Peek Front Element of Queue in DSA Python - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to peek the front element of the queue using list indexing.

DSA Python

queue = [10, 20, 30, 40]
front = queue[1]
print(front)

Drag options to blanks, or click blank then click option'

A[0]

B[-1]

C[1]

D[:1]

Attempts:

3 left

💡 Hint

Common Mistakes

Using negative index which gives the last element instead of the front.

Using slice notation which returns a list, not a single element.

The front element of a queue stored as a list is at index 0.

2fill in blank

medium

Complete the code to peek the front element of the queue using the deque from collections.

DSA Python

from collections import deque
queue = deque([5, 15, 25])
front = queue[1]
print(front)

Drag options to blanks, or click blank then click option'

Apeek()

Bpopleft()

Cpop()

D[0]

Attempts:

3 left

💡 Hint

Common Mistakes

Using popleft() which removes the element instead of peeking.

Using pop() which removes the last element.

Deque supports indexing, so queue[0] gives the front element without removing it.

3fill in blank

hard

Fix the error in the code to peek the front element of the queue without removing it.

DSA Python

queue = [100, 200, 300]
front = queue.[1]()
print(front)

Drag options to blanks, or click blank then click option'

Apop

Bfront

Cpeek

Dpopleft

Attempts:

3 left

💡 Hint

Common Mistakes

Using pop() removes the element, not just peeks.

Using popleft() is not valid for lists.

Python lists do not have a peek() method, so this code will cause an error. The correct way is to use indexing, but since the blank expects a method, peek is the intended fix to illustrate the error.

4fill in blank

hard

Fill both blanks to create a function that returns the front element of a queue without removing it.

DSA Python

def peek_queue(queue):
    if len(queue) [1] 0:
        return queue[2]
    return None

Drag options to blanks, or click blank then click option'

A>

B==

C[0]

D[-1]

Attempts:

3 left

💡 Hint

Common Mistakes

Using '==' instead of '>' causes the function to fail for non-empty queues.

Using index -1 returns the last element, not the front.

The function checks if the queue length is greater than 0, then returns the first element at index 0.

5fill in blank

hard

Fill all three blanks to create a dictionary comprehension that maps each queue's name to its front element if not empty.

DSA Python

queues = {'q1': [1, 2], 'q2': [], 'q3': [3, 4]}
fronts = {name: queue[1] for name, queue in queues.items() if len(queue) [2] 0 and queue[3]

Drag options to blanks, or click blank then click option'

A[0]

B>

C!= []

D== []

Attempts:

3 left

💡 Hint

Common Mistakes

Using '==' instead of '!=' causes empty queues to be included incorrectly.

Using index -1 returns the last element, not the front.

The comprehension gets the first element of each non-empty queue. It checks length > 0 and queue not equal to empty list.

Practice

1. What does the peek operation do in a queue?

easy

A. Returns the front element without removing it

B. Removes the front element from the queue

C. Adds an element to the back of the queue

D. Checks if the queue is empty

Solution

1. Step 1: Understand the peek operation

   The peek operation looks at the front element of the queue without changing the queue.

2. Step 2: Compare with other operations

   Unlike dequeue, peek does not remove the element; it only returns it.

3. Final Answer:

   Returns the front element without removing it -> Option A

4. Quick Check:

   Peek = front element without removal [OK]

Hint: Peek shows front without removing it [OK]

Common Mistakes:

• Confusing peek with dequeue
• Thinking peek adds elements
• Assuming peek checks emptiness

2. Which of the following is the correct syntax to peek the front element of a queue named q in Python using collections.deque?

easy

A. front = q[0]

B. front = q.peek()

C. front = q.front()

D. front = q.get()

Solution

1. Step 1: Recall deque indexing

   In Python's deque, the front element is accessed by index 0, like q[0].

2. Step 2: Check other options

   Methods peek(), front(), and get() do not exist for deque.

3. Final Answer:

   front = q[0] -> Option A

4. Quick Check:

   deque front element = q[0] [OK]

Hint: Use q[0] to peek front in deque [OK]

Common Mistakes:

• Using non-existent peek() method
• Trying q.front() which is invalid
• Using q.get() which is for other structures

3. What will be the output of the following code?

from collections import deque
q = deque([10, 20, 30])
print(q[0])
q.popleft()
print(q[0])

medium

A. 30\n20

B. 10\n10

C. 20\n30

D. 10\n20

Solution

1. Step 1: Peek front before removal

   Initially, q = [10, 20, 30], so q[0] is 10.

2. Step 2: Remove front and peek again

   After q.popleft(), q becomes [20, 30], so q[0] is now 20.

3. Final Answer:

   10\n20 -> Option D

4. Quick Check:

   First peek 10, after removal peek 20 [OK]

Hint: Peek before and after popleft to see front change [OK]

Common Mistakes:

• Assuming front stays same after popleft
• Confusing popleft with pop
• Printing wrong elements

4. Identify the error in the following code that tries to peek the front element of a queue:

from queue import Queue
q = Queue()
q.put(5)
q.put(10)
print(q[0])

medium

A. print statement syntax error

B. Queue is empty, cannot peek

C. Queue object does not support indexing

D. put() method is incorrect

Solution

1. Step 1: Understand Queue class behavior

   Python's queue.Queue does not support direct indexing like q[0].

2. Step 2: Identify correct peek method

   To peek, one must use q.queue[0] or other methods, but direct q[0] causes error.

3. Final Answer:

   Queue object does not support indexing -> Option C

4. Quick Check:

   queue.Queue no indexing [OK]

Hint: queue.Queue objects can't use q[0] indexing [OK]

Common Mistakes:

• Trying to index queue.Queue directly
• Assuming put() is wrong
• Thinking queue is empty

5. You have a queue implemented using a list in Python: q = [5, 10, 15]. You want to peek the front element safely without causing an error if the queue is empty. Which code snippet correctly does this?

hard

A. front = q.pop(0)

B. front = q[0] if len(q) > 0 else None

C. front = q[-1]

D. front = q.get()

Solution

1. Step 1: Check for empty queue before peeking

   Accessing q[0] when q is empty causes error, so check length first.

2. Step 2: Understand other options

   pop(0) removes element, not peek; q[-1] accesses last element; q.get() is invalid for list.

3. Final Answer:

   front = q[0] if len(q) > 0 else None -> Option B

4. Quick Check:

   Safe peek with length check [OK]

Hint: Check length before accessing q[0] to avoid errors [OK]

Common Mistakes:

• Using pop(0) which removes element
• Accessing q[0] without checking empty
• Using q.get() on list