Hi i am a beginner in programming C++ language and am having a bug in the doubly linked list program. The problem is while printing the list in fwd direction the prgm prints the contents just fine. however while trying to print the list in reverse direction nothing gets printed. Any comments on why this is happening would greatly b appreciated. Thanks

#include <iostream.h>
#include <process.h>
#include <conio.h>

class node
{
public:
int data;
node *prev,*next;
};

class doubly
{
private:
    node *head;
public:
    doubly()
    {
    head=NULL;
    }
    void create();
    void display();
};


void doubly::create()
{
int val;
node *temp;
temp=new node;
cout<<"\nEnter value:";
cin>>val;
if(head==NULL)
{
temp->data=val;
temp->prev=NULL;
temp->next=NULL;
head=temp;
return;
}
else
temp->data=val;
temp->prev=NULL;
temp->next=head;
temp->next->prev=temp;
head=temp;
//display();
}//create

void doubly::display()
{
node *temp;
temp=head;

 while(temp!=NULL)
 {
 cout<<temp->data<<"\t";
 temp=temp->next;
 }
 cout<<"\n";
 cout<<"\nnow printing in reverse order";

 while(temp->prev!=NULL)
 {
 cout<<temp->data<<"\t";
 temp=temp->prev;
 }


}//display

void main()
{
int ch;
doubly s;
clrscr();
do
{
cout<<"\n1)Add";
cout<<"\n2)Display";
cout<<"\n3)Exit";
cout<<"\nEnter Choice:";
cin>>ch;

switch(ch)
{
case 1:s.create();
       break;

case 2:s.display();
       break;
case 3:exit(0);
       break;
}
}while(ch!=3);
}//main

Edited 3 Years Ago by Reverend Jim: Fixed formatting

Just looking at the code for display, I'm surprised it doesn't crash.

void doubly::display()
{
  node *temp;
  temp=head;

  while(temp!=NULL)
  {
    cout<<temp->data<<"\t";
    temp=temp->next;
  }
  cout<<"\n";
  cout<<"\nnow printing in reverse order";

  while(temp->prev!=NULL)
  {
    cout<<temp->data<<"\t";
    temp=temp->prev;
  }
}

When you quit the first loop, temp will be NULL, as per the loop condition. Then when you try to start the second loop, you'll be accessing invalid memory trying to dereference NULL (and should crash). You'll can change the first loop to go until temp->next == NULL or you can keep a tail pointer in addition to the head one (where tail would record the last node in the list).

Side notes:
- <iostream.h> is deprecated. Use <iostream> and the std:: namespace in the future.
- <conio.h> is compiler specific and most people here would not be able to run your code.
- Why do you need <process.h>? Unless I missed something...
- Please use [code] and [/code] tags when you post code on the forums, as it preserves formatting (especially indentation) ;)

Thanxs for your reply. I made a stupid error! Anyway i am using the Turbo C 3.0 Compiler which is over a decade old. The exit(0) function requires the process.h file

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