Queue operations (enqueue, dequeue) in Data Structures Theory - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

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

Data Structures Theory

queue.[1](element)

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

Adequeue

Bappend

Cenqueue

Dpop

Attempts:

3 left

💡 Hint

Common Mistakes

Using dequeue or pop which remove elements instead of adding.

In many programming languages, adding an element to the end of a list or queue is done using append. This simulates the enqueue operation.

2fill in blank

medium

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

Data Structures Theory

removed = queue.[1](0)

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

Aappend

Benqueue

Cdequeue

Dpop

Attempts:

3 left

💡 Hint

Common Mistakes

Using append which adds elements instead of removing.

Using pop(0) removes the first element from the list, simulating the dequeue operation in a queue.

3fill in blank

hard

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

Data Structures Theory

removed = queue.[1](0)

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

Aenqueue

Bdequeue

Cpop

Dappend

Attempts:

3 left

💡 Hint

Common Mistakes

Using enqueue or append which add elements instead of removing.

Using pop(0) removes the first element from the list, simulating the dequeue operation in a queue.

4fill in blank

hard

Fill both blanks to create a dictionary comprehension that maps each element to its position in the queue.

Data Structures Theory

{element: index for index, element in enumerate(queue) if index [1] [2]

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A<

B5

C>

D3

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong comparison operators or numbers that don't filter correctly.

This comprehension creates a dictionary of elements with their indexes only if the index is less than 5.

5fill in blank

hard

Fill all three blanks to create a dictionary comprehension that maps uppercase elements to their indexes if the index is greater than 2.

Data Structures Theory

{ [1]: [2] for [3], element in enumerate(queue) if [3] > 2 }

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

Aelement.upper()

Belement

Cindex

Di

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up element and index variables or forgetting to convert to uppercase.

The comprehension maps the uppercase version of each element to its index, filtering only elements with index greater than 2.

Practice

1. What does the enqueue operation do in a queue?

easy

A. Adds an item to the rear of the queue

B. Removes an item from the front of the queue

C. Checks if the queue is empty

D. Returns the size of the queue

Solution

1. Step 1: Understand enqueue operation

   Enqueue means to add an element to the queue.

2. Step 2: Identify where enqueue adds

   In a queue, new items are added at the rear (end).

3. Final Answer:

   Adds an item to the rear of the queue -> Option A

4. Quick Check:

   Enqueue = Add rear [OK]

Hint: Enqueue always adds at the back of the queue [OK]

Common Mistakes:

• Confusing enqueue with dequeue
• Thinking enqueue removes items
• Assuming enqueue adds at the front

2. Which of the following is the correct syntax to remove an element from a queue in most programming languages?

easy

A. queue.dequeue()

B. queue.enqueue()

C. queue.pop()

D. queue.push()

Solution

1. Step 1: Identify the operation to remove from queue

   Removing an element from a queue is called dequeue.

2. Step 2: Match correct method name

   The standard method to remove is dequeue(), not pop() or push().

3. Final Answer:

   queue.dequeue() -> Option A

4. Quick Check:

   Remove from front = dequeue() [OK]

Hint: Dequeue removes from front, enqueue adds at rear [OK]

Common Mistakes:

• Using pop() which is for stacks
• Confusing enqueue and dequeue methods
• Using push() which adds to stack or queue

3. Consider a queue initially empty. After these operations:
enqueue(5), enqueue(10), dequeue(), enqueue(15), dequeue()
What is the next element to be dequeued?

medium

A. 5

B. 10

C. Queue is empty

D. 15

Solution

1. Step 1: Track queue after each operation

   Start empty: []
   enqueue(5): [5]
   enqueue(10): [5, 10]
   dequeue(): removes 5, queue is [10]
   enqueue(15): [10, 15]
   dequeue(): removes 10, queue is [15]

2. Step 2: Identify next element to dequeue

   Next element at front is 15.

3. Final Answer:

   15 -> Option D

4. Quick Check:

   Remaining front element = 15 [OK]

Hint: Follow FIFO order step-by-step [OK]

Common Mistakes:

• Forgetting to remove elements on dequeue
• Mixing order of enqueue and dequeue
• Assuming queue empties after two dequeues

4. What is wrong with this queue operation sequence?

queue = []
queue.dequeue()

medium

A. Nothing is wrong; it works fine

B. Dequeue on empty queue causes error

C. Queue should be a stack for dequeue

D. Enqueue is missing before dequeue

Solution

1. Step 1: Understand initial queue state

   The queue is empty: [].

2. Step 2: Analyze dequeue on empty queue

   Trying to dequeue when empty causes underflow or error because no element exists to remove.

3. Final Answer:

   Dequeue on empty queue causes error -> Option B

4. Quick Check:

   Empty dequeue = error [OK]

Hint: Never dequeue from an empty queue [OK]

Common Mistakes:

• Assuming dequeue returns null or zero
• Confusing queue with stack behavior
• Ignoring empty state checks

5. You have a queue with elements [2, 4, 6, 8]. You perform these operations:
dequeue(), enqueue(10), dequeue(), enqueue(12), dequeue()
What is the state of the queue after these operations?

hard

A. [4, 6, 8, 10, 12]

B. [6, 8, 10, 12]

C. [8, 10, 12]

D. [10, 12]

Solution

1. Step 1: Track queue after each operation

   Start: [2, 4, 6, 8]
   dequeue(): removes 2 -> [4, 6, 8]
   enqueue(10): add 10 -> [4, 6, 8, 10]
   dequeue(): removes 4 -> [6, 8, 10]
   enqueue(12): add 12 -> [6, 8, 10, 12]
   dequeue(): removes 6 -> [8, 10, 12]

2. Step 2: Confirm final queue state

   The queue now contains [8, 10, 12].

3. Final Answer:

   [8, 10, 12] -> Option C

4. Quick Check:

   Final queue after operations = [8, 10, 12] [OK]

Hint: Remove front, add rear stepwise [OK]

Common Mistakes:

• Not removing correct elements on dequeue
• Adding new elements at front instead of rear
• Losing track of queue order