inserting a node into a linked list - c

Question

Assume structure list and node are defined as
struct list {struct node *a;};
struct node { int value;
struct node *next;};
The following function inserts integer e into l as the first element
void insert_first(int e, struct list *l){
struct node * r = malloc(sizeof(struct node));
r->value = e;
r->next = l->a;
l->a = r;}
Example: original list "b": 1 2 3 4
after calling insert_first(3,*b)
list "b": 3 1 2 3 4
insert_first is pretty straightfoward; however, I am having a hard time trying to figure out how to write a function insert_last which inserts a number as the last element of the list.
Example: original list "b": 1 2 3 4
after calling insert_last(3,*b)
list "b": 1 2 3 4 3
Thanks for any help in advance.

Answer 1

One way of doing it is to iterate over the list until you find the tail. Something like this:
void insert_last(int e, struct list *l)
{
// "iter" Will iterate over the list.
struct node *iter = l->a;
struct node *new_node = malloc(sizeof(struct node));
// Advice: ALWAYS check, if malloc returned a pointer!
if(!new_node) exit(1); // Memory allocation failure.
new_node->value = e;
new_node->next = NULL;
if(iter){
// Loop until we found the tail.
// (The node with no next node)
while(iter->next) iter = iter->next;
// Assign the new tail.
iter->next = new_node;
}else{
// The list was empty, assign the new node to be the head of the list.
l->a = new_node;
}
}
EDIT: Something I saw in your code, that really tickles me: ALWAYS check, when using malloc, whether you actually got a pointer back or not (check if the pointer is NULL). If malloc failes to allocate memory, be it for a lack thereof or some other critical error, it will toss you a NULL pointer. If you do not check for that, you might end up running in to some very nasty, hard to detect bugs. Just a little reminder!

Answer 2

You need to save original HEAD node and traverse through list . Hope this code will help you.
struct node {
int value;
struct node *next;
};
struct list {struct node *a;};
struct node *insert_last(int e, struct list *l) {
/* Store the initial head of the list */
struct list *org_head = head;
struct node *r = malloc(sizeof(struct node));
r->value = e;
r->next = NULL /* Assign next pointer of current node to NULL */
/* If the head is initially NULL, then directly return the new created node (r) as the head of a linked list with only one node */
if(head == NULL)
{
return r;
}
/* While we do not reach the last node, move to the next node */
while(head -> next != NULL)
head = head -> next;
/* Assign the 'next' pointer of the current node to the "r" */
head->next = r;
/* return the original head that we have stored separately before */
return org_head;
}