Creating and printing linked list from user input C++ - c++

Question

#include <iostream>
using namespace std;
int main()
{
struct node
{
int data;
node * next;
};
node * head;
node * n;
node * temp;
node * q;
int number;
cout << "Enter numbers";
cin >> number;
n = new node;
n->data = number;
head = n;
temp = n;
while (cin >> number)
{
while (number != -500)
{
n = new node;
n->data = number;
temp->next = n;
temp = n;
}
}
while (head != NULL)
{
cout << head->data;
head = head->next;
}
}
I don't understand why this would not work. The program creates a new node then sets whatever the user entered equivalent to the variable data of that new node then it makes head and temp point to the new node. Then it gets the users second input and compares it to -500 and if it evaluates as true it creates a new node puts the data of the second input into the variable data then it links the first node and second node together then it makes temp point to the second node. If the condition of the 2nd while loop is false it goes to the third which is where it is suppose to print the list.

Answer 1

Who sets the last node's next to NULL?
At n = new node; n->next is not NULL but undefined, in Debug versions usually it is 0xcccccccc or something similar value to make it visible that it is not initialized. If you try to dereference it, you will get an access violation.

Answer 2

while (cin >> number) { // this loop is endless, because you can always read the user input data (unless some exception happen)
while (number != -500)
{
n = new node; // you already have data allocated. no need to allocate it once more
n->data = number;
temp->next = n;
temp = n;
// as was already mentioned: set n->next to NULL
} }
if you want to break after you checked that the number is not -500, then you can do the following instead:
while (cin >> number) {
if (number != -500) { ...; // do your stuff
break;
}
}
And, by the way, you have a memory leak. If you use flat C pointers then consider delete operator to clear your memory. For that purpose, you need to know where your list starts(basically, head) and iterate through a whole list invoking delete node.
Please, also consider, that it is a bad code style to write something like:
while (cin >> number)

Answer 3

You can also try this-
This code makes a linked list on user input and printlinkedlist (function) print the linked list created.
#include <bits/stdc++.h>
using namespace std;
struct node
{
int data;
node* next;
};
void printlinkedlist(node* node)
{
int c=0; //taken just for good looking output
while(node!=NULL)
{
if(c>0)
{
cout<<"->"<<node->data;
node = node->next;
}
else
{
cout<<node->data;
node = node->next;
c++;
}
}
}
int main() {
int n;
cout<<"Enter no. of nodes=";
cin>>n; //User enters number of nodes he want.
int num,c=0; //initialized c for setting head with second node..
node* head = new node; //initialized head node
node * temp = new node; //initialized temp node
cin>>num;
head->data=num;
for(int i=2;i<=n;i++)
{
if(c==0)
{
cin>>num;
temp->data=num;
head->next=temp; //head point to second node i.e. temp
c++;
}
else
{
cin>>num;
node * temp1 = new node; //initialize other temp node for every value
temp1->data=num;
temp->next=temp1; //point to temp1 to temp
temp=temp1; //set temp as temp1
}
}
printlinkedlist(head);
}

Answer 4

#include<iostream>
using namespace std;
class node{
public:
int data;
node* next;
};
// a function to create linked list passing pointer to pinter
void create_list(node**head)
{
int num;//num is the data
cin>>num ;
node*new_node=new node();//
new_node->data=num;
new_node->next=NULL;
node*temp;
if(*head==NULL)
{
*head=temp=new_node;
}
else{
temp->next=new_node;
temp=new_node;
}
}
void print_list(node* head)
{
if(head == NULL)
{
cout<<"empty list"<<endl;
}
node *ptr = NULL;
ptr=head;
cout<<"data in the list: "<<endl;
while(ptr!=0)
{
cout<<ptr->data<<endl;
ptr=ptr->next;
}
}
int main()
{
int n,i; //
cout<<"enter the number of nodes:"<<endl;
cin>>n;
cout<<"enter data as num:"<<endl;
node* head=NULL;
for(i=0;i<n;i++)
{
create_list(&head);//passing the address of head
}
print_list(head);
return 0;
}