Queue Implementation Using Array in DSA Python - Execution Trace

Concept Flow - Queue Implementation Using Array

Initialize empty array queue

↓

Enqueue: Add element at end

↓

Check if queue is full?

Yes→Reject enqueue

No↓

Dequeue: Remove element from front

↓

Check if queue is empty?

Yes→Reject dequeue

No↓

Shift front pointer forward

↓

Repeat operations as needed

This flow shows how elements are added at the end and removed from the front of an array-based queue, with checks for full and empty states.

Execution Sample

DSA Python

queue = []
front = 0
rear = -1

# Enqueue 10
rear += 1
queue.append(10)

# Enqueue 20
rear += 1
queue.append(20)

# Dequeue
if front <= rear:
    item = queue[front]
    front += 1

This code enqueues two elements and dequeues one by moving the front pointer.

Execution Table

Step	Operation	Queue Array	Front Pointer	Rear Pointer	Visual State
1	Initialize queue	[]	0	-1	Queue: [] Front=0 Rear=-1
2	Enqueue 10	[10]	0	0	Queue: [10] Front=0 Rear=0
3	Enqueue 20	[10, 20]	0	1	Queue: [10, 20] Front=0 Rear=1
4	Dequeue (remove 10)	[10, 20]	1	1	Queue: [10, 20] Front=1 Rear=1 (10 removed)
5	Enqueue 30	[10, 20, 30]	1	2	Queue: [10, 20, 30] Front=1 Rear=2
6	Dequeue (remove 20)	[10, 20, 30]	2	2	Queue: [10, 20, 30] Front=2 Rear=2 (20 removed)
7	Dequeue (remove 30)	[10, 20, 30]	3	2	Queue: [10, 20, 30] Front=3 Rear=2 (30 removed)
8	Dequeue attempt on empty queue	[10, 20, 30]	3	2	Queue empty: Front > Rear, cannot dequeue

💡 Step 8 stops because front pointer (3) is greater than rear pointer (2), meaning queue is empty.

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	After Step 5	After Step 6	After Step 7	After Step 8
queue	[]	[10]	[10, 20]	[10, 20]	[10, 20, 30]	[10, 20, 30]	[10, 20, 30]	[10, 20, 30]
front	0	0	0	1	1	2	3	3
rear	-1	0	1	1	2	2	2	2

Key Moments - 3 Insights

Why does the front pointer move forward instead of removing the element physically?

Because in array-based queue, we just move the front pointer to mark the next element as front. The old element stays but is ignored. See Step 4 and Step 6 in execution_table where front moves but array stays same.

What happens when front pointer becomes greater than rear pointer?

It means the queue is empty and no elements can be dequeued. This is shown in Step 8 where dequeue is rejected because front=3 > rear=2.

Why does rear pointer start at -1?

Rear starts at -1 to indicate the queue is empty initially. When first element is enqueued, rear moves to 0. See Step 1 and Step 2 in execution_table.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of front pointer after Step 5?

A 0

B 1

C 2

D 3

💡 Hint

Check the 'Front Pointer' column at Step 5 in execution_table.

At which step does the queue become empty?

A Step 6

B Step 8

C Step 7

D Step 4

💡 Hint

Look for when front pointer is greater than rear pointer in execution_table.

If we enqueue 40 after Step 7, what will be the rear pointer value?

A 3

B 2

C 4

D 5

💡 Hint

Rear pointer increments by 1 on enqueue; see rear pointer changes in variable_tracker.

Concept Snapshot

Queue using array:
- Enqueue adds element at rear (end)
- Dequeue removes element from front
- Use front and rear pointers
- Queue empty if front > rear
- Rear starts at -1, front at 0

Full Transcript

This visualization shows how a queue works using an array. We add elements at the rear end and remove from the front. The front and rear pointers track where to remove and add. Initially, the queue is empty with rear at -1 and front at 0. When we enqueue, rear moves forward and element is added. When we dequeue, front moves forward to ignore the removed element. The array keeps all elements but only those between front and rear are valid. When front pointer passes rear pointer, the queue is empty and no dequeue is possible. This simple pointer movement avoids shifting elements in the array.