Dequeue Operation in DSA Python - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to add an element to the front of the dequeue.

DSA Python

from collections import deque

queue = deque()
queue.[1](10)
print(list(queue))

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

Aappendleft

Bappend

Cpop

Dpopleft

Attempts:

3 left

💡 Hint

Common Mistakes

Using append instead of appendleft adds element to the end, not front.

Using pop or popleft removes elements instead of adding.

The method appendleft adds an element to the front of the dequeue.

2fill in blank

medium

Complete the code to remove an element from the rear of the dequeue.

DSA Python

from collections import deque

queue = deque([1, 2, 3])
removed = queue.[1]()
print(removed)
print(list(queue))

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

Apopleft

Bpop

Cappend

Dappendleft

Attempts:

3 left

💡 Hint

Common Mistakes

Using popleft removes from the front, not the rear.

Using append or appendleft adds elements instead of removing.

The pop method removes and returns the element from the rear of the dequeue.

3fill in blank

hard

Fix the error in the code to remove an element from the front of the dequeue.

DSA Python

from collections import deque

queue = deque([5, 6, 7])
removed = queue.[1]()
print(removed)
print(list(queue))

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

Apopleft

Bappend

Cappendleft

Dpop

Attempts:

3 left

💡 Hint

Common Mistakes

Using pop removes from the rear, not front.

Using append or appendleft adds elements instead of removing.

popleft removes and returns the element from the front of the dequeue.

4fill in blank

hard

Fill both blanks to add 20 to the rear and remove from the front of the dequeue.

DSA Python

from collections import deque

queue = deque([10, 15])
queue.[1](20)
removed = queue.[2]()
print(removed)
print(list(queue))

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

Aappend

Bpop

Cpopleft

Dappendleft

Attempts:

3 left

💡 Hint

Common Mistakes

Using pop instead of popleft removes from rear, not front.

Using appendleft adds to front, not rear.

append adds to the rear, popleft removes from the front.

5fill in blank

hard

Fill all three blanks to add 5 to front, remove from rear, and then add 10 to rear.

DSA Python

from collections import deque

queue = deque([1, 2, 3])
queue.[1](5)
removed = queue.[2]()
queue.[3](10)
print(removed)
print(list(queue))

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

Aappendleft

Bpop

Cappend

Dpopleft

Attempts:

3 left

💡 Hint

Common Mistakes

Using popleft instead of pop removes from front, not rear.

Using append instead of appendleft adds to rear, not front.

appendleft adds to front, pop removes from rear, append adds to rear.

Practice

1. What is the main feature of a deque (double-ended queue) compared to a regular queue?

easy

A. It only stores unique elements

B. Elements can only be added at the rear

C. Elements can be added or removed from both ends

D. Elements can only be removed from the front

Solution

1. Step 1: Understand queue vs deque

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

2. Step 2: Identify deque feature

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

3. Final Answer:

   Elements can be added or removed from both ends -> Option C

4. Quick Check:

   Deque = double-ended queue [OK]

Hint: Deque means double-ended, so both ends work [OK]

Common Mistakes:

• Thinking deque works like a normal queue
• Assuming elements can only be added at one end
• Confusing deque with stack

2. Which of the following is the correct syntax to add an element '5' to the front of a deque named d in Python?

easy

A. d.push_front(5)

B. d.addfront(5)

C. d.insert_front(5)

D. d.appendleft(5)

Solution

1. Step 1: Recall deque methods in Python

   Python's collections.deque uses appendleft() to add at front.

2. Step 2: Check method names

   Other options are not valid deque methods in Python.

3. Final Answer:

   d.appendleft(5) -> Option D

4. Quick Check:

   appendleft adds to front [OK]

Hint: Use appendleft() to add front in Python deque [OK]

Common Mistakes:

• Using non-existent methods like push_front
• Confusing append() with appendleft()
• Trying to use list methods on deque

3. What will be the output of the following code?

from collections import deque
d = deque([1, 2, 3])
d.appendleft(0)
d.pop()
print(list(d))

medium

A. [0, 1, 2]

B. [1, 2, 3, 0]

C. [0, 1, 2, 3]

D. [1, 2]

Solution

1. Step 1: Initial deque and appendleft

   Start with deque [1, 2, 3]. appendleft(0) adds 0 at front: [0, 1, 2, 3].

2. Step 2: pop removes from rear

   pop() removes last element 3: [0, 1, 2].

3. Final Answer:

   [0, 1, 2] -> Option A

4. Quick Check:

   appendleft + pop = [0,1,2] [OK]

Hint: appendleft adds front, pop removes rear [OK]

Common Mistakes:

• Thinking pop removes from front
• Ignoring appendleft effect
• Confusing list(d) output

4. Identify the error in the following code snippet:

from collections import deque
d = deque([10, 20, 30])
d.removeleft()
print(d)

medium

A. removeleft() is not a valid deque method

B. deque cannot be initialized with a list

C. print(d) should be print(list(d))

D. No error, code runs fine

Solution

1. Step 1: Check deque methods

   deque has no method named removeleft(); correct method to remove front is popleft().

2. Step 2: Validate other parts

   deque can be initialized with list; print(d) works; so no other errors.

3. Final Answer:

   removeleft() is not a valid deque method -> Option A

4. Quick Check:

   Use popleft() to remove front [OK]

Hint: Use popleft(), not removeleft() [OK]

Common Mistakes:

• Using non-existent removeleft() method
• Thinking print(d) causes error
• Confusing initialization syntax

5. You have a deque d = deque([1, 2, 3, 4]). You perform these operations:

d.pop()
d.appendleft(0)
d.popleft()
d.append(5)
print(list(d))

What is the final state of the deque?

hard

A. [0, 1, 2, 3, 5]

B. [1, 2, 3, 5]

C. [2, 3, 4, 5]

D. [0, 2, 3, 5]

Solution

1. Step 1: Trace each operation on deque

   Start: [1, 2, 3, 4]
   d.pop() removes 4: [1, 2, 3]
   d.appendleft(0): [0, 1, 2, 3]
   d.popleft() removes 0: [1, 2, 3]
   d.append(5): [1, 2, 3, 5]

2. Step 2: Confirm final deque state

   After all operations, deque is [1, 2, 3, 5].

3. Final Answer:

   [1, 2, 3, 5] -> Option B

4. Quick Check:

   pop + appendleft + popleft + append = [1,2,3,5] [OK]

Hint: Apply operations step-by-step on deque [OK]

Common Mistakes:

• Forgetting popleft removes front
• Mixing order of operations
• Assuming appendleft stays after popleft