Error while parsing through the linked list - c++

Question

Exception thrown at 0x00A12849 in DLL.exe: 0xC0000005: Access violation reading ocation 0xDDDDDDDD.
The error occurs at the parse function and I have no clue why. I wrote the exact same code as in the book they only wrote the del function and not the parse function. pls tell why this error is occurring in this code and how to avoid such in future. also help to write a proper parse function.
thanks to everyone in advance
#include<iostream>
using namespace std;
struct list
{
int data;
list *next;
list *prev;
};
list* createNode()
{
list *node = new list;
node->next = NULL;
node->prev = NULL;
return node;
}
void insert(list**head,int data,int position)
{
int k = 1;
list *current, *temp, *node = createNode();
current = NULL;
temp = *head;
node->data = data;
if (position==1)
{
node->next = *head;
*head = node;
if (*head)
{
(*head)->prev = node;
}
}
else
{
while (temp->next!= NULL&&k < (position-1))
{
temp = temp->next;
k++;
}
node->next = temp->next;
node->prev = temp->prev;
if (temp->next)
temp->next->prev = node;
temp->next= node;
}
}
void del(list**head, int position)
{
int k = 1;
list *current, *temp;
current = NULL;
temp = *head;
if (position == 1)
{
*head = (*head)->next;
if (*head != NULL)
(*head)->prev = NULL;
delete temp;
return;
}
while (k < position&&temp->next != NULL)
{
temp = temp->next;
k++;
}
current = temp->prev;
current->next = temp->next;
if (temp->next)
temp->next->prev = current;
delete temp;
return;
}
void parse(list * temp)
{
while (temp != NULL)
{
cout << "->" << temp->data;
temp = temp->next;
}
}
void main()
{
struct list *head = NULL;
start:
int choice;
cin >> choice;
switch (choice)
{
case 1: int data, position;
cout << "\ndata:"; cin >> data; cout << "\nposition:";
cin>>position;
insert(&head, data, position);
goto start;
case 2:parse(head);
goto start;
case 3: cout << "\nposition:"; cin >> position;
del(&head, position);
goto start;
default: break;
}
exit(0);
}

Answer 1

These steps are in the wrong sequence and make the node its own prev:
*head = node;
if (*head)
{
(*head)->prev = node;
}
Then you also set prev incorrectly here:
node->prev = temp->prev;
The following code is not necessarily incorrect on its own, but it is a needlessly dangerous use of prev that fails do to the bugs that make prev wrong, but also fails in some cases of bad input (invalid position).
while (k < position&&temp->next != NULL)
{
temp = temp->next;
k++;
}
current = temp->prev;
You said that del function was in the book. Maybe you don't have a choice, but you apparently are using a pretty bad book. That function (and your insert function) are written in a beginner style that is not a good way to learn to program. Beginner problems are appropriate for learning, but some aspects of typical beginner solutions should be avoided from the beginning. One of the most important skills in programming is finding the similarities that let you combine related operations into more general operations. Breaking out special cases (such as first position in those functions) is a natural beginner mistake that you should be unlearning, not copying from a book.
You are already using the extra level of indirection ** that enables combining the cases, so you should take full advantage of that, rather than drop the advantage at the first opportunity:
void insert(list**head,int data,int position)
{
list *p = NULL;
while ( (position--)>0 && *head )
{
p = *head;
head = &( p->next );
}
list *node = createNode();
node->prev = p;
node->next = *head;
node->data = data;
*head = node;
}
Notice how many cases are mixed in so they all share the same code. That style cuts down on undiscovered bugs. Learning linked lists is a tiny corner of learning to program. Learning to combine and generalize operations is learning to program.