Queues (first-in, first-out) in Intro to Computing - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to add an item to the end of the queue.

Intro to Computing

queue = []
queue.[1]('apple')

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

Apop

Bappend

Cremove

Dinsert

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'pop' removes an item instead of adding.

Using 'remove' deletes a specific item, not add.

Using 'insert' needs an index and is more complex.

To add an item to the end of a list (queue), we use append. This keeps the first-in, first-out order.

2fill in blank

medium

Complete the code to remove the first item from the queue.

Intro to Computing

queue = ['apple', 'banana', 'cherry']
first_item = queue.[1](0)

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

Apop

Bappend

Cremove

Dinsert

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'append' adds items instead of removing.

Using 'remove' needs the item value, not index.

Using 'insert' adds items, not removes.

To remove the first item from the queue, use pop(0). This removes and returns the item at index 0, following FIFO.

3fill in blank

hard

Fix the error in the code to correctly remove the first item from the queue.

Intro to Computing

queue = ['dog', 'cat', 'bird']
removed = queue.[1]

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

Adelete(0)

Bpop

Cremove(0)

Dpop(0)

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'pop()' removes the last item, not the first.

Using 'remove(0)' causes an error because remove expects a value.

Using 'delete(0)' is not a valid list method.

The pop method needs the index to remove the first item. pop(0) removes the first element. pop() without arguments removes the last item.

4fill in blank

hard

Fill both blanks to create a queue comprehension that keeps only items longer than 3 characters.

Intro to Computing

queue = ['cat', 'lion', 'dog', 'tiger']
filtered = [item for item in queue if len(item) [1] [2]]

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

A>

B3

C<

D==

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<' keeps shorter items, not longer.

Using '==' keeps only items exactly length 3.

Swapping the number and operator causes wrong filtering.

The comprehension keeps items where the length is greater than 3, so use len(item) > 3.

5fill in blank

hard

Fill all three blanks to create a dictionary showing each item and its position in the queue.

Intro to Computing

queue = ['red', 'blue', 'green']
positions = [1]: [2] for [3], color in enumerate(queue)}

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

A{color

Bindex

Dcolor

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping key and value reverses the dictionary.

Using wrong variable names causes errors.

Missing braces or colons breaks syntax.

The dictionary comprehension uses {color: index for index, color in enumerate(queue)} to map each color to its position.

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