Queues (first-in, first-out) in Intro to Computing - Draw & Build Visually

Draw This - beginner

Draw a flowchart that shows how a queue works with the following steps: 1. Add three people named Alice, Bob, and Carol to the queue in that order. 2. Remove one person from the front of the queue. 3. Add one more person named Dave to the queue. 4. Remove two people from the front of the queue. Show the queue contents after each step.

10 minutes

Hint 1

Hint 2

Hint 3

Hint 4

Grading Criteria

Start and End symbols present

Actions for adding and removing people shown as rectangles

Arrows correctly show the flow from one step to the next

Queue contents shown after each add or remove action

Queue updates correctly reflect first-in, first-out order

Solution

Initialize empty queue

↓

Add Alice to queue

↓

Add Bob to queue

↓

Add Carol to queue

↓

Remove one person from front

↓

Add Dave to queue

↓

Remove one person from front

↓

Remove one person from front

This flowchart starts with an empty queue.

Step 1: We add Alice, Bob, and Carol one by one. The queue grows as Alice, Bob, Carol.

Step 2: We remove one person from the front. Since Alice was first, she leaves. Queue is now Bob, Carol.

Step 3: We add Dave to the back. Queue is Bob, Carol, Dave.

Step 4: We remove two people from the front. Bob leaves first, then Carol. Queue is now Dave.

This shows the first-in, first-out behavior of queues clearly.

Variations - 2 Challenges

[intermediate] Draw a flowchart for a queue where you add four people (Anna, Ben, Cara, Dan) and then remove three people one by one, showing the queue after each step.

[advanced] Draw a flowchart for a queue that starts empty, adds two people, removes one, adds two more, then removes all remaining people one by one. Show the queue contents after each step.

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