C: using sentinels on lists

Question

Hello so I am learning linked lists and I have to write several functions by using a sentinel. I have this definition: A sentinel is a fake element that is the first element of the list. An empty list is then a single sentinel and not a NULL pointer.

I need initialise the sentinel of an empty list

void list_init(struct list *list);

and check if the list is empty (return true is empty)

int list_is_empty(struct list *list);

but I am completely lost, could you help me out, thanks!

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score 2 · Accepted Answer · answered Dec 05 '18 at 13:52

2

A linked list node always has a next member

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

When you create your sentinel you initialize next to NULL

void list_init(struct list *list)
{
    list->data = -1;
    list->next = NULL;
}


struct list *head = malloc(sizeof(struct list));
list_init(head);

Now the head has a next member of NULL so all you have to do is check if next equals NULL

int list_is_empty(struct list *list)
{
    if (list->next == NULL) return 1;

    return 0;
}

One you add one node head->next becomes NOT NULL and you will know the list is not empty. But you have to make sure that you always pass the head to the list_functions.

answered Dec 05 '18 at 13:52

Serve Laurijssen

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thank you very much, but can you explain me why you did list->data = -1; why set it to -1 ? – sltcv Dec 05 '18 at 14:50
that was just a random value – Serve Laurijssen Dec 05 '18 at 14:51
but then, what if in the list i have other values of -1 how do we know which one is the sentinel ? what is the point of using a sentinel – sltcv Dec 05 '18 at 14:55
the int is not the sentinel, the next pointer is. The int is just useless for the first node – Serve Laurijssen Dec 05 '18 at 15:07
i don't understand what actually is a "sentinel" ? it is a node ? what is its role – sltcv Dec 05 '18 at 15:11
sentinel is a node with a special value which has no meaning to the program. Google "sentinel C" – Serve Laurijssen Dec 06 '18 at 06:56

C: using sentinels on lists

1 Answers1