Double Ended Queue Deque in DSA Python - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create an empty deque using Python's collections module.

DSA Python

from collections import deque

my_deque = deque([1])
print(my_deque)

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

ANone

B[]

C{}

D()

Attempts:

3 left

💡 Hint

Common Mistakes

Passing None which causes error

To create an empty deque, you pass an empty list [] to deque().

2fill in blank

medium

Complete the code to add an element 10 to the right end of the deque.

DSA Python

from collections import deque

my_deque = deque([])
my_deque.[1](10)
print(my_deque)

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

Aappendleft

Bpop

Cpopleft

Dappend

Attempts:

3 left

💡 Hint

Common Mistakes

Using pop or popleft which remove elements

Using appendleft which adds to the left end

Use append to add an element to the right end of the deque.

3fill in blank

hard

Fix the error in the code to remove an element from the left end of the deque.

DSA Python

from collections import deque

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

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

Apopleft

Bremove

Cpop

Ddelete

Attempts:

3 left

💡 Hint

Common Mistakes

Using pop which removes from right end

Using remove which removes by value, not position

Using delete which is not a deque method

Use popleft() to remove and return the element from the left end of the deque.

4fill in blank

hard

Fill both blanks to rotate the deque one step to the right.

DSA Python

from collections import deque

my_deque = deque([1, 2, 3, 4])
my_deque.[1]([2])
print(my_deque)

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

Arotate

Bappend

C1

D-1

Attempts:

3 left

💡 Hint

Common Mistakes

Using append instead of rotate

Using negative number to rotate right

Use rotate(1) to move all elements one step to the right.

5fill in blank

hard

Fill all three blanks to create a deque, add 5 to the left, and remove from the right.

DSA Python

from collections import deque

my_deque = deque([1])
my_deque.[2](5)
removed = my_deque.[3]()
print(removed)
print(my_deque)

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

A[]

Bappendleft

Cpop

D[1, 2, 3]

Attempts:

3 left

💡 Hint

Common Mistakes

Starting with empty deque instead of list

Using append instead of appendleft

Using popleft instead of pop

Start with deque([1, 2, 3]), add 5 to the left with appendleft, then remove from right with pop.

Practice

1. What is a deque in Python's collections module?

easy

A. A list that automatically sorts elements when inserted

B. A queue that only allows insertion at the rear and removal from the front

C. A stack that only allows insertion and removal from the top

D. A double-ended queue allowing insertion and removal from both ends

Solution

1. Step 1: Understand the deque concept

   A deque is a data structure that supports adding and removing elements from both the front and the back efficiently.

2. Step 2: Compare with other data structures

   Unlike a queue or stack, deque allows operations on both ends, not just one end.

3. Final Answer:

   A double-ended queue allowing insertion and removal from both ends -> Option D

4. Quick Check:

   Deque = double-ended queue [OK]

Hint: Remember: deque means double-ended queue [OK]

Common Mistakes:

• Confusing deque with stack or queue
• Thinking deque only works like a queue
• Assuming deque sorts elements automatically

2. Which of the following is the correct way to import and create an empty deque in Python?

easy

A. import collections\nd = collections.deque[]

B. from collections import deque\nd = deque()

C. from collections import Deque\nd = Deque()

D. import deque\nd = deque()

Solution

1. Step 1: Check the correct import statement

   The deque class is in the collections module and must be imported as from collections import deque.

2. Step 2: Verify deque creation syntax

   Creating an empty deque is done by calling deque() with parentheses, not brackets.

3. Final Answer:

   from collections import deque\nd = deque() -> Option B

4. Quick Check:

   Correct import and empty deque creation [OK]

Hint: Use 'from collections import deque' and call deque() [OK]

Common Mistakes:

• Using uppercase 'Deque' instead of 'deque'
• Trying to import deque directly without collections
• Using square brackets [] instead of parentheses ()

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

from collections import deque
d = deque([1, 2, 3])
d.appendleft(0)
d.append(4)
print(list(d))

medium

A. [4, 1, 2, 3, 0]

B. [1, 2, 3, 0, 4]

C. [0, 1, 2, 3, 4]

D. [1, 2, 3, 4, 0]

Solution

1. Step 1: Trace deque initialization and appendleft

   Deque starts as [1, 2, 3]. After appendleft(0), it becomes [0, 1, 2, 3].

2. Step 2: Trace append operation

   After append(4), 4 is added to the right end, making deque [0, 1, 2, 3, 4].

3. Final Answer:

   [0, 1, 2, 3, 4] -> Option C

4. Quick Check:

   appendleft adds front, append adds end [OK]

Hint: appendleft adds front, append adds end [OK]

Common Mistakes:

• Confusing appendleft with append
• Assuming append adds to front
• Not converting deque to list before printing

4. Find the error in the following code snippet:

from collections import deque
d = deque()
d.appendleft(1)
d.append(2)
d.pop()
d.pop()
d.pop()

medium

A. Calling pop() on an empty deque causes an IndexError

B. appendleft() is not a valid method for deque

C. deque() cannot be initialized empty

D. append() requires two arguments

Solution

1. Step 1: Analyze deque operations

   Deque starts empty, then 1 is added to front, 2 to end. After two pops, deque is empty.

2. Step 2: Identify error on third pop

   The third pop tries to remove from empty deque, causing IndexError.

3. Final Answer:

   Calling pop() on an empty deque causes an IndexError -> Option A

4. Quick Check:

   pop on empty deque throws error [OK]

Hint: pop() on empty deque causes error [OK]

Common Mistakes:

• Thinking appendleft() is invalid
• Assuming deque can't be empty
• Believing append() needs two arguments

5. You have a deque initialized as d = deque([10, 20, 30, 40]). You want to rotate it so that the element 30 becomes the first element. Which code snippet achieves this correctly?

hard

A. d.rotate(-2)

B. d.rotate(2)

C. d.rotate(-3)

D. d.rotate(3)

Solution

1. Step 1: Understand deque rotation

   Positive rotate moves elements to the right; negative rotate moves to the left.

2. Step 2: Calculate rotation to bring 30 to front

   30 is at index 2. Rotating left by 2 (rotate(-2)) moves 30 to front.

3. Final Answer:

   d.rotate(-2) -> Option A

4. Quick Check:

   Negative rotate shifts left, bringing 30 front [OK]

Hint: Negative rotate shifts left; positive shifts right [OK]

Common Mistakes:

• Using positive rotate instead of negative
• Confusing rotation direction
• Not counting index positions correctly