Create and Initialize Doubly Linked List in DSA C - Execution Trace

Concept Flow - Create and Initialize Doubly Linked List

Define Node Structure

↓

Create Head Node with data

↓

Set head.prev = NULL

↓

Set head.next = NULL

↓

Initialize list with single node

↓

Done: List ready with one node

Start by defining the node structure, then create the first node (head) with data, set its previous and next pointers to NULL, and finish with a list containing one node.

Execution Sample

DSA C

struct Node {
    int data;
    struct Node* prev;
    struct Node* next;
};

struct Node* head = malloc(sizeof(struct Node));
head->data = 10;
head->prev = NULL;
head->next = NULL;

This code creates a doubly linked list with one node containing data 10, and both pointers set to NULL.

Execution Table

Step	Operation	Nodes in List	Pointer Changes	Visual State
1	Define struct Node	None	None	List is empty
2	Allocate memory for head node	Node(?, prev=garbage, next=garbage)	head points to new node	head -> [ ? | garbage | garbage ]
3	Assign data = 10 to head	Node(10, prev=garbage, next=garbage)	head->data = 10	head -> [ 10 | garbage | garbage ]
4	Set head->prev = NULL	Node(10, prev=NULL, next=garbage)	head->prev = NULL	head -> [ 10 | NULL | garbage ]
5	Set head->next = NULL	Node(10, prev=NULL, next=NULL)	head->next = NULL	head -> [ 10 | NULL | NULL ]
6	Initialization complete	Node(10, prev=NULL, next=NULL)	No change	head -> [ 10 | NULL | NULL ]

💡 List initialized with one node; both prev and next pointers are NULL.

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	After Step 5	Final
head	NULL	Points to allocated node	Points to node with data=10	Points to node with data=10, prev=NULL	Points to node with data=10, prev=NULL, next=NULL	Points to node with data=10, prev=NULL, next=NULL
head->data	Undefined	Undefined	10	10	10	10
head->prev	Undefined	garbage	garbage	NULL	NULL	NULL
head->next	Undefined	garbage	garbage	garbage	NULL	NULL

Key Moments - 3 Insights

Why do we set both head->prev and head->next to NULL?

Because this is the first and only node, it has no previous or next nodes. Setting these pointers to NULL clearly marks the ends of the list, as shown in execution_table steps 4 and 5.

What happens if we forget to allocate memory for the head node?

Without allocating memory (step 2), head would point to NULL or garbage, causing errors when accessing or assigning data. The list would not be properly created.

Why is the data assigned after allocating memory but before setting pointers?

We must first have a valid node (step 2) to store data. Assigning data before allocation would cause errors. Setting pointers after data assignment (steps 4 and 5) ensures the node is fully initialized.

Visual Quiz - 3 Questions

Look at the execution_table at step 3, what is the value of head->data?

A NULL

B Undefined

C 10

D 0

💡 Hint

Check the 'Pointer Changes' and 'Visual State' columns at step 3 in execution_table.

At which step does head->prev get set to NULL?

A Step 3

B Step 4

C Step 2

D Step 5

💡 Hint

Look at the 'Operation' column in execution_table to find when head->prev is assigned.

If we skip setting head->next to NULL, what would the Visual State show after step 5?

A head -> [ 10 | NULL | garbage ]

B head -> [ 10 | garbage | NULL ]

C head -> [ 10 | NULL | NULL ]

D head -> [ 10 | garbage | garbage ]

💡 Hint

Consider what happens if head->next is not initialized; check pointer defaults in variable_tracker.

Concept Snapshot

Create a struct Node with data, prev, next pointers.
Allocate memory for head node.
Assign data to head->data.
Set head->prev = NULL and head->next = NULL.
This creates a doubly linked list with one node.
Pointers NULL mark list ends.

Full Transcript

To create and initialize a doubly linked list, first define the node structure with data and two pointers: prev and next. Then allocate memory for the head node. Assign the desired data value to head->data. Set both head->prev and head->next to NULL because this is the only node, so it has no neighbors. This process results in a doubly linked list with a single node, where both ends are clearly marked by NULL pointers.