head)
{
Node *temp = head;
while(temp!=NULL)
{
cout << temp->data << " ";
temp = temp->next;
}
}
int main()
{
addFront(&head, 1);
addFront(&head, 2);
addFront(&head, 3);
traverse(head);
}
Argument of addFront is input as well as output. (Node *head)
It needs to be passed as referenced

C++ program to Reverse a linked list from position m to n

I am stuck on this simple question for hours...can someone plz help...where i am going wrong?
Question : Reverse a linked list from position m to n. Do it in-place and in one-pass.
For example:
Given 1->2->3->4->5->NULL, m = 2 and n = 4,
return 1->4->3->2->5->NULL.
Note: 1 ≤ m ≤ n ≤ length of list
#include<stdio.h>
#include<bits/stdc++.h>
using namespace std;
struct node
{
int data;
struct node *next;
};
typedef struct node ListNode;
node *newNode(int key)
{
node *temp = new node;
temp->data = key;
temp->next = NULL;
return temp;
}
ListNode* reverseUpto(ListNode *head, int size)
{
if(size<=1)
return head;
ListNode *cur=head,*newhead=NULL,*temp;
for(int i=0;i<size;i++)
{
temp=cur;
cur=cur->next;
temp->next=newhead;
newhead=temp;
}
head->next=cur;
return newhead;
}
ListNode* reverseBetween(ListNode* A, int m, int n)
{
ListNode *head=A;
if(m==n)
return A;
ListNode *dummyhead=newNode(0);
dummyhead->next=head;
ListNode *prev=dummyhead;
ListNode *cur=head;
int counter=1;
while(counter<m)
{
printf("counter= %d prev=%d cur=%d\n",counter,prev->data,cur->data);
counter++;
prev=cur;
cur=cur->next;
}
prev->next=NULL;
ListNode * retPtr=reverseUpto(cur,m-n+1);
prev->next=retPtr;
return A;
}
void printlist(node *head)
{
while(head != NULL)
{
cout << head->data << " ";
head = head->next;
}
cout << endl;
}
int main()
{
node *head1 = newNode(1);
head1->next = newNode(2);
head1->next->next = newNode(3);
head1->next->next->next = newNode(4);
head1->next->next->next->next = newNode(5);
head1->next->next->next->next->next = newNode(6);
head1->next->next->next->next->next->next = newNode(7);
head1->next->next->next->next->next->next->next = newNode(8);
cout << "Given linked list\n";
printlist(head1);
head1=reverseBetween(head1,3,5);
cout << "\nReversed linked list\n";
printlist(head1);
return 0;
}
I am getting same output as my input!!....where is the pitfall?
ListNode * retPtr=reverseUpto(cur,m-n+1);
modify to
ListNode * retPtr=reverseUpto(cur,n-m+1);

Simple linked list in C++

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

C++ doubly linked list

I got a problem with my doubly linked list. How can i make the input unique ( i don`t want it to be repeated )
for example i can input 1 and then again 1 i will have a list of 1 and 1. I need to forbid this somehow :) so the list can contain only not repeating numbers.
#include <cstdlib>
#include <iostream>
using namespace std;
struct node
{
int data;
node* next;
node* prev;
};
class Node
{
public:
Node();
~Node();
void setKopa();
void printForward();
private:
node* head;
node* tail;
node* n;
};
Node::Node()
{
setKopa();
}
Node::~Node()
{
delete n;
}
void Node::setKopa()
{
int lenght;
do
{
cout << "Input list lenght (how many elements): ";
cin >> lenght;
if(lenght<2)
cout << "Error list has to have atleast 2 elements!" <<endl;
}
while(lenght<2);
int fill;
cout << "Input "<< lenght <<" elements: "<<endl;
for (int i=0; i<lenght; i++)
{
cin>>fill;
n = new node;
n->data = fill;
if (i==0)
{
n->prev = NULL;
head = n;
tail = n;
}
else if (i+1==lenght)
{
n->prev = tail;
tail->next = n;
tail = n;
tail->next = NULL;
}
else
{
n->prev = tail;
tail->next = n;
tail = n;
}
}
}
void Node::printForward()
{
node* temp = head;
while(temp != NULL)
{
cout << temp->data << " ";
temp = temp-> next;
}
cout << endl;
}
int main()
{
Node a;
a.printForward();
system("pause");
return 0;
}
When you read input, go through the list to see if the input is already there.
With that (simple) answer out of the way, I would like to address some other things regarding your code. The first is that you have a memory leak in that you never delete the list. The second is that you don't need the class member variable n, it might as well be a local variable inside the setKopa loop.
Your way of adding new nodes is also, well, weird. It should, in my opinion, be more general instead of using the loop counter to check what to do. What I suggest is that you make a member function to add new nodes, taking the integer data as argument. This way you can call this function to add nodes anywhere, and not just in the setKopa function. In fact, I think the list should not handle that input at all, instead it should be a free-standing function called from main and which calls the addNode function.
Also the node structure doesn't need to be in the global namespace, it could be a private structure in the Node class. And speaking of the Node class, shouldn't it really be called List instead?
So if I may suggest, you might want to do something like this:
#include <iostream>
class List
{
public:
List()
: head(nullptr), tail(nullptr)
{}
~List();
void addNode(const int data);
void printAll() const;
private:
struct node
{
node()
: next(nullptr), prev(nullptr)
{}
node* next;
node* prev;
int data;
};
node* head;
node* tail;
};
List::~List()
{
for (node* next, *cur = head; cur; cur = next)
{
next = cur->next;
delete cur;
}
}
void List::addNode(const int data)
{
node* n = new node;
n->data = data;
if (tail == nullptr)
{
// First node in list
head = tail = n;
}
else
{
n->prev = tail;
tail->next = n;
tail = n;
}
}
void List::printAll() const
{
std::cout << "{ ";
for (node* cur = head; cur != nullptr; cur = cur->next)
std::cout << cur->data << ' ';
std::cout << "}\n";
}
int main()
{
List list;
for (int i = 0; i < 10; ++i)
list.addNode(i);
list.printAll();
}
The above code should print
{ 0 1 2 3 4 5 6 7 8 9 }
Replace the node-adding loop with your own.

Linked list problem

I got some problem with the linked list I've written. I dunno if it's either my insert function that the problem, or if it's my traverse function that's not correct. I hope for some input. A side note, I'm initalising the list in main now since I don't know if my initNode function is correct.
#include <iostream>
using namespace std;
typedef struct Node
{
int data;
Node *next;
};
void initNode(Node *head)
{
head = new Node;
head->next = NULL;
}
void insertNode(Node *head, int x)
{
Node *temp;
temp = new Node;
temp->data = x;
temp->next = head;
head = temp;
}
void traverse(Node *head)
{
Node *temp;
temp = head;
if(head == NULL)
{
cout << "End of list. " << endl;
}
else
{
while(temp != NULL)
{
cout << temp->data << " ";
temp = temp->next;
}
}
}
int main()
{
Node *head;
head = NULL;
insertNode(head, 5);
insertNode(head, 5);
traverse(head);
return 0;
}
Your head isn't being returned to main from insertNode. Note that even though head is a pointer, the pointer itself is a value and any changes to the pointer value are not reflected in main. The simplest solution is to pass back the updated value of head:
Node *insertNode(Node *head, int x)
{
...
return head;
}
And update it in main:
head = insertNode(head, 5);
The other common way of doing this is to pass a pointer to a pointer and update it directly:
void insertNode(Node **head, int x)
{
Node *temp;
temp = new Node;
temp->data = x;
temp->next = *head;
*head = temp;
}
And call it like this:
insertNode(&head, 5);
The way you have you initNode function written will result in memory leaks. You've passed in a pointer, but you need to pass in a reference to a pointer. (Same issue that James and casablanca mentioned for insertNode.)