Linked List Implementation using C

“In their most basic sense, linked list and blockchain are very similar. A node (or block in case of the latter) contains the address (or hash) of the next node.”

My first encounter with linked list was in the 2nd semester of my undergrad study. Since we only use C/C++ in our university, I felt like I had to make some adjustments on my usual programming environment; I usually use Python in Pycharm and Swift on XCode. Unbeknown to me, XCode also support C/C++, with built-in realtime error checker!. So yeah, in the end, I use XCode instead of VSCode or CLion.

As stated in the title, I used C, not C++, so there are no classes and objects.

The first step is to create a struct containing the placeholder of the data and pointer to the next node (AKA the struct itself).

struct Node { 
         int data;
         struct Node *next;
} *previous, *start, *current;

What are those three on the last line?

Those are struct variables; instance of the struct itself. Remember, when we defined Node using keyword struct, we are creating a blueprint of a collection of variables. Those three variables are the instance of the struct.
Typing them that way is the same thing as typing these:

struct Node *previous, *start, *current;

inside the main function.

If you’re familiar with object-oriented programming, we are essentially creating a class and objects. Only this time, a struct and struct variables.

Now, of course, we want to start filling the linked list with data. But how?

start = NULL; 

while(1) {
         //allocate memory for current node
         current = malloc(sizeof(struct Node));

         //take input from user
         scanf("%d", &current->data);
         
         //just enter 123123 to end this cycle
         if(current->data == 123123) {
             break;
         }

         //make sure the current node is the last node
         current->next = NULL;
         
         //for the first time, make the current node
         //as the starting node
         if (start == NULL) start = current;
         
         //for the subsquent iterations, make the tail
         //of the previous node points to the current node
         else previous->next = current;
         
         //take the content of current node 
         //into the previous node
         previous = current;
} 

That’s it!

After entering 123123 to end the cycle, if we want to traverse through all the nodes, we just need to do

struct Node *n = start;
     
while (n!= NULL) {
         printf("%d ", n->data);
         n = n->next;
 } 

Why do we use n!=NULL?

The last node in our linked list terminates the cycle after we enter

current->next = NULL;

hence the *next pointer of our last node doesn’t point anywhere. So when the while loop reaches n==NULL, it terminates itself.

I will post more code on insertion and deletion in the next blog post.