Peek Front Element of Queue in DSA Python - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of peeking the front element after enqueue operations?

Consider a queue where you enqueue the elements 10, 20, and 30 in that order. What will be the front element after these operations?

DSA Python

from collections import deque

queue = deque()
queue.append(10)
queue.append(20)
queue.append(30)
front = queue[0]
print(front)

A 10

B 20

C 30

D IndexError

💡 Hint

Remember, the front element is the first one added that has not been removed.

In a queue, elements are added at the back and removed from the front. After adding 10, 20, and 30, the front is still 10 because it was the first added and not removed.

❓ Predict Output

intermediate

What happens when peeking an empty queue?

What will happen if you try to peek the front element of an empty queue implemented using deque?

DSA Python

from collections import deque

queue = deque()
front = queue[0]
print(front)

A 0

B None

C IndexError

D AttributeError

💡 Hint

Think about what happens when you access an index that does not exist in a list or deque.

Trying to access queue[0] when the queue is empty raises an IndexError because there is no element at index 0.

🔧 Debug

advanced

Identify the error when peeking front element in this queue implementation

This code tries to peek the front element of a queue implemented as a list. What error will it produce?

DSA Python

queue = []
queue.append(5)
queue.append(10)
front = queue.pop(0)
print(front)
print(queue[0])

A No error, prints 5 then 5

B IndexError on print(queue[0])

C TypeError on queue.pop(0)

D No error, prints 5 then 10

💡 Hint

Check what happens to the queue after popping the front element and then accessing queue[0].

The first pop(0) removes 5, so front is 5. Then print(queue[0]) tries to print the new front, which is 10. But if the queue had only one element, accessing queue[0] after pop(0) would cause IndexError. Here, since queue has 10 after pop, no error occurs. So the correct answer is no error and prints 5 then 10.

❓ Predict Output

advanced

What is the output after multiple enqueue and dequeue operations?

Given the following operations on a queue, what will be printed as the front element?

DSA Python

from collections import deque

queue = deque()
queue.append(1)
queue.append(2)
queue.append(3)
queue.popleft()
queue.append(4)
front = queue[0]
print(front)

A 2

B 1

C 3

D 4

💡 Hint

Remember that popleft removes the front element.

Initially queue is [1,2,3]. popleft removes 1, so queue becomes [2,3]. Then 4 is appended, queue is [2,3,4]. The front element is queue[0] which is 2.

🧠 Conceptual

expert

Which data structure property ensures peek front is O(1)?

Why does a queue implemented with a linked list or deque allow peeking the front element in constant time?

A Because the queue uses hashing for quick access

B Because the front element is directly referenced without traversal

C Because the queue elements are stored in contiguous memory

D Because the queue stores elements in sorted order

💡 Hint

Think about how the front element is accessed in these data structures.

In linked lists and deques, the front element is directly accessible via a pointer or index, so peeking does not require traversing the structure, making it O(1).