C linked Lists/-> Operator

Question

So I've been trying to understand the concepts of linked lists (Been looking at some example code, I found this one on the internet. Now if I could kindly have someone confirm if I have grasped some of the concepts correctly. I will draw diagrams of what I think each linke of code does.

#include <stdio.h>
#include <stdlib.h>

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

int main (int argc, char *argv[]){

   struct ListItem a;  
   a.data = 0;
   a.next = NULL;
   struct ListItem b;
   b.data = 1;
   b.next = NULL;
   struct ListItem c;
   c.data = 2;
   c.next = NULL;

   a.next = &b;
   b.next = &c;

   int counter = 0;
   struct ListItem *i;
   for (i = &a; i != NULL; i = i->next){

      printf("Item %d value is %d\n\r", counter, i->data);
      counter++;
   }


   return 0;
}

---

Snippet of code 1:

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

This creates a structure called ListItems. There are two components to the structure, a component to store the data and a pointer to another structure of type struct ListItem. I visualise the linked list like this:

Is this a correct way of visualising it?

---

Code snippet 2:

struct ListItem a;  
a.data = 0;
a.next = NULL;
struct ListItem b;
b.data = 1;
b.next = NULL;
struct ListItem c;
c.data = 2;
c.next = NULL;

Yup, I know it can be cut shorter but I just did it this way to see if I can understand the concept. Now this snippet creates a variable "a", "b" and "c" of type struct ListItem. Then it sets the first member (data) of each structure to 0, 1, 2 respectively and the second member (next) to point to NULL. So my visualisation now is like this:

Now MORE QUESTIONS:

Question 1: When we are initially the pointer to NULL, it points to nothing correct? Why do we do this? Wasn't it originally pointed to nothing?

---

Snippet 3:

   a.next = &b;
   b.next = &c;

This let's next in each variable a, b (which is a structure) point to the address memory location of b and c respectively.

My visualisation:

Question: How can it do this? Isn't the structure itself stored over multiple memory address (4 for int, etc)

---

Snippet 4:

   int counter = 0;
   struct ListItem *i;
   for (i = &a; i != NULL; i = i->next){

      printf("Item %d value is %d\n\r", counter, i->data);
      counter++;
   }

Here is the snippet I've been a bit confused with. Now, we set aside an integer called counter and initailise this to zero. Also, we create a variable called i that points to the type struct ListItem. Now could someone explain to me the for loop? I am a bit confused on what it's doing. In particular, i=i->next, I'm not familiar with this. I know it's equivalent to i=(*i).next but not sure what it really does. Could someone create a quick diagram?

ALSO: If anyone has any good resources/links (no pun intended) to some useful websites to help me understand linked lists a bit better feel free to post them.

Answer 1

Q1: an uninitialized pointer does not point to "nothing". If you don't set its value, it can (and will) point to just about anything, including (but not limited to) a nul value, out-of-bounds memory, or the last password you entered on a website.

It's really no different than asking what the value of your counter is when you do

int counter;
...
printf ("counter is %d\n", counter);

Setting the next pointer to NULL, therefore, serves two distinct purposes. First, you are sure it is a "known" value, not any random one. Second, by convention a NULL value means that it does not point to any valid location.

(It is a 'convention' because NULL effectively stores the numerical value '0'. This, as a memory address -- since you are talking about pointers -- is perfectly valid. There are just not many daily uses for it.)