I am a beginner in C++ and need help in many things. Well, for the starters, I have been working on Linked List and not really getting why my header(the first pointer which points towards first node) keep on rotating. I am just pointing it towards first node plus my display node is just displaying last node, why is it so?. Please tell me where I am wrong. Thank you in advance
#include <iostream>
#include <conio.h>
using namespace std;
struct Node
{
int data;
Node *link;
};
Node* create_Node()
{
int no_of_nodes;
Node *header = new Node;
Node *ptr = new Node;
header = ptr;
cout << "Enter no of nodes:";
cin >> no_of_nodes;
cout << "Enter data:";
for(int n = 0; n < no_of_nodes; n++)
{
cin >> ptr->data;
Node *temp = new Node;
ptr->link = temp;
temp = ptr;
}
ptr->link = NULL;
return ptr;
}
void display_link_list(Node * list)
{
Node *temp = new Node;
temp = list;
while(temp != NULL)
{
if(temp->link != NULL)
{
cout << "List:" << list->data << endl;
temp = temp->link;
}
}
}
int main()
{
Node *n = new Node;
n = create_Node();
display_link_list(n);
getch();
return 0;
}
Welcome to C++. My advice here is to break the Linked list into two. First the Nodes and then a List struct.
struct Node
{
int data;
Node *next;
Node(int data) : data(data), next(NULL) {}
};
struct List {
Node* tail;
Node* head;
List() : head(NULL), tail(NULL) {}
void insert(int data) {
if(head==NULL) {
head = new Node(data);
tail = head;
} else {
tail->next = new Node(data);
tail = tail->next;
}
}
};
Now you can insert one element into the list at a time and use head to print the list from beginning to end.
Something basic that you need to understand:
When you do Node* p = new Node, you are setting variable p to point to the start address of a piece of memory, the size of which being equal to sizeof(Node).
Now, when you then do p = something else (which often appears in your code), you are essentially overriding the previous value of p with some other value. It is like doing:
int i = 5;
i = 6;
So your code does not do what you're expecting to begin with.
In addition to that, what's bad about overriding the first value with a second value in this case, is the fact that the first value is the address of a dynamically-allocated piece of memory, that you will need to delete at a later point in your program. And once you've used p to store a different value, you no longer "remember" that address, hence you cannot delete that piece of memory.
So you should start by fixing this problem in each of the following places:
Node *header = new Node; // Variable 'header' is assigned
header = ptr; // Variable 'header' is reassigned
Node *temp = new Node; // Variable 'temp' is assigned
temp = list; // Variable 'temp' is reassigned
Node *n = new Node; // Variable 'n' is assigned
n = create_Node(); // Variable 'n' is reassigned
Related
I've been studying about dynamic allocated lists (the stuff we are discussing right now in class is pretty outdated tbh) and I can't seem to access the next node in a list. (I've just began learning this topic)
The problem is that the while loop that I'm using for going through the list never stops. I'm definitely missing something and I don't understand what.
struct node {
int info;
node* next;
};
int main()
{
node *p, *prim=NULL;
p = prim;
node* t;
t = new node;
p->next = t;
while (p != NULL)
{
cout << "test";
p = p -> next;
}
return 0;
}
Here is the code.
Why does my program not output anything and also tells me "it exited with code -1073741819" instead of 0?
Thanks.
////edit: I forgot to tell you that I've tried this way too
struct node {
int info;
node* next;
};
int main()
{
node *p, *prim=NULL;
p = prim;
node* t;
t = new node;
prim->next = t;
while (p != NULL)
{
cout << "test";
p = p -> next;
}
return 0;
}
Let's analyze your code:
node *p, *prim=NULL; // prim is NULL, p not yet initnialized
p = prim; // p now equal to NULL as well
node* t;
t = new node; // t is allocated
p->next = t; // NULL->next = t
So you're crashing on a null pointer when you try to dereference p->next for assignment.
It looks like you're trying to setup a basic linked list and loop through it. Your loop looks good, but you forgot the part where you actually setup the list!
That could look like:
struct node {
int info;
node* next;
//Add a quick constructor to make creating new nodes easy
node(int i) : info(i), next(nullptr) { }
};
int main()
{
//Start out with 3 itmes
// This list will look like:
// head -> 5 -> 3 -> 1
node *head = new node(5);
head->next = new node(3);
head->next->next = new node(1);
//Loop through the list and print each value
for(node *p = head; p; p = p->next) {
std::cout << p->info << std::endl;
}
//Don't forget to delete the memory you allocated to prevent a leak!
for(node *p = head; p;) {
node *temp = p->next;
delete p;
p = temp;
}
return 0;
}
See it run: https://ideone.com/WngD3b
The following code is for a basic circular linked list, but when one inputs a large value for n(e.g 8 digits) it throws the "abort signal from abort(3) (sigabrt)" error. I'm not sure what it means and would love some guidance about fixing this with regard to my code.
Thank you!
#include<bits/stdc++.h>
using namespace std;
//First I created a structure for a node in a circular linked list
struct Node
{
int data;
struct Node *next;
};
// function to create a new node
Node *newNode(int data)
{
Node *temporary = new Node;
temporary->next = temporary;
temporary->data = data;
return temporary;
}
// This function finds the last man standing in
//the game of elimination
void gameOfElimination(int m, int n)
{
//first I created a circular linked list of the size which the user inputted
Node *head = newNode(1);
Node *prev = head;
//this loop links the previous node to the next node, and so on.
for (int index = 2; index <= n; index++)
{
prev->next = newNode(index);
prev = prev->next;
}
prev->next = head; //This connects the last and first nodes in our linked list together.
//when only one node is left, which is our answer:
Node *ptr1 = head, *ptr2 = head;
while (ptr1->next != ptr1)
{
int count = 1;
while (count != m)
{
ptr2 = ptr1;
ptr1 = ptr1->next;
count++;
}
/* Remove the m-th node */
ptr2->next = ptr1->next;
ptr1 = ptr2->next;
}
printf ("%d\n ",
ptr1->data);
}
//main program which takes in values and calls the function:
int main()
{
int n, p;
cin>>n>>p;
int m=p+1;
gameOfElimination(m, n);
return 0;
}
SIGABRT is generally issued when there are memory issues (heap corruption being quite common). In your code, I see only the new() operator being called, but you aren't deleting any unused nodes from your linked list! Seems like you're exhausting the memory allocated to your process.
You might be running out of memory. Check your ram usage during the execution of your program, that might lead to something.
enter code here
#include<bits/stdc++.h>
using namespace std;
class Node{
public:
int data;
Node *next;
};
void traverse(Node *head)
{
while (head != NULL)
{
/* code */
cout<<head->data<<"->";
head = head->next;
}
cout<<"NULL"
}
int main()
{
Node *head = new Node();
Node *second = new Node();;
Node *third = new Node();;
Node *fourth = new Node();;
head->data = 5;
head->next = second;
//cout<<head->data;
second->data=10;
second->next=third;
third->data = 15;
third->next = fourth;
fourth->data = 20;
fourth->next= NULL;
traverse(head);
return 0;
}```
Could anyone tell me if this is the basic idea of linked lists? What are the pros and cons to this method and what are best practices when implementing linked lists in C++? Im new to data structures so this is my first approach. If there is a better way to do this same thing, please let me know. Additionally, how would you create the nodes dynamically without hard coding it? Thanks.
#include <iostream>
#include <string>
using namespace std;
struct node {
int x;
node *next;
};
int main()
{
node *head;
node *traverser;
node *n = new node; // Create first node
node *t = new node; // create second node
head =n; //set head node as the first node in out list.
traverser = head; //we will first begin at the head node.
n->x = 12; //set date of first node.
n->next = t; // Create a link to the next node
t->x = 35; //define date of second node.
t->next = 0; //set pointer to null if this is the last node in the list.
if ( traverser != 0 ) { //Makes sure there is a place to start
while ( traverser->next != 0 ) {
cout<< traverser->x; //print out first data member
traverser = traverser->next; //move to next node
cout<< traverser->x; //print out second data member
}
}
traverser->next = new node; // Creates a node at the end of the list
traverser = traverser->next; // Points to that node
traverser->next = 0; // Prevents it from going any further
traverser->x = 42;
}
for tutorial purpose, you can work out this example:
#include <iostream>
using namespace std;
struct myList
{
int info;
myList* next;
};
int main()
{
//Creation part
myList *start, *ptr;
char ch = 'y';
int number;
start = new myList;
ptr = start;
while (ptr != NULL)
{
cout << "Enter no. ";
cin >> ptr->info;
cout << "Continue (y/n)? ";
cin >> ch;
if (ch == 'y')
{
ptr->next = new myList;
ptr = ptr->next;
}
else
{
ptr->next = NULL;
ptr = NULL;
}
}
//Traversal part begins
cout << "Let's start the list traversal!\n\n";
ptr = start;
while (ptr!=NULL)
{
cout << ptr->info << '\n';
ptr = ptr->next;
}
}
It allocates memory dynamically for as many elements as you want to add.
I'd prefer to make a linked list class. This eliminates the need to call 'new' more than once. A nice implementation with examples can be found here.
You are in fact already doing dynamic allocation. So, not sure what you are asking for. But if you want to define functions to add new nodes to your linked list (or delete a node etc.), this can be a probable solution:
The location nodes get inserted/deleted is dependent on the type of data-structure. In a queue, new nodes will get added to the end; at the top in case of a stack. A function that adds a node to the top, simulating STACK push operation:
void pushNode(node **head, int Value) {
node *newNode = new node;
newNode->x = Value;
newNode->next = *head;
*head = newNode;
}
It would be called like pushNode(&head, 15) where 'head' would be defined as node *head = NULL. The root head should initially be set to NULL. After this operation head will point to the newly added node (top of stack).
The approach would be very similar for other data-structures (viz. queues) and works fine. But as you are using C++, I would suggest to define a class for your linked-list and define these functions as methods. That way, it will be more convenient and less error-prone.
Even better use std::list. It's the standard thing, so much portable and robust than a custom implementation.
You can also do it in this way
#include <iostream>
using namespace std;
struct Node{
int data;
Node* next;
};
void createList(Node** head ,Node* temp){
int n;
char ch;
temp = *head;
while(temp != NULL){
cout<<"Enter The Value ";
cin>>temp->data;
cout<<"DO you want to continue(y/n)";
cin>>ch;
if(ch=='Y' || ch == 'y'){
temp->next = new Node;
temp = temp->next;
}else{
temp->next = NULL;
temp = NULL;
}
}
}
void ShowList(Node* head){
cout<<"your list :"<<endl;
while(head != NULL){
cout<<head->data<<" ";
head = head->next;
}
}
int main()
{
//Creation part
Node *head, *temp;
createList(&head,temp);
ShowList(head);
}
I was writing a simple function to insert at the end of a linked list on C++, but finally it only shows the first data. I can't figure what's wrong. This is the function:
void InsertAtEnd (node* &firstNode, string name){
node* temp=firstNode;
while(temp!=NULL) temp=temp->next;
temp = new node;
temp->data=name;
temp->next=NULL;
if(firstNode==NULL) firstNode=temp;
}
What you wrote is:
if firstNode is null, it's replaced with the single node temp which
has no next node (and nobody's next is temp)
Else, if firstNode is not null, nothing happens, except that the temp
node is allocated and leaked.
Below is a more correct code:
void insertAtEnd(node* &first, string name) {
// create node
node* temp = new node;
temp->data = name;
temp->next = NULL;
if(!first) { // empty list becomes the new node
first = temp;
return;
} else { // find last and link the new node
node* last = first;
while(last->next) last=last->next;
last->next = temp;
}
}
Also, I would suggest adding a constructor to node:
struct node {
std::string data;
node* next;
node(const std::string & val, node* n = 0) : data(val), next(n) {}
node(node* n = 0) : next(n) {}
};
Which enables you to create the temp node like this:
node* temp = new node(name);
You've made two fundamental mistakes:
As you scroll through the list, you roll off the last element and start constructing in the void behind it. Finding the first NULL past the last element is useless. You must find the last element itself (one that has its 'next' equal NULL). Iterate over temp->next, not temp.
If you want to append the element at the end, you must overwrite the last pointer's NULL with its address. Instead, you write the new element at the beginning of the list.
void InsertAtEnd (node* &firstNode, string name)
{
node* newnode = new node;
newnode->data=name;
newnode->next=NULL;
if(firstNode == NULL)
{
firstNode=newnode;
}
else
{
node* last=firstNode;
while(last->next != NULL) last=last->next;
last->next = newnode;
}
}
Note, this gets a bit neater if you make sure never to feed NULL but have all lists always initialized with at least one element. Also, inserting at the beginning of list is much easier than appending at the end: newnode->next=firstNode; firstNode=newnode.
The last element in your list never has it's next pointer set to the new element in the list.
The problem is that you are replacing the head of the linked list with the new element, and in the process losing the reference to the actual list.
To insert at the end, you want to change the while condition to:
while(temp->next != null)
After the loop, temp will point to the last element in the list. Then create a new node:
node* newNode = new node;
newNode->data = name;
newNode->next = NULL;
Then change temps next to this new node:
temp->next = newNode;
You also do not need to pass firstNode as a reference, unless you want NULL to be treated as a linked list with length 0. In that case, you will need to significantly modify your method so it can handle the case where firstNode is NULL separately, as in that case you cannot evaluate firstNode->next without a segmentation fault.
If you don't want to use reference pointer, you could use pointer to pointer. My complete code goes like below:
void insertAtEnd(struct node **p,int new_data)
{
struct node *new_node=(struct node *)malloc(sizeof(struct node));
new_node->data=new_data;
new_node->next=NULL;
if((*p)==NULL)//if list is empty
{
*p=new_node;
return;
}
struct node* last=*p;//initailly points to the 1st node
while((last)->next != NULL)//traverse till the last node
last=last->next;
last->next=new_node;
}
void printlist(struct node *node)
{
while(node != NULL);
{
printf("%d->",node->data);
node=node->next;
}
}
int main()
{
struct node *root=NULL;
insertAtEnd(&root,1);
insertAtEnd(&root,2);
insertAtEnd(&root,3);
insertAtEnd(&root,4);
insertAtEnd(&root,5);
printlist(root);
return 0;
}
Understanding the need of the below two variables is key to understanding the problem:
struct node **p: Because we need to link it from the root node created in the main.
struct node* last: Because if not used, the original content will be changed with the contents of the next node inside the while loop. In the end only 2 elements will be printed, the last 2 nodes, which is not desired.
void addlast ( int a)
{
node* temp = new node;
temp->data = a;
temp->next = NULL;
temp->prev=NULL;
if(count == maxnum)
{
top = temp;
count++;
}
else
{
node* last = top;
while(last->next)
last=last->next;
last->next = temp;
}
}
#include <bits/stdc++.h>
using namespace std;
class Node
{
public:
int data;
Node *next;
};
void append(Node *first, int n)
{
Node *foo = new Node();
foo->data = n;
foo->next = NULL;
if (first == NULL)
{
first = foo;
}
else
{
Node *last = first;
while (last->next)
last = last->next;
last->next = foo;
}
}
void printList(Node *first)
{
while (first->next != NULL)
{
first = first->next;
cout << first->data << ' ';
}
}
int main()
{
Node *node = new Node();
append(node, 4);
append(node, 10);
append(node, 7);
printList(node);
return 0;
}
Output: 4 10 7
You can use this code:
void insertAtEnd(Node* firstNode, string name)
{
Node* newn = new Node; //create new node
while( firstNode->next != NULL ) //find the last element in yur list
firstNode = firstNode->next; //he is the one that points to NULL
firstNode->next = newn; //make it to point to the new element
newn->next = NULL; //make your new element to be the last (NULL)
newn->data = name; //assign data.
}
void InsertAtEnd (node* &firstNode, string name){
node* temp=firstNode;
while(temp && temp->next!=NULL) temp=temp->next;
node * temp1 = new node;
temp1->data=name;
temp1->next=NULL;
if(temp==NULL)
firstNode=temp1;
else
temp->next= temp1;
}
while loop will return at temp==null in your code instead you need to return last node pointer from while loop like this
while(temp && temp->next!=NULL) temp=temp->next;
and assign a new node to next pointer of the returned temp node will add the data to the tail of linked list.
#include <iostream>
using namespace std;
struct Node
{
int item; // storage for the node's item
Node* next; // pointer to the next node
};
Node* addNode(Node*& head, int data , int& count)
{
Node * q; // new node
q = new Node; // allocate memory for the new mode
q->item = data; // inserting data for the new node
q->next = head; // point to previous node ?? how would i do that? ( am i doing it correctly?)
count++; // keep track of number of node
head = q;
return q;
}
int main()
{
int a, count=0;
int data;
bool repeat;
Node *head= NULL;
//^^ assuming thats creating the first node ^^
do
{
cout << "please enter the data for the next node" <<endl;
cin >> data;
addNode(head, data, count);
cout << "do you wish to enter another node? (enter true or false)" << endl;
cin >>repeat;
}
while (repeat == true);
// assuming this is the print function
while(head != NULL)
{
cout << "output" << temp->item << endl;
cout << temp->next << endl;
}
system("pause");
return 0;
}
okey i tried adding a new element in the list how would i move the head around like a LIFO memory (stack) so the last element is on the very top..
any help would be appreciated ! The pointers and the nodes are messing with my brain lately ....
temp is an uninitialized pointer. So -
temp-> item = a; // temp is not initialized or pointing to a memory location
// that has Node object to use operator ->
First, temp needs to be allocated memory location using new.
temp = new Node;
temp -> item = a;
And now assign it head. Similarly allocate memory for the child nodes too in the while loop. And return all the resources acquired from child to head using delete before program termination.
You seem to have some misunderstandings here:
Your "head" is the start of the list. It's always the start.
You add append elements to a linked list by assigning them to the last node's next pointer.
Third, you're not allocating anything.
Node *head= new Node();
Node *temp = new Node();
cout<<"enter something into data"<<endl;
cin >> a ;
temp->item = a;
head->next = temp;
Now ... to add the next thing, you either need to keep track of the last node (tail), or traverse the list to find the last node.
Node *nextNode = new Node();
nextNode->item = 0.0;
Node *i;
for (i = head; i->next != null; i = i->next);
i->next = nextNode;
This is O(n) execution time. By keeping track of the tail you make it O(1):
Node *head= new Node();
Node *tail = head;
Node *temp = new Node();
cout<<"enter something into data"<<endl;
cin >> a ;
temp->item = a;
tail->next = temp;
tail = temp;
Node *nextNode = new Node();
nextNode->item = 0.0;
tail->next = nextNode;
tail = nextNode;
EDIT: As pointed out, if you want to prepend to the list, you would:
temp->next = head;
head = temp;
Since I'm not sure every answer completely answers it, here's a linked list implementation (written without testig:
// your (correct) structure
struct Node
{
float item; // storage for the node's item
Node* next; // pointer to the next node
};
Now we need two pointers somewhere to look after the list:
/* some pointers */
struct List
{
Node* head;
Node* tail;
};
Now we need to create some elements. As others have said, you can do that with new:
/* create some elements we want to link in */
Node* elem1 = new Node();
Node* elem2 = new Node();
Node* elem3 = new Node();
/* maybe even set their properties! */
elem1->item = 3.14;
elem2->item = 3.14;
elem3->item = 3.14;
Notice how I didn't try to add these elements to a list yet? That's because I've got a function in mind which looks like this:
void addtolist(List &list, Node* node)
{
/* if no head, initialise the list */
if ( list->head == NULL )
{
list->head = node;
list->tail = node;
}
else if ( list->head != NULL && list->tail != NULL )
{
/* access the tail element and set its
next to this ptr.
Move tail to this node */
list->tail->next = node;
list->tail = node;
}
else
{
/* probably raise an exception! */
}
}
You can call this by doing this:
List l;
addtolist(l, elem1); /* etc */
Deleting elements is somewhat more tricky, since you have to go to that element, remember its previous element, grab it's next element, join them up and delete the Node* you're on.
Now for traversing lists... your terminology HEAD|TAIL reminds me of Erlang and tail recursion, where the current element is referred to as the head and the remainder the tail. If I write:
Node* cur = l.head;
while ( cur != NULL )
{
// do something with cur.item ?
cur = cur->next;
}
You can see this happening. Replacing cur with head here would be harmless thanks to the List struct.
Finally, I've used a very C-like approach here, but there's scope for templates:
template<typename T>
struct node
{
T item; // storage for the node's item
Node<T>* next; // pointer to the next node
};
and encapsulating the List struct as a class:
template<typename T>
class List
{
protected:
Node<T>* head;
Node<T>* tail;
public:
void addtolist(Node<T>* node);
Node<T>* gethead();
Node<T>* gettail();
}
Which brings you a little bit closer to std::list.
Additionally note that you are doing an implicit cast from int to float on
temp-> item = a;
as a is an int, while temp->item is a double.
To solve your problem: You want to allocate a new structure before accessing temp, thus
temp = new Node();