Python Coding challenge - Day 1195| What is the output of the following Python Code?

 

Code :

from collections import deque d = deque(maxlen=4) for i in range(6): d.append(i) print(list(d))

Explanation:

🔹 1. Importing deque

from collections import deque

✅ Explanation

deque means Double Ended Queue.

It allows insertion and deletion from both ends efficiently.

Here we'll use a special feature called maxlen.

Think of it like a train with only 4 seats.

Train Capacity = 4

🚆 [ _ | _ | _ | _ ]

No matter how many passengers come, only 4 passengers can stay.

🔹 2. Creating a Fixed-Size Queue

d = deque(maxlen=4)

✅ Explanation

A deque is created with a maximum capacity of 4.

Current state:

Capacity = 4

[]

Important rule:

If queue becomes full,

new element enters,

oldest element automatically leaves.

Unlike a normal list, you never get an overflow error.

🔹 3. Starting the Loop

for i in range(6):

✅ Explanation

range(6) generates:

0

1

2

3

4

5

Python will execute the loop 6 times.

🔹 4. First Iteration (i = 0)

d.append(0)

Queue before:

[]

Queue after:

[0]

Seats occupied:

🚆 [0 | _ | _ | _]

Still space available.

🔹 5. Second Iteration (i = 1)

d.append(1)

Queue:

[0,1]

Visual:

🚆 [0 | 1 | _ | _]

Still not full.

🔹 6. Third Iteration (i = 2)

d.append(2)

Queue:

[0,1,2]

Visual:

🚆 [0 | 1 | 2 | _]

One seat remains.

🔹 7. Fourth Iteration (i = 3)

d.append(3)

Queue becomes:

[0,1,2,3]

Visual:

🚆 [0 | 1 | 2 | 3]

Now the queue is completely full.

Capacity:

4 / 4

🔹 8. Fifth Iteration (i = 4)

d.append(4)

Here's the interesting part.

Current queue:

[0,1,2,3]

But there is no empty seat.

So Python automatically removes the oldest element.

Oldest element:

0

After removing 0:

[1,2,3]

Now 4 is inserted.

Final queue:

[1,2,3,4]

Visual:

Before

🚆 [0 | 1 | 2 | 3]

Passenger 4 arrives

↓

Passenger 0 leaves automatically

↓

🚆 [1 | 2 | 3 | 4]

This behavior is called a Sliding Window.

🔹 9. Sixth Iteration (i = 5)

d.append(5)

Current queue:

[1,2,3,4]

Again queue is full.

Oldest passenger:

1

Leaves automatically.

New passenger:

5

enters.

Final queue:

[2,3,4,5]

Visual:

Before

🚆 [1 | 2 | 3 | 4]

Passenger 5 arrives

↓

Passenger 1 leaves

↓

🚆 [2 | 3 | 4 | 5]

🔹 10. Printing the Queue

print(list(d))

✅ Explanation

The deque is converted into a list.

Final output:

[2, 3, 4, 5]

🎯 Final Output

[2, 3, 4, 5]