Queues (first-in, first-out) in Intro to Computing - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ trace

intermediate

Trace the queue operations

Consider a queue where the following operations happen in order:

1. Enqueue 5
2. Enqueue 10
3. Dequeue
4. Enqueue 15
5. Dequeue

What is the content of the queue after these operations?

A [5, 15]

B [5, 10, 15]

C [15]

D [10, 15]

💡 Hint

Remember, a queue removes items in the order they were added (first-in, first-out).

Initially, the queue is empty. After enqueue 5 and 10, queue is [5, 10]. Dequeue removes 5, queue becomes [10]. Enqueue 15 adds 15 at the end, queue is [10, 15]. Dequeue removes 10, queue becomes [15].

🧠 Conceptual

intermediate

Understanding queue behavior

Which real-life example best represents how a queue works?

A A line of people waiting to buy tickets where the first person in line is served first

B A stack of plates where you take the top plate first

C A bookshelf where you pick any book you want

D A pile of clothes where you pick the last folded item first

💡 Hint

Think about who gets served first in each example.

A queue follows first-in, first-out. The line of people waiting to buy tickets is a perfect example because the first person to get in line is the first to be served.

❓ identification

advanced

Identify the queue operation causing error

Given a queue with elements [1, 2, 3], which operation will cause an error if performed next?

A Dequeue (remove the first element)

B Enqueue 4 (add element 4 at the end)

C Dequeue four times in a row

D Peek (view the first element without removing)

💡 Hint

Think about what happens if you remove more elements than the queue contains.

Options A, B, and C are valid on a queue with elements [1, 2, 3]. Dequeue four times in a row removes 1, 2, 3, then attempts to dequeue from an empty queue, causing an error.

❓ Comparison

advanced

Compare queue and stack behavior

Which statement correctly compares a queue and a stack?

A Both queue and stack remove elements in the order they were added

B Queue removes elements in first-in, first-out order; stack removes elements in last-in, first-out order

C Stack removes elements in first-in, first-out order; queue removes elements in last-in, first-out order

D Queue and stack both remove elements randomly

💡 Hint

Think about which element is removed first in each data structure.

A queue removes the oldest element first (FIFO), while a stack removes the most recently added element first (LIFO).

🚀 Application

expert

Determine the final queue content after mixed operations

A queue starts empty. The following operations happen:

1. Enqueue 'A'
2. Enqueue 'B'
3. Dequeue
4. Enqueue 'C'
5. Enqueue 'D'
6. Dequeue
7. Enqueue 'E'
8. Dequeue

What is the content of the queue after all these operations?

A ['D', 'E']

B ['C', 'D', 'E']

C ['B', 'C', 'D', 'E']

D ['E']

💡 Hint

Track each enqueue and dequeue step carefully, remembering FIFO order.

Step by step:
1. Queue: ['A']
2. Queue: ['A', 'B']
3. Dequeue removes 'A', Queue: ['B']
4. Enqueue 'C', Queue: ['B', 'C']
5. Enqueue 'D', Queue: ['B', 'C', 'D']
6. Dequeue removes 'B', Queue: ['C', 'D']
7. Enqueue 'E', Queue: ['C', 'D', 'E']
8. Dequeue removes 'C', Queue: ['D', 'E']

Practice

1.

What does a queue data structure follow?

Choose the best description.

easy

A. First in, first out (FIFO)

B. Last in, first out (LIFO)

C. Random order

D. Sorted order

Solution

1. Step 1: Understand queue behavior

   A queue works like a line where the first person to join is the first to leave.

2. Step 2: Match behavior to options

   This matches the FIFO principle, meaning first in, first out.

3. Final Answer:

   First in, first out (FIFO) -> Option A

4. Quick Check:

   Queue = FIFO [OK]

Hint: Queues always remove the oldest item first [OK]

Common Mistakes:

• Confusing queue with stack (LIFO)
• Thinking queue removes newest item first
• Assuming queue sorts items automatically

2.

Which of the following is the correct way to enqueue an item 5 into a queue represented as a list named q in Python?

easy

A. q.append(5)

B. q.pop(0)

C. q.insert(0, 5)

D. q.remove(5)

Solution

1. Step 1: Understand enqueue operation

   Enqueue means adding an item to the back of the queue.

2. Step 2: Identify correct list method

   In Python, append() adds an item to the end of the list, which matches enqueue.

3. Final Answer:

   q.append(5) -> Option A

4. Quick Check:

   Enqueue = append at end [OK]

Hint: Use append() to add at queue's end [OK]

Common Mistakes:

• Using pop(0) which removes front item
• Using insert(0, 5) which adds at front
• Using remove(5) which deletes item by value

3.

Given the queue q = [10, 20, 30], what will be the queue after performing q.pop(0)?

medium

A. [10, 20, 30]

B. [10, 20]

C. [20, 30]

D. [30]

Solution

1. Step 1: Understand pop(0) on list

   pop(0) removes the first item from the list, which is 10 here.

2. Step 2: Remove first item and check remaining

   Removing 10 leaves [20, 30] in the queue.

3. Final Answer:

   [20, 30] -> Option C

4. Quick Check:

   pop(0) removes front item [OK]

Hint: pop(0) removes front item in queue [OK]

Common Mistakes:

• Removing last item instead of first
• Confusing pop() with append()
• Expecting queue to remain unchanged

4.

Consider this Python code snippet:

q = []
q.pop(0)

What error will occur and why?

medium

A. TypeError because pop needs an argument

B. IndexError because popping from empty queue

C. ValueError because 0 is not in the list

D. No error, queue becomes empty

Solution

1. Step 1: Analyze pop(0) on empty list

   pop(0) tries to remove the first item, but the list is empty.

2. Step 2: Identify error type

   Removing from empty list causes an IndexError in Python.

3. Final Answer:

   IndexError because popping from empty queue -> Option B

4. Quick Check:

   pop empty list = IndexError [OK]

Hint: Cannot pop from empty queue [OK]

Common Mistakes:

• Thinking pop(0) needs no argument error
• Confusing ValueError with IndexError
• Assuming no error occurs

5.

You have a queue q = [1, 2, 3, 4]. You perform these operations:

q.append(5)
q.pop(0)
q.append(6)
q.pop(0)

What is the final state of the queue?

hard

A. [4, 5, 6]

B. [2, 3, 4, 5, 6]

C. [1, 2, 3, 4, 5, 6]

D. [3, 4, 5, 6]

Solution

1. Step 1: Perform first append and pop

   Start: [1, 2, 3, 4]
   append(5) -> [1, 2, 3, 4, 5]
   pop(0) removes 1 -> [2, 3, 4, 5]

2. Step 2: Perform second append and pop

   append(6) -> [2, 3, 4, 5, 6]
   pop(0) removes 2 -> [3, 4, 5, 6]

3. Final Answer:

   [3, 4, 5, 6] -> Option D

4. Quick Check:

   Queue after operations = [3, 4, 5, 6] [OK]

Hint: Track each enqueue and dequeue step-by-step [OK]

Common Mistakes:

• Forgetting to remove front item after pop
• Mixing order of operations
• Assuming append adds to front