Traverse backward DLL after deleting first node - c

Question

I have doubly linked list set up using an insertion method which works.
Using this struct:
struct NODE
{
struct NODE *fwd;
struct NODE *bwd;
int value;
};
typedef struct NODE Node;
And this initialization:
void initializeList(Node *rt)
{
rt->fwd = NULL;
rt->bwd = NULL;
rt->value = 0;
}
My main function is:
main()
{
Node root;
int j;
initializeList(&root);
for (j = 0; j < 15; j++)
insertFirst(&root, j);
printf("The list traversed forward \n");
traverseForward(root);
printf("\n");
printf("The list traversed backward \n");
traverseBackward(root);
printf("\n");
printf("After first deletion traverse forward\n");
deleteFirst(&root);
traverseForward(root);
printf("\n");
printf("After first deletion traverse backwards\n");
printf("\n");
traverseBackward(root);
}
My deletefirst function is:
void deleteFirst(Node *rt)
{
Node *newnode = rt;
Node *tmp = NULL;
if (newnode != NULL)
{
if (newnode->fwd != NULL)
{
newnode = newnode->fwd;
tmp = newnode->bwd;
*rt = *newnode;
}
else
tmp = newnode;
}
free(tmp);
}
Insert function:
int insertFirst(Node *rt, int val)
{
Node *node = (Node *)malloc(sizeof(Node));
if (node == NULL) return 0;
node->value = val;
/* For a previously empty list */
if (rt->fwd == NULL)
{
rt->fwd = node;
rt->bwd = node;
node->fwd = NULL;
node->bwd = NULL;
}
/* For a list with at least one node */
else
{
/* previous first node now new node's successor */
rt->fwd->bwd = node;
node->fwd = rt->fwd;
/* no predecessor to the new first node */
node->bwd = NULL;
/* root points to this new first */
rt->fwd = node;
}
return 1;
}
Traverse functions:
void traverseForward(Node root)
{
Node *q = root.fwd;
while (q)
{
printf("%d ", q->value);
q = q->fwd;
}
}
void traverseBackward(Node root)
{
Node *q = root.bwd;
while (q)
{
printf("%d ", q->value);
q = q->bwd;
}
}
The system prints out the list traversed forward
14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
The system prints out the list traversed backward
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
After first deletion forward traversal
13 12 11 10 9 8 7 6 5 4 3 2 1 0
After first deletion backward traversal
(Nothing is printed)
I cant figure out how to adjust the pointers to get it to work.

Answer 1

I believe the problem is because the odd way you are storing the head and tail pointers to the start and end of the list in a root node and then mixing that interchangeably with the actual nodes of the list. This confusion could be eliminated if you just have simple Node *head and Node *tail pointers, but that is only a suggestion.
In your delete function, you set the root node to effectively rt->fwd, which is fine for forwards traversal as rt->fwd->fwd is what is desired, but you forget to consider the value of rt->fwd->bwd which is always pointing at NULL for the first item in the list (as there is nothing technically before this node), not the actual tail of the list which is the desired functionality by that logic.
This obviously causes a problem when you try to use it to iterate backwards as it thinks the list is empty. You should change your deletion code to something like this:
void deleteFirst(Node *rt)
{
if (rt == NULL)
{
return; /* Return if an invalid root was passed in */
}
if (rt->fwd == NULL) {
return; /* Return if the list is already empty */
}
Node *tmpfwd = rt->fwd; /* Store the current "head" */
rt->fwd = rt->fwd->fwd; /* Set the root's head to the current head's next node */
rt->fwd->bwd = NULL; /* Set the new head's previous node to NULL as it is the start of the list */
free(tmpfwd); /* Delete the old "head" */
}
There are a lot of other problems here regarding edge cases and things the comments have mentioned (just the overall design makes this very problematic) but I think this is the problem you are having currently.