Inserting a node in a linked list - c

Question

I had this exercice that asks me to create a function that adds a number in a new node to the head of a linked list based on a struct containing one integer. This is the struct:
struct Node
{
int data;
struct Node *next;
};
no problem till now. so I created one that takes 2 arguments: the integer to add and the pointer to the head of linked list, but it didn't work. this is my code:
void push(struct Node* head, int new_data)
{
struct Node* new_node = (struct Node*) malloc(sizeof(struct Node));
new_node->data = new_data;
new_node->next = head;
head = new_node;
}
So, what I did is that I made the new_node point to the same node pointed by head, after that I make the new node the new head of the linked list. it seems very logical, although it didn't work. on the other hand, when I give the function the adress of head pointer instead of the pointer itself, it does work:
void push(struct Node** head_ref, int new_data)
{
/* 1. allocate node */
struct Node* new_node = (struct Node*) malloc(sizeof(struct Node));
/* 2. put in the data */
new_node->data = new_data;
/* 3. Make next of new node as head */
new_node->next = (*head_ref);
/* 4. move the head to point to the new node */
(*head_ref) = new_node;
}
and this is the main for the function with double **:
int main()
{
struct Node* head = NULL;
push(&head,7);
push(&head,6);
push(&head,3);
return 0;
}
I understand that the second function should work, but I don't see why it is necessary to use the adress of head and not head itself. I would be glad if anybody could explain the reason to me, Thanks.

Answer 1

but I don't see why it is necessary to use the adress of head and not head itself.
In plain c code you can't have references (as in contrast to c++), but just pointers.
The value stored in the head pointer variable should be changed from within the call to push(), so you need to pass the address of the head variable to change the (single * pointer) value.
int main()
{
struct Node* head = NULL;
push(&head,7);
// ...
}
void push(struct Node** head_ref, int new_data)
{
// ...
/* 3. Make next of new node as head */
new_node->next = (*head_ref); // Dereferencing head_ref yields the current
// content of head
/* 4. move the head to point to the new node */
(*head_ref) = new_node; // Store the newly allocated memory address
// into the head pointer
}
As you were tagging your question c++ originally this isn't necessary using c++ code.
You can take the pointer parameter by reference as well:
void push(struct Node*& head_ref, int new_data)
// ^
{
// ...
/* 3. Make next of new node as head */
new_node->next = head_ref;
/* 4. move the head to point to the new node */
head_ref = new_node; // <<<<<<<<<<
}
int main() {
struct Node* head = nullptr;
push(head,7);
push(head,6);
push(head,3);
return 0;
}