Queues (first-in, first-out) in Intro to Computing - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a queue in computing?

A queue is a way to organize items where the first item added is the first one to be removed. This is called first-in, first-out (FIFO). Imagine a line at a store checkout where the first person to get in line is the first to be served.

beginner

Explain the term FIFO in simple words.

FIFO means First-In, First-Out. It means the first thing that goes in is the first thing that comes out, like a queue of people waiting for a bus.

beginner

What are the two main operations of a queue?

The two main operations are:
1. Enqueue: Adding an item to the back of the queue.
2. Dequeue: Removing the item from the front of the queue.

intermediate

Why is a queue different from a stack?

A queue removes items in the order they were added (FIFO), like a line of people. A stack removes the last item added first (LIFO), like a stack of plates where you take the top plate first.

beginner

Give a real-life example of a queue.

A real-life example is waiting in line at a coffee shop. The first person to get in line is the first to get served, and new people join at the end of the line.

What does FIFO stand for in queues?

A Fast Input, Fast Output

B First-In, Final-Out

C First-In, First-Out

D Fast-In, Fast-Out

FIFO means the first item added is the first item removed, like a queue of people.

Which operation adds an item to the back of a queue?

A Enqueue

B Push

C Dequeue

D Pop

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

In a queue, which item is removed first?

A The last item added

B A random item

C The middle item

D The first item added

Queues follow FIFO, so the first item added is removed first.

Which data structure removes the last item added first?

A Stack

B Linked List

C Array

D Queue

A stack uses LIFO (Last-In, First-Out), removing the last item added first.

What real-life situation is like a queue?

A Stack of books

B Line at a ticket counter

C Pile of clothes

D Randomly picking cards

A line at a ticket counter is a queue where the first person in line is served first.

Describe how a queue works using a real-life example.

Think about how people wait in line for something.

Explain the difference between a queue and a stack.

Compare a line of people to a stack of plates.

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