Circular doubly linked list in C delete function - c

Question

I have a circular doubly linked list.
The deletefront() function is not working: the output is wrong. What is the mistake?
The other functions are working. But I get a wrong output after displaying after calling deletefront function. The 100 value which should be deleted is still appearing. Please correct it.
I have included the C source code:
// circular doubly linked list
#include <stdio.h>
#include <stdlib.h>
typedef struct node {
int data;
struct node *rlink;
struct node *llink;
} node;
node *head = NULL;
node *getnode(int ele) {
node *ptr;
ptr = (node *)malloc(sizeof(node));
if (ptr == NULL) {
printf("memory not alloc");
exit(0);
}
if (ptr != NULL) {
ptr->data = ele;
ptr->rlink = NULL;
ptr->llink = NULL;
}
return ptr;
}
void insertfront(int ele) {
node *newnode;
newnode = getnode(ele);
if (head == NULL) {
head = newnode;
head->rlink = head;
head->llink = head;
} else {
head->llink = newnode;
newnode->rlink = head;
head = newnode;
}
}
void insertend(int ele) {
node *newnode;
newnode = getnode(ele);
if (head == NULL) {
head = newnode;
head->rlink = head;
head->llink = head;
} else {
node *temp = head;
do {
temp = temp->rlink;
} while (temp != head->llink);
newnode->rlink = temp->rlink;
temp->rlink = newnode;
newnode->llink = temp;
}
}
int lenlist() {
node *temp;
int count = 0;
temp = head;
do {
temp = temp->rlink;
count++;
} while (temp != head);
return count;
}
void insertatpos(int ele,int pos) {
if (pos == 1) {
insertfront(ele);
} else
if (pos == (lenlist() + 1)) {
insertend(ele);
} else
if (pos > 1 && pos <= (lenlist() + 1)) {
node *prev, *curr;
node *newnode = getnode(ele);
int count = 1;
curr = head;//curr points to 1st node
do {
prev = curr;
count++;
curr = curr->rlink;
if (count == pos) {
prev->rlink = newnode;
newnode->llink = prev;
newnode->rlink = curr;
curr->llink = newnode;
}
} while (curr != head);
} else {
printf("invalid position");
}
}
void delfront() {
if (head == NULL)
printf("empty list");
node *aux;
node *lastnode, *secondnode;
aux = head;
lastnode = head->llink;
secondnode = head->rlink;
secondnode->llink = lastnode;
lastnode->rlink = secondnode;
free(aux);
head = secondnode;
}
void display() {
node *aux = head;
do {
printf("%d->", aux->data);
aux = aux->rlink;
} while (aux != head);
printf("\n");
}
int main() {
insertfront(100);
insertend(20);
printf("\n%d\n", lenlist());
insertatpos(45, 2);
display();
delfront();
display();
}

Answer 1

The problem is not in the deletefront() function, instead, you have missed updating a few links in the insertfront() and insertend() functions.
I have updated the code here and also added the comment where I made the changes. Try to visualise it using an example.
However, I suggest that you solve such issues using a debugger or go through the code with a sample test case. It will improve you debugging as well as coding skills!

Answer 2

Your code have a lot of mistakes.
// circular doubly linked list
#include <stdio.h>
#include <stdlib.h>
/*i changed the names of your pointers here*/
typedef struct node {
int data;
struct node *prev, *next;
} node;
/*
node *head = NULL;
This will be removed.
Avoid using globals as much as you can.
*/
/*
This function is unecessary.
node *createNode(int ele) {
node *ptr;
ptr = (node *)malloc(sizeof(node));
if (ptr == NULL) {
printf("memory not alloc");
exit(0);
}
if (ptr != NULL) {
ptr->data = ele;
ptr->rlink = NULL;
ptr->llink = NULL;
}
return ptr;
}
*/
char insertFront(node **head, int ele) {
node *newNode=malloc(sizeof(node));
If (newNode==NULL) return 0;
newNode->data=ele;
if (*head){
newNode->next=*head;
newNode->prev=(*head)->prev;
(*head)->prev->next=newNode;
(*head)->prev=newNode;
} else {
newNode->next=newNode;
newNode->prev=newNode;
}
*head=newNode;
return 1;
}
char insertEnd(node **head, int ele) {
node *newNode=malloc(sizeof(node));
If (newNode==NULL) return 0;
newNode->data=ele;
if (*head){
newNode->next=*head;
newNode->prev=(*head)->prev;
(*head)->prev->next=newNode;
(*head)->prev=newNode;
} else {
newNode->next=newNode;
newNode->prev=newNode;
*head=newNode;
}
return 1;
}
/*You could simple create a struct list that would have as members
the head of your list and its height to avoid calculating it each time
you want it but anyway. I will fix that.
int lenList(node *head) {
if (*head==NULL) return 0;
node *temp=head;
int count = 0;
do {
temp = temp->next;
count++;
} while (temp != head);
return count;
}
*/
char insertNatP(node **head, int ele, int pos) {
If (pos<1 || pos>lenList(head)){
printf("Invalid Position\n");
return 0;
}
int i;
for(i=0; i<pos-1; head=&head->next, i++);
node *newNode=malloc(sizeof(node));
If (newNode==NULL){
printf("Memory could not be allocated\n");
return 0;
}
newNode->data=ele;
If (*head!=NULL){
newNode->prev=(*head)->prev;
(*head)->prev->next=newNode;
(*head)->prev=newNode
newNode->next=*head;
} else {
newNode->prev=newNode;
newNode->next=newNode;
}
*head=newNode;
return 1;
}
char delFront(node **head) {
if (*head == NULL) return 0;
node garbage=*head;
*head=(*head)->next;
if (*head==garbage) *head=NULL; else{
(*head)->prev=garbage->prev;
garbage->prev->next=*head;
}
free(garbage);
return 1;
}
void printList(node *list) {
if (list==NULL) return;
node *sentinel=list->prev;
while (list!=sentinel) {
printf("%d->", list->data);
list=list->next;
}
printf("%d\n", list->data);
}
int main() {
node *l1=NULL;
insertFront(&l1, 100);
insertEnd(&l1, 20);
printf("\n%d\n", lenList(l1));
insertNatP(&l1, 45, 2);
printList(l1);
delFront(&l1);
printList(l1);
}
Try this