Thread: Help with Doubly Linked Lists!

Here's a very primitive code for a doubly linked list. Although its only very rudimentary, whats wrong with this code? the code's supposed to print the elements sequentially starting from the first node and later print them in reverse.

Code:

#include<stdio.h>
#include<conio.h>

typedef struct tagnode              
{
        int data;
        struct tagnode *next;
        struct tagnode *prev;
}node;                       

node* create(int);
void print(node *);
int main(void)
{
   int n;
   node *head; 
    printf("Enter number of nodes to create");
    scanf("%d",&n);
    head=create(n);
    print(head);
   return 0;
}


node* create(int n)
{
 int data,i;
 
 node *localhead=(node *)malloc(sizeof(node));
 node *movinghead=localhead;
 movinghead->prev=NULL;
 movinghead->next=NULL;
 printf("\nEnter data:");
 scanf("%d",&data);
 movinghead->data=data;
 for(i=1;i<n;i++)
 {
   movinghead->next=(node *)malloc(sizeof(node));
   movinghead->next->prev=movinghead;
   movinghead=movinghead->next;
   scanf("%d",&data);
   movinghead->data=data;
   movinghead->next=NULL;
 }
return localhead;
} 

void print(node *p)
{
   node *movinghead;
   for(movinghead=p;movinghead!=NULL;movinghead=movinghead->next)
   {
    printf("\n%d",movinghead->data);
   }
   
   while(movinghead->prev!=NULL)
   {
     printf("%d",movinghead->data);
     movinghead=movinghead->prev;
} 
getch();
}

Any help would be appreciated!!

Originally Posted by quzah

How about you tell me? What's it not doing that it's supposed to, or doing that it shouldn't be?

It prints the elements of the list entered properly. But it doesn't print them in the reverse order as expected from the code starting at line 56. How should I modify it to print them in reverse?(if at all a modification is absolutely necessary)

And yet another cross-poster

Originally Posted by quzah

Code:

   for(movinghead=p;movinghead!=NULL;movinghead=movinghead->next)
   {
    printf("\n%d",movinghead->data);
   }
    
   while(movinghead->prev!=NULL)

When does the first loop end? How does that affect the second loop?


Quzah.

The first loop ends as soon as movinghead becomes NULL(which is the case for the last node in the list) By my logic, at this time, movinghead now points to the last element in the list. All I have attempted from line 56 onwards in to traverse the list in the reverse order and then print them as movinghead goes up the list.


Also-I forgot to include stdlib.h in the above code but even after doing so, the same problem exists.

Btw,while debugging, I got a segmentation fault around line 56 if that helps.

Originally Posted by rags_to_riches

And yet another cross-poster

I don't see anything wrong with that. It's not like I disrespect people's valuable feedback and opinions; its just that I sometimes like to have more than one viewpoints on a particular problem. Besides, is there any guarantee that users on one forum will necessarily be registered on other forums as well?

Anyway found my mistake. While exiting the for loop, movinghead became equal to NULL. So in the next loop, any reference to movinghead resulted in the compiler trying to access a NULL address something like a segmentation fault!

This thread may be marked 'closed'!

Here's the modified code:

Code:

void print(node *p)
{
   node *movinghead;
   for(movinghead=p;movinghead->next!=NULL;movinghead=movinghead->next)//can't use test condition as movinghead!=NULL because for the last 
                                                                       //iteration,movinghead=movinghead->next becomes invalid since 
                                                                      //movinghead->next is NULL so can't print this 'NULL' value.In other words,
                                                                      //stop when movinghead points to the last node.
                                                                      
   {
    printf("%d",movinghead->data);
   }
    printf("%d",movinghead->data);                                     //print last node data(notice that there is no need to execute
                                                                      //movinghead=movinghead->next once more.This is done only if reverse
                                                                      //printing of list is required.Otherwise incorporate this print stmt
                                                                      //in for loop only.
   
   while(movinghead!=NULL)                                      
   {
     printf("%d",movinghead->data);
     movinghead=movinghead->prev;
   }
getch();
}