Pointer-to-Pointer and linked list, passing parameters per value

Question

Background is that I am experimenting with pointer-to-pointer in C by implementing a linked list. My question is regarding the difference in the two pieces of code and why the first one is giving expected output, but not the other one. Why does the first piece of code not advance head "outsite" the function which code 2 seems to do?

void add_node(int x, Node **head){
if(*head == NULL){
    Node *new_node = (Node*) malloc(sizeof(new_node));
    new_node->x = x;
    *head = new_node;
} else {
    Node *n = *head;

    while(n->next != NULL){
        n = n->next;
    }

    Node *new_node = (Node*) malloc(sizeof(new_node));
    new_node->x = x;
    n->next = new_node;
}
}

Output is as expected if I add 4 elements and after each addition print the list: 1 | 12 | 123 | 1234

void add_node(int x, Node **head){
if(*head == NULL){
    Node *new_node = (Node*) malloc(sizeof(new_node));
    new_node->x = x;
    *head = new_node;
} else {
    while((*head)->next != NULL){
        *head = (*head)->next;
    }

    Node *new_node = (Node*) malloc(sizeof(new_node));
    new_node->x = x;
    (*head)->next = new_node;
}
}

Output is following: 1 | 12 | 23 | 34

`Node *new_node = (Node*) malloc(sizeof(*new_node)); new_node->x = x;new_node->next=NULL;` — BLUEPIXY, Nov 29 '15 at 21:33
`*head = (*head)->next;` this update `*head`, It is no longer at the top of the list. — BLUEPIXY, Nov 29 '15 at 21:35
Allocation size is incorrect, should be `malloc(sizeof(Node))` — chqrlie, Nov 29 '15 at 21:45

score 2 · Accepted Answer · answered Nov 29 '15 at 21:39

2

In the first example you are using the pointer n to travel the linked list, you're assigning it to n->next, which is exactly what you want to do to travel a linked list. In the second example, you are changing what the head pointer is pointing to:

*head = (*head)->next;

You're essentially moving the beginning of the linked list to another node, that's why you're having such behavior.

answered Nov 29 '15 at 21:39

hit

36
1

Ah, I see, makes sense. What confused my in the beginning was why they were different but I see it now. Is it a good way to do it or are there any obvious better ways? – TutenStain Nov 29 '15 at 21:53
If you believe this is correct, Mark it as accepted please :) – rcmgleite Nov 29 '15 at 22:07

score 0 · Answer 2 · answered Nov 29 '15 at 21:36

0

Evaluate input 1,2,3 and focus on the head.

while((*head)->next != NULL){
    *head = (*head)->next;
}
Node *new_node = (Node*) malloc(sizeof(new_node));
        new_node->x = x;
        (*head)->next = new_node;

leaves the head pointing somewhere, which your output is depicting ;)

For input 1 and 1,2 the conditions are not met and you escape.

answered Nov 29 '15 at 21:36

Naman

27,789
26
218
353

1

Allocation size is incorrect, should be `malloc(sizeof(Node))` – chqrlie Nov 29 '15 at 21:44

Pointer-to-Pointer and linked list, passing parameters per value

2 Answers2