Head Node not updated outside Scope [Single Linked List]

Question

I have explained where I'm currently stuck at the bottom.

This structure below is my node .

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

struct node *head;

I have already alocated memory to head node.

head = malloc(sizeof(struct node));

And created a bunch of other nodes linked to it by

struct node *temp;
for (int i=0; i < no_of_nodes - 1; i++)
    {
        struct node *n = malloc(sizeof(struct node));
        temp -> next = n;
        printf("Enter Node %d data : ", i+1);
        scanf("%d", &(n -> data));

        temp = n;
    }
    temp -> next = NULL;

Then my printNodes function

void printNodes(struct node *n)
{
    //printf("%d", n -> data);
    while(n != NULL)
    {   
        if (n -> next != NULL)
            printf("%d%s", n -> data, " -> ");
        else
            printf("%d%s", n -> data, " -> NULL
");
        n = n->next;
    }
}

The output will look like : 6 -> 7 -> 8 -> 9 -> 3 -> NULL

Now, i'm trying to insert a node at head node, i.e it becomes the head node and the original head becomes the second node.

printf("Enter the head position (obviously 0) : ");
scanf("%d", &pos);
insertNode(head, pos);
printNodes(head);

The insertNode function :

void insertNode (struct node *head, int pos)
{
    struct node *n;
    n = malloc(sizeof(struct node));
    printf("Enter the Node data : ");
    scanf("%d", &(n -> data));

    if (pos == 0)
    {
        printf("Pos was 0, so HEAD
");
        n -> next = head;
        head = n;
        printNodes(head);
    }
}

Ok, so the problem is that, the printNodes(head) actually prints the as intended.. but the printNodes(head);after insertNodes(head, pos) doesn't print the updated Linked List, why is the head not being updated ? Where did I do wrong ?

The output I get is, Output:

Enter the Head position (obviously 0) : 0

Enter the Node data : 34

Pos was 0, so HEAD

34 -> 6 -> 7 -> 8 -> 9 -> 3 -> NULL [from printNodes(head) inside insertNodes(head, pos)]

6 -> 7 -> 8 -> 9 -> 3 -> NULL [from printNodes(head) after insertNodes(head, pos)]

If anyone wants to look at the full code, here you go.

Any help is appreciated.. Thank you in advance.

user_2877241 · Answer

The function insertNode accepts the pointer to the head node by value. That is it deals with a copy of the value of the original pointer to the head node. Changing the copy does not reflect on the original pointer. The original pointer stays unchanged.

You need to pass the pointer by reference.

In C passing an object by reference means passing it indirectly through a pointer to it. Dereferencing the pointer you will get a direct access to the referenced object.

The function can look at least the following way

int insertNode( struct node **head, size_t pos )
{
    struct node *n = malloc( sizeof( struct node ) );
    int success = n != NULL;

    if ( success )
    {
        printf( "Enter the Node data : " );
        scanf( "%d", &n->data );

        while ( *head != NULL && pos-- )
        {
            head = &( *head )->next;
        }

        n->next = *head;
        *head = n;
    }

    return success;
}

And the function is called like

insertNode( &head, pos );

Head Node not updated outside Scope [Single Linked List]

1 Answers