Deque (double-ended queue) in Data Structures Theory - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the sentence to describe a deque.

Data Structures Theory

A deque allows insertion and removal of elements from both the [1] and the [2].

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

Aback

Bmiddle

Cfront

Drandom

Attempts:

3 left

💡 Hint

Common Mistakes

Thinking elements can be added or removed from the middle.

Confusing deque with a stack or queue that only works on one end.

A deque (double-ended queue) supports adding and removing elements from both the front and the back ends.

2fill in blank

medium

Complete the sentence to explain deque operations.

Data Structures Theory

The operation to add an element at the back of a deque is called [1].

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

Apush_back

Bdequeue

Cenqueue

Dpop_front

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'pop_front' which means removing from front.

Confusing enqueue and dequeue which are queue operations.

In a deque, adding an element at the back is called push_back.

3fill in blank

hard

Fix the error in the statement about deque removal.

Data Structures Theory

Removing an element from the front of a deque is done using [1].

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

Apush_back

Bpush_front

Cpop_front

Dpop_back

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'push_front' which adds instead of removes.

Confusing 'pop_back' which removes from the back.

To remove an element from the front of a deque, the correct operation is pop_front.

4fill in blank

hard

Fill both blanks to describe deque usage.

Data Structures Theory

A deque can be used as a [1] or a [2] because it supports insertion and removal from both ends.

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

Astack

Bqueue

Clist

Dtree

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing 'list' or 'tree' which are different data structures.

Not realizing deque can behave like both stack and queue.

A deque can act like a stack (LIFO) or a queue (FIFO) because it allows adding and removing elements from both ends.

5fill in blank

hard

Fill all three blanks to complete the Python deque example.

Data Structures Theory

from collections import [1]
d = [1]()
d.[2](5)  # add to back
d.pop[3]()  # remove from front

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

Adeque

Bappend

Cleft

Dlist

Attempts:

3 left

💡 Hint

Common Mistakes

Using list instead of deque, which is inefficient for front operations.

Using appendleft for back or pop for front.

Forgetting the specific method names in Python's deque.

In Python, deque from collections is used. append(5) adds to the back, and popleft() removes from the front.

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