What is wrong in below linked list program in cpp? - c++

Question

I'm facing weird problem that the only 1st element of the list is get printed. I have writing linked list program after long time. thanks for the help. Is there something wrong with printAll function or add function in list class. I have tried printing previous elements while adding new one & it works. So, I'm not getting why only 1st element .ie. head is getting printed & head->next seems to be null.
#include<iostream>
using namespace std;
class Node{
public: int data;
public: Node *next;
public: Node(int data){
this->data = data;
this->next = NULL;
}
};
class List{
Node *head, *trav;
public: List(){
this->head = NULL;
this->trav = NULL;
};
void add(int data){
if(this->head==NULL && this->trav==NULL){
cout<<"inside the if block"<<endl;
this->head = new Node(data);
this->trav = this->head->next;
}
else{
cout <<"inside the else block"<<endl;
this->trav = new Node(data);
this->trav = this->trav->next;
}
}
void printAll(){
this->trav = this->head;
while(this->trav!=NULL){
cout<<this->trav->data<<endl;
this->trav = this->trav->next;
}
}
};
int main(){
List list;
list.add(2);
list.add(3);
list.add(4);
list.add(5);
list.printAll();
cout<<sizeof(list);
}

Answer 1

The add() method else part is not linking list properly, do some paper work.
Here is the working one, I tried to explain in comments.
void add(int data){
if(this->head==NULL && this->trav==NULL){ /* for 1st node */
cout<<"inside the if block"<<endl;
this->head = new Node(data);
this->trav = this->head->next;
}
else{
this->new_node = new Node(data); /*new_node */
cout <<"inside the else block"<<endl;
this->trav = head;/*temp var to point to ast node */
while(this->trav->next!=NULL) {
this->trav = this->trav->next;
}
this->trav->next = this->new_node; /*adding at end */
this->new_node->next = NULL; /*new_node next make it NULL */
}
}

Answer 2

Your add function doesn't link anything. Whenever you get into the else block trav is NULL and you set it equal a new node. But you never link that new node to the previous last node.
Normally trav would be named tail and point to the last element so that you can link a new element to the current last element.
Something like:
if(this->head==NULL && this->trav==NULL){
cout<<"inside the if block"<<endl;
this->head = new Node(data);
this->trav = this->head;
}
else{
cout <<"inside the else block"<<endl;
this->trav->next = new Node(data);
this->trav = this->trav->next;
}
Edit
OP commented that trav is not considered a tail pointer but just a pointer to traverse the list.
So therefore the answer is different as the code need to find the current tail using a loop.
Something like:
if(this->head==NULL){
cout<<"inside the if block"<<endl;
this->head = new Node(data);
}
else{
cout <<"inside the else block"<<endl;
this->trav = this->head;
while(this->trav->next)
{
this->trav = this->trav->next;
}
this->trav->next = new Node(data);
}
However, notice:
If trav is "just" a pointer to traverse the list, there is no real purpose in making it a member of List. Simply use a local variable inside the functions that need to traverse the list.
Since your code adds new elements to end-of-list it's often a very good idea to have a tail pointer as member in List. Especially if the list can hold many elements and you frequently add new elements.
Your code use this->some_member in many places where it's not needed. Avoiding that will make your code easier to read.

Answer 3

You never set the next pointer in Node.
Your code also has less-than-ideal indentation, uses raw pointers in places where you could use unique_ptr, doesn't initialise variables in constructors the C++ way, uses NULL instead of nullptr, and performs undefined behaviour by falling off the bottom without returning a value in a function declared to return a value (main(), specifically).

Answer 4

The problem is in your add() method.
Replace it with this
void add(int data){
if(this->head == NULL){ /*If head is null, init it and trav node*/
cout<<"inside the if block"<<endl;
this->head = new Node(data); /*init head*/
this->trav = this->head; /*point to head of list*/
} else{
cout <<"inside the else block"<<endl;
this->trav->next = new Node(data); /* add new elem to next of trav*/
this->trav = this->trav->next; /*move trav to next node i.e. reset */
}
}