Peek Front Element of Queue in DSA C - Execution Trace

Concept Flow - Peek Front Element of Queue

Start with Queue

↓

Check if Queue is empty?

Yes→Return error or NULL

No↓

Access front element

↓

Return front element value

↓

Done

The flow checks if the queue is empty. If not, it accesses and returns the front element without removing it.

Execution Sample

DSA C

int peekFront(Queue* q) {
    if (q->front == NULL) return -1; // empty
    return q->front->data;
}

This code returns the value at the front of the queue or -1 if the queue is empty.

Execution Table

Step	Operation	Nodes in Queue	Pointer Changes	Visual State
1	Start with queue having nodes 10 -> 20 -> 30	10 -> 20 -> 30 -> NULL	front points to node with 10, rear points to node with 30	[front]10 -> 20 -> 30 -> NULL
2	Check if queue is empty (front == NULL?)	10 -> 20 -> 30 -> NULL	No pointer changes	[front]10 -> 20 -> 30 -> NULL
3	Access front element data	10 -> 20 -> 30 -> NULL	No pointer changes	[front]10 -> 20 -> 30 -> NULL
4	Return front element value: 10	10 -> 20 -> 30 -> NULL	No pointer changes	[front]10 -> 20 -> 30 -> NULL

💡 Peek operation ends after returning front element value without modifying queue.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
front	Node(10)	Node(10)	Node(10)	Node(10)	Node(10)
rear	Node(30)	Node(30)	Node(30)	Node(30)	Node(30)
front->data	10	10	10	10	10

Key Moments - 3 Insights

Why do we check if front == NULL before peeking?

Because if front is NULL, the queue is empty and there is no element to peek. This prevents accessing invalid memory. See execution_table step 2.

Does peeking remove the front element from the queue?

No, peeking only reads the front element's value without changing the queue structure or pointers. See execution_table steps 3 and 4.

What happens if we try to peek an empty queue?

The function returns -1 (or an error) indicating the queue is empty, as shown in the code and explained in execution_table step 2.

Visual Quiz - 3 Questions

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

A 20

B 10

C 30

D -1

💡 Hint

Check the 'front->data' row in variable_tracker after step 3.

At which step does the code confirm the queue is not empty?

A Step 1

B Step 3

C Step 2

D Step 4

💡 Hint

Look at the 'Operation' column in execution_table where the empty check happens.

If the queue was empty, what would the function return?

A -1

B 10

C 0

D NULL

💡 Hint

See the code in execution_sample and explanation in key_moments about empty queue.

Concept Snapshot

Peek Front Element of Queue:
- Check if queue is empty (front == NULL)
- If empty, return error (e.g., -1)
- Else, return front->data without removing it
- Queue remains unchanged after peek
- Useful to see next element without dequeueing

Full Transcript

Peeking the front element of a queue means looking at the first element without removing it. The code first checks if the queue is empty by seeing if the front pointer is NULL. If it is empty, it returns -1 to indicate no element is present. Otherwise, it returns the data stored in the front node. The queue structure and pointers do not change during this operation. This lets us see the next element to be dequeued without modifying the queue.