How to delete nodes in a doubly linked list - c

Question

I'm having trouble deleting nodes in a doubly linked list, the program crash and i can't figure out the problem. Can you please help me?
This is the full code that creates new nodes, views them and deletes them.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
struct Test
{
int id;
};
typedef struct Node {
struct Test structure;
struct Node * next;
struct Node *prev;
}TNode;
typedef TNode* Node;
void NewNode(struct Test p, Node *pp)
{
Node temp;
temp = (Node)malloc(sizeof(struct Node));
temp->structure = p;
temp->next = *pp;
temp->prev = NULL;
if(*pp != NULL)
{
(*pp)->prev = temp;
}
*pp = temp;
}
void ReadStructure(struct Test * p)
{
printf("\nID:");
scanf(" %d", &p->id);
}
void ViewList(Node node)
{
Node temp;
while(node != NULL)
{
temp = node->prev;
if(node->prev == NULL)
{
printf("Prev = NULL\n");
}
else
{
printf("Prev: %d\n", temp->structure.id);
}
printf("Curr: %d\n", node->structure.id);
node = node->next;
}
}
void Delete(Node * head, Node del)
{
if(*head == NULL || del == NULL)
{
return;
}
if(*head == del)
{
*head = del->next;
}
if(del->next != NULL)
{
del->next->prev = del->prev;
}
if(del->prev != NULL)
{
del->prev->next = del->next;
}
free(del);
return;
}
int Menu()
{
int c;
printf("*** M E N U ***\n"
"1 - New Node\n"
"2 - View List\n"
"3 - Delete\n"
"0 - Exit\n"
"\n>> ");
scanf(" %d", &c);
return c;
}
int main()
{
int c;
struct Test test;
Node list = NULL;
Node del = NULL;
do {
c = Menu();
switch (c)
{
case 1: ReadStructure(&test);
NewNode(test, &list); break;
case 2: ViewList(list); break;
case 3: printf("\nElement to Delete: ");
scanf("%d", &del->structure.id);
Delete(&list, del); break;
default: c = 0;
}
} while (c != 0);
return 0;
}
I think the problem is related to the scanf() for the Node del, but i'm not sure. When i just pass list or list->next as second argument to the function Delete() it works. Is everything all right with the code?

Answer 1

int main()
{
...
Node del = NULL;
...
scanf("%d", &del->structure.id);
Your program should crash here. You are dereferencing a null pointer.
Probably you will need to read the user input into a temporary id variable, then search the list for a matching item, and if you find one, then you can try deleting it.

Answer 2

Ok, i added a function that searches for the node to be deleted and i modified a bit the Delete() function, this is the workaround, thank you for the suggestions:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
struct Test
{
int id;
};
typedef struct Node {
struct Test structure;
struct Node * next;
struct Node *prev;
}TNode;
typedef TNode* Node;
void NewNode(struct Test p, Node *pp)
{
Node temp;
temp = (Node)malloc(sizeof(struct Node));
temp->structure = p;
temp->next = *pp;
temp->prev = NULL;
if(*pp != NULL)
{
(*pp)->prev = temp;
}
*pp = temp;
}
void ReadStructure(struct Test * p)
{
printf("\nID:");
scanf(" %d", &p->id);
}
void ViewList(Node node)
{
Node temp;
while(node != NULL)
{
temp = node->prev;
if(node->prev == NULL)
{
printf("Prev = NULL\n");
}
else
{
printf("Prev: %d\n", temp->structure.id);
}
printf("Curr: %d\n", node->structure.id);
node = node->next;
}
}
Node SearchNode(Node head)
{
int d;
printf("\nElement to Delete:");
scanf("%d", &d);
while(head != NULL)
{
if(head->structure.id == d)
{
return head;
}
head = head->next;
}
printf("\nNo Element [%d] Found", d);
return NULL;
}
void Delete(Node * head, struct Test temp)
{
Node del = SearchNode(*head);
if(*head == NULL || del == NULL)
{
return;
}
if(*head == del)
{
*head = del->next;
}
if(del->next != NULL)
{
del->next->prev = del->prev;
}
if(del->prev != NULL)
{
del->prev->next = del->next;
}
free(del);
return;
}
int Menu()
{
int c;
printf("\n*** M E N U ***\n"
"1 - New Node\n"
"2 - View List\n"
"3 - Delete\n"
"0 - Exit\n"
"\n>> ");
scanf(" %d", &c);
return c;
}
int main()
{
int c;
struct Test test, del;
Node list = NULL;
do {
c = Menu();
switch (c)
{
case 1: ReadStructure(&test);
NewNode(test, &list); break;
case 2: ViewList(list); break;
case 3: Delete(&list, del); break;
default: c = 0;
}
} while (c != 0);
return 0;
}

Answer 3

del value is NULL, but you reference it when you delete.
What you need is to search a node in the list for the givent id and then delete it.