Deque (double-ended queue) in Data Structures Theory - Mini Project: Build & Apply

Deque (double-ended queue)

📖 Scenario: Imagine you are managing a line of customers waiting for service at a bank. Sometimes customers join the line at the back, and sometimes VIP customers join at the front. Also, customers can leave from either end when served.

🎯 Goal: You will build a simple representation of a deque (double-ended queue) using a list. You will add customers to both ends and remove customers from both ends, simulating how a deque works.

📋 What You'll Learn

Create a list called deque with initial customers 'Alice' and 'Bob'

Create a variable called vip_customer with the value 'Eve'

Add the vip_customer to the front of the deque

Remove the last customer from the deque

💡 Why This Matters

🌍 Real World

Deques are used in real life to manage lines where people can join or leave from either end, like customer service lines or task scheduling.

💼 Career

Understanding deques helps in programming tasks that require flexible data structures, such as managing tasks, undo features, or buffering data streams.

1

Create the initial deque

Create a list called deque with the exact customers 'Alice' and 'Bob' in that order.

Data Structures Theory

# Create the deque list with 'Alice' and 'Bob'
deque = []  # Your code here

Hint

Use square brackets to create a list with two strings: 'Alice' and 'Bob'.

2

Add a VIP customer variable

Create a variable called vip_customer and set it to the string 'Eve'.

Data Structures Theory

deque = ['Alice', 'Bob']
# Create vip_customer variable with value 'Eve'
vip_customer = ''  # Your code here

Hint

Assign the string 'Eve' to the variable vip_customer.

3

Add VIP customer to the front of the deque

Use the insert method on deque to add vip_customer at the front (index 0).

Data Structures Theory

deque = ['Alice', 'Bob']
vip_customer = 'Eve'
# Insert vip_customer at the front of deque
# Your code here

Hint

Use deque.insert(0, vip_customer) to add Eve at the front.

4

Remove the last customer from the deque

Use the pop method on deque without arguments to remove the last customer.

Data Structures Theory

deque = ['Alice', 'Bob']
vip_customer = 'Eve'
deque.insert(0, vip_customer)
# Remove the last customer from deque
# Your code here

Hint

Use deque.pop() to remove the last item from the list.

Practice

1. What is a key feature of a deque compared to a regular queue?

easy

A. It allows adding and removing elements from both ends.

B. It only allows adding elements at the rear.

C. It only allows removing elements from the front.

D. It stores elements in sorted order automatically.

Solution

1. Step 1: Understand queue vs deque

   A regular queue allows adding at the rear and removing from the front only.

2. Step 2: Identify deque's special feature

   A deque allows both adding and removing elements from front and rear ends.

3. Final Answer:

   It allows adding and removing elements from both ends. -> Option A

4. Quick Check:

   Deque = double-ended queue = both ends operations [OK]

Hint: Deque means double-ended: both front and rear operations [OK]

Common Mistakes:

• Thinking deque behaves exactly like a queue
• Assuming deque only adds at rear
• Confusing deque with stack

2. Which of the following is the correct operation to add an element at the front of a deque?

easy

A. push(element)

B. add_front(element)

C. enqueue(element)

D. insert_end(element)

Solution

1. Step 1: Recall deque operations

   Common deque operations include add_front, add_rear, remove_front, remove_rear.

2. Step 2: Match operation to adding at front

   The operation to add at front is named add_front(element).

3. Final Answer:

   add_front(element) -> Option B

4. Quick Check:

   Add front = add_front() method [OK]

Hint: Add front uses add_front(), not enqueue or push [OK]

Common Mistakes:

• Confusing enqueue with add_front
• Using push which is stack operation
• Assuming insert_end adds at front

3. Consider this sequence of operations on an empty deque:
add_rear(10)
add_front(20)
add_rear(30)
remove_front()
What is the element removed by remove_front()?

medium

A. 20

B. 10

C. 30

D. None, deque is empty

Solution

1. Step 1: Track elements after each operation

   Start empty. add_rear(10) -> deque: [10]. add_front(20) -> deque: [20, 10]. add_rear(30) -> deque: [20, 10, 30].

2. Step 2: Identify element removed by remove_front()

   remove_front() removes from front, which is 20.

3. Final Answer:

   20 -> Option A

4. Quick Check:

   Front element removed = 20 [OK]

Hint: Track deque ends step-by-step to find removed element [OK]

Common Mistakes:

• Removing from rear instead of front
• Confusing order after add_front and add_rear
• Assuming remove_front removes last added element

4. A programmer wrote this code snippet for a deque:
deque = []
deque.add_front(5)
deque.add_rear(10)
What is the error in this code?

medium

A. No error, code is correct

B. add_rear should be add_back

C. List object has no method add_front

D. Deque must be imported from collections

Solution

1. Step 1: Check data structure type and methods

   Python list does not have add_front or add_rear methods.

2. Step 2: Identify correct usage for deque

   Deque methods like appendleft() and append() exist in collections.deque, not in list.

3. Final Answer:

   List object has no method add_front -> Option C

4. Quick Check:

   List lacks add_front method error [OK]

Hint: List has no add_front; use collections.deque for deque methods [OK]

Common Mistakes:

• Assuming list supports deque methods
• Thinking add_rear is named add_back
• Ignoring need to import deque class

5. You want to implement an undo feature using a deque. Which deque operation sequence best supports undoing the last action?

hard

A. add_front(action), remove_rear() to undo

B. add_front(action), remove_front() to undo

C. add_rear(action), remove_front() to undo

D. add_rear(action), remove_rear() to undo

Solution

1. Step 1: Understand undo behavior

   Undo removes the most recent action added, so last-in-first-out behavior is needed.

2. Step 2: Match deque operations to LIFO

   Adding actions at rear and removing from rear simulates stack behavior for undo.

3. Final Answer:

   add_rear(action), remove_rear() to undo -> Option D

4. Quick Check:

   Undo = last action removed from rear [OK]

Hint: Undo removes last action added at rear using remove_rear() [OK]

Common Mistakes:

• Removing from front instead of rear for undo
• Adding actions at front but removing from front
• Confusing undo with queue FIFO behavior