How do we display a linked list? - c++

Question

So I created the following bit of code but the display function is giving me problems. It breaks into an infinite loop each time I try using it. Could someone please have a look at it and tell me what's going wrong ?
#include<iostream.h>
#include<conio.h>
struct node{
int info;
node *next;
}*ptr,*start,*temp;
node* create_new()
{
ptr=new node;
cout<<"\nEnter the data: ";
cin>>ptr->info;
ptr->next=NULL;
return ptr;
}
void insert_at_beg()
{
ptr=create_new();
if(start==NULL)
{
start=ptr;
}
if(start!=NULL)
{
ptr->next=start;
start=ptr;
}
}
void display()
{
temp=start;
while(temp->next!=NULL)
{
cout<<"\t"<<temp->info;
temp=temp->next;
}
}
void insert_at_end()
{
if(start==NULL)
{
start=ptr;
}
if(start!=NULL)
{
ptr=create_new();
temp=start;
while(temp->next!=NULL)
{
temp=temp->next;
}
temp->next=ptr;
}
}
void delete_from_end()
{
if(start==NULL)
{
cout<<"NULL LL";
}
else
{
temp=start;
while(temp->next!=NULL)
{
ptr=temp;
temp=temp->next;
}
ptr->next=NULL;
delete temp;
}
}
void delete_from_beg()
{
if(start==NULL)
cout<<"\nNULL LL";
else
start=start->next;
}
void delete_from_mid()
{
int el;
if(start==NULL)
{
cout<<"\nNULL LL";
}
else
{
cout<<"\nEnter element that you want to delete: ";
cin>>el;
temp=start;
while(temp->next!=NULL&&temp->info!=el)
{
ptr=temp;
temp=temp->next;
}
ptr->next=temp->next;
delete temp;
}
}
void main()
{
clrscr();
start=NULL;
temp=NULL;
ptr=NULL;
insert_at_beg();
display();
getch();
}

Answer 1

Your bug is in this code:
void insert_at_beg()
{
ptr=create_new();
if(start==NULL)
{
start=ptr;
}
if(start!=NULL)
{
ptr->next=start;
start=ptr;
}
}
The first time in start will be null, so you'll do start=ptr. However, now if(start!=NULL) will be true, so you'll do ptr->next=start. Since ptr and start both point to the same thing you'll create an infinite loop.

Answer 2

Firstly I would not recommend you to use global variables especially when you have functions that themselves output parameters of the same type (and value).
The problem according to me is in the function insert_at_beg():
// yes start is NULL initially
if(start==NULL)
{
start=ptr; // now start is not NULL!!
}
//This statement will also be entered.
if(start!=NULL)
{
ptr->next=start;
start=ptr;
}
}
Instead use an else
if(start==NULL)
{
start=ptr; // now start is not NULL!!
}
else
{
ptr->next=start;
start=ptr;
}
}
Also, instead of:
#include<iostream.h>
#include<conio.h>
Use just #include<iostream>

Answer 3

Your bug that cause infinite loop is somewhere else (insert_at_beg(), I saw that someone already added the details so I won't do this too) in the code.
You still have a problem in your code:
void display()
{
temp=start;
while(temp->next!=NULL)
{
cout<<"\t"<<temp->info;
temp=temp->next;
}
}
You will not print the last elemnt. You stop when the next of the current element (temp) is NULL.
Change this to
while(temp) // equivalent to while(temp !=NULL)

Answer 4

Your display function can be simplified:
void display()
{
node * p = start;
while (p != NULL)
{
cout << "\t" << p->info;
p = p->next;
}
cout << endl; // The side effect of this is to print blank line when empty list.
}