inserting node at the beginning of a circular linked list using void function

Question

I m trying to insert a node at the beginning of a circular linked list using a void function. When i pass pointer to pointer head in the function because i have to change the head itself, it is throwing me some error on line 24 where i marked it with comment : "the error is here". Before passing a pointer to pointer, it was inserting the element at the end Here's the complete code :

 #include <stdio.h>
#include <stdlib.h>

struct Node
{
    int data;
    struct Node *next;
};

void circularLLTraversal(struct Node *head)
{
    struct Node *ptr = head;
    do
    {
        printf("Element: %d\n", ptr->data);
        ptr = ptr->next;
    } while (ptr != head);
}

void insertionAtBeginning(struct Node **head, int data)
{
    struct Node *ptr = (struct Node *)malloc(sizeof(struct Node));
    ptr->data = data;
    struct Node *p = *head->next; //The error is here
    while (p->next != *head)
    {
        p = p->next;
    }
    // At this point p points to the last node of the circular linked list

    p->next = ptr;
    ptr->next = *head;
    *head = ptr;
}

int main()
{

    struct Node *head;
    struct Node *second;
    struct Node *third;
    struct Node *fourth;

    head = (struct Node *)malloc(sizeof(struct Node));
    second = (struct Node *)malloc(sizeof(struct Node));
    third = (struct Node *)malloc(sizeof(struct Node));
    fourth = (struct Node *)malloc(sizeof(struct Node));

    head->data = 4;
    head->next = second;

    second->data = 3;
    second->next = third;

    third->data = 6;
    third->next = fourth;

    fourth->data = 1;
    fourth->next = head;

    printf("Circular Linked List before insertion: \n");
    circularLLTraversal(head);

    insertionAtBeginning(&head, 8);

    printf("Circular Linked List after insertion: \n");
    circularLLTraversal(head);

    return 0;
}

Please let me know where m i wrong with pointer to pointer, this really makes me scratch my head.

why are you using a double pointer in the function for the head? and you are not passing the address of the head in the function call. — Jaya Shankar B
– Jaya Shankar B, Commented Oct 15, 2022 at 14:46
Using the double pointer because i have to change the head to the ptr. — Neilson Programmer
– Neilson Programmer, Commented Oct 15, 2022 at 14:48
You need to wrap the *head with parenthesis to fix the issue (*head)->next refer the following link :gcc.gnu.org/bugzilla/…*server%20in%20parentheses%20but%20the%20error%20message%20should%20still%20be%20amended%20imho. — Jaya Shankar B
– Jaya Shankar B, Commented Oct 15, 2022 at 15:02
You don't need a loop to find the end of the list when you are inserting at the beginning. — Jonathan Leffler
– Jonathan Leffler, Commented Oct 15, 2022 at 15:36
we need to find the end because this is a circular linked list where the last node points to the head. so we need to loop till the last node's next doesn't becomes the head. — Neilson Programmer
– Neilson Programmer, Commented Oct 16, 2022 at 7:10

Jaya Shankar B · Accepted Answer · 2022-10-15 15:04:17Z

1

You need to wrap the *head with parenthesis to fix the issue (*head)->next

refer the following link :https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91134#:~:text=The%20fix%20programming%20side%20is%20of%20course%20just%20wrapping%20*server%20in%20parentheses%20but%20the%20error%20message%20should%20still%20be%20amended%20imho.

The code is

void insertionAtBeginning(struct Node **head, int data)
{
    struct Node *ptr = (struct Node *)malloc(sizeof(struct Node));
    ptr->data = data;
    struct Node *p = (*head)->next; // Fix
    while (p->next != *head)
    {
        p = p->next;
    }
    // At this point p points to the last node of the circular linked list

    p->next = ptr;
    ptr->next = *head;
    *head = ptr;
}

It could be due the priority order

answered Oct 15, 2022 at 15:04

Jaya Shankar B

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1 Comment

Neilson Programmer Over a year ago

Yes...it worked the issue was with parenthesis . Thanx a lot :)

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