Enqueue Operation in DSA C - Execution Trace

Concept Flow - Enqueue Operation

Create new node with data

↓

Is queue empty?

Yes→Set head and tail to new node

No↓

Set tail.next to new node

↓

Update tail to new node

↓

Done

This flow shows how a new node is created and added at the end of the queue, updating pointers accordingly.

Execution Sample

DSA C

void enqueue(Queue* q, int value) {
    Node* newNode = malloc(sizeof(Node));
    newNode->data = value;
    newNode->next = NULL;
    if (q->tail == NULL) {
        q->head = newNode;
        q->tail = newNode;
    } else {
        q->tail->next = newNode;
        q->tail = newNode;
    }
}

This code adds a new element at the end of the queue, updating head and tail pointers.

Execution Table

Step	Operation	Nodes in List	Pointer Changes	Visual State
1	Create new node with data=10	None	newNode created	null
2	Check if queue is empty (tail == NULL)	None	tail is NULL (empty queue)	null
3	Set head and tail to newNode	10	head -> newNode, tail -> newNode	10 -> null
4	Create new node with data=20	10	newNode created	10 -> null
5	Check if queue is empty (tail == NULL)	10	tail not NULL	10 -> null
6	Set tail->next to newNode	10, 20	tail->next -> newNode	10 -> 20 -> null
7	Update tail to newNode	10, 20	tail -> newNode	10 -> 20 -> null
8	Create new node with data=30	10, 20	newNode created	10 -> 20 -> null
9	Check if queue is empty (tail == NULL)	10, 20	tail not NULL	10 -> 20 -> null
10	Set tail->next to newNode	10, 20, 30	tail->next -> newNode	10 -> 20 -> 30 -> null
11	Update tail to newNode	10, 20, 30	tail -> newNode	10 -> 20 -> 30 -> null
12	End of enqueue operations	10, 20, 30	head -> 10, tail -> 30	10 -> 20 -> 30 -> null

💡 All enqueue operations completed; tail updated to last node; queue is not empty.

Variable Tracker

Variable	Start	After Step 3	After Step 7	After Step 11	Final
head	NULL	Node(10)	Node(10)	Node(10)	Node(10)
tail	NULL	Node(10)	Node(20)	Node(30)	Node(30)
newNode	NULL	Node(10)	Node(20)	Node(30)	NULL
Queue Size	0	1	2	3	3

Key Moments - 3 Insights

Why do we check if tail is NULL before adding a new node?

Because if tail is NULL, the queue is empty, so both head and tail must point to the new node. This is shown in execution_table step 2 and 3.

Why do we set tail->next to the new node when queue is not empty?

Because the new node must be linked after the current tail to maintain the queue order. This happens in steps 6 and 10.

Why do we update tail to the new node after linking it?

Because tail always points to the last node in the queue, so after adding a new node, tail must move to it. See steps 7 and 11.

Visual Quiz - 3 Questions

Look at the execution_table at step 3, what does the queue look like?

A null

B 10 -> null

C 20 -> null

D 10 -> 20 -> null

💡 Hint

Check the Visual State column at step 3 in execution_table.

At which step does the tail pointer first move from NULL to a node?

A Step 3

B Step 6

C Step 1

D Step 7

💡 Hint

Look at Pointer Changes column to see when tail changes from NULL.

If we enqueue a new node after step 11, what will tail point to?

A Node with data 10

B Node with data 30

C Newly created node

D Node with data 20

💡 Hint

Tail always points to the last node added, see variable_tracker after step 11.

Concept Snapshot

Enqueue Operation in Queue:
- Create new node with data
- If queue empty (tail == NULL), set head and tail to new node
- Else, link new node after tail (tail->next = new node)
- Update tail to new node
- Queue grows at tail end maintaining FIFO order

Full Transcript

The enqueue operation adds a new element at the end of the queue. First, a new node is created with the given data. If the queue is empty, both head and tail pointers are set to this new node. Otherwise, the current tail's next pointer is set to the new node, and then tail is updated to point to the new node. This maintains the order of elements in the queue, ensuring FIFO behavior. The execution table shows each step of this process, including pointer updates and the visual state of the queue. The variable tracker records how head, tail, and newNode pointers change after each operation. Key moments clarify why checking for an empty queue is necessary and why tail must be updated after adding a node. The visual quiz tests understanding of these steps and pointer changes.