Dequeue Operation in DSA C# - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

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

DSA C#

queue.[1]();

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

AEnqueue

BClear

CPeek

DDequeue

Attempts:

3 left

💡 Hint

Common Mistakes

Using Enqueue instead of Dequeue

Using Peek which only views the first element without removing it

The Dequeue method removes and returns the first element of the queue.

2fill in blank

medium

Complete the code to check the first element without removing it.

DSA C#

var first = queue.[1]();

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

APeek

BDequeue

CContains

DClear

Attempts:

3 left

💡 Hint

Common Mistakes

Using Dequeue which removes the element

Using Contains which checks for existence but does not return the element

The Peek method returns the first element without removing it from the queue.

3fill in blank

hard

Fix the error in the code to remove an element from the queue.

DSA C#

queue.[1]();

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

ADequeue

BRemove

CPop

DDelete

Attempts:

3 left

💡 Hint

Common Mistakes

Using Pop which is for stacks

Using Remove or Delete which are not valid queue methods

The correct method to remove the first element from a queue in C# is Dequeue. Methods like Pop, Remove, or Delete do not exist for the Queue class.

4fill in blank

hard

Complete the code to safely remove the first element only if the queue is not empty.

DSA C#

if (queue.[1] > 0) { queue.[2](); }

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

ACount

BDequeue

CPeek

DEnqueue

Attempts:

3 left

💡 Hint

Common Mistakes

Using Peek instead of Dequeue (Peek doesn't remove)

Forgetting to check Count and causing exceptions on empty queue

Check queue.Count > 0 before calling Dequeue() to avoid InvalidOperationException when the queue is empty.

5fill in blank

hard

Complete the code to empty the queue by repeatedly removing elements from the front.

DSA C#

while (queue.[1] > 0) { queue.[2](); }

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

ACount

BDequeue

CPeek

DClear

Attempts:

3 left

💡 Hint

Common Mistakes

Using Clear() which empties directly without dequeuing

Using Peek() which doesn't remove elements

Loop with queue.Count > 0 and call Dequeue() to remove elements one by one until the queue is empty.

Practice

1. What does the Dequeue operation do in a queue data structure?

easy

A. Returns the number of elements in the queue

B. Adds an element to the end of the queue

C. Checks if the queue is empty

D. Removes and returns the element at the front of the queue

Solution

1. Step 1: Understand the queue structure

   A queue works like a line where the first person in line is served first (FIFO).

2. Step 2: Define the Dequeue operation

   Dequeue removes and returns the first element in the queue, following FIFO.

3. Final Answer:

   Removes and returns the element at the front of the queue -> Option D

4. Quick Check:

   Dequeue = Remove front element [OK]

Hint: Dequeue always removes from the front [OK]

Common Mistakes:

• Confusing Dequeue with Enqueue (adding element)
• Thinking Dequeue checks queue size
• Assuming Dequeue removes from the end

2. Which of the following is the correct syntax to remove an element from a Queue<int> named q in C#?

easy

A. q.Remove()

B. q.Pop()

C. q.Dequeue()

D. q.Delete()

Solution

1. Step 1: Recall C# Queue methods

   The method to remove and return the front element in C# Queue is Dequeue().

2. Step 2: Check each option

   Only q.Dequeue() matches the correct method name and syntax.

3. Final Answer:

   q.Dequeue() -> Option C

4. Quick Check:

   Queue removal method = Dequeue() [OK]

Hint: Use Dequeue() to remove front element in C# Queue [OK]

Common Mistakes:

• Using Remove() which does not exist for Queue
• Using Pop() which is for stacks
• Using Delete() which is invalid

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

Queue<string> q = new Queue<string>();
q.Enqueue("apple");
q.Enqueue("banana");
q.Enqueue("cherry");
string first = q.Dequeue();
Console.WriteLine(first);
Console.WriteLine(q.Peek());

medium

A. apple\nbanana

B. banana\ncherry

C. apple\ncherry

D. cherry\napple

Solution

1. Step 1: Trace Enqueue operations

   Queue after enqueues: front -> apple, banana, cherry -> rear.

2. Step 2: Dequeue removes 'apple'

   Dequeue returns 'apple' and removes it from queue.

3. Step 3: Peek returns new front 'banana'

   After dequeue, front is 'banana', so Peek returns 'banana'.

4. Final Answer:

   apple banana -> Option A

5. Quick Check:

   Dequeue = apple, Peek = banana [OK]

Hint: Dequeue returns front; Peek shows new front [OK]

Common Mistakes:

• Confusing Peek with Dequeue
• Assuming Dequeue removes from rear
• Mixing order of elements

4. Identify the error in the following C# code that uses a queue:

Queue<int> q = new Queue<int>();
q.Enqueue(10);
q.Enqueue(20);
int x = q.Dequeue();
int y = q.Dequeue();
int z = q.Dequeue(); // Error here
Console.WriteLine(z);

medium

A. Queue does not support Dequeue method

B. Dequeue called on empty queue causes InvalidOperationException

C. Enqueue method is used incorrectly

D. Console.WriteLine cannot print integers

Solution

1. Step 1: Count elements and Dequeue calls

   Queue has 2 elements; 3 Dequeue calls are made.

2. Step 2: Third Dequeue on empty queue causes error

   Calling Dequeue on empty queue throws InvalidOperationException.

3. Final Answer:

   Dequeue called on empty queue causes InvalidOperationException -> Option B

4. Quick Check:

   Empty queue Dequeue = Exception [OK]

Hint: Dequeue on empty queue throws exception [OK]

Common Mistakes:

• Assuming Dequeue returns default value on empty
• Thinking Enqueue is incorrect
• Believing Console.WriteLine can't print int

5. You have a queue with elements [5, 10, 15, 20] (front to rear). You perform these operations:
1. Dequeue
2. Enqueue(25)
3. Dequeue
4. Enqueue(30)
What is the final state of the queue (front to rear)?

hard

A. [15, 20, 25, 30]

B. [10, 15, 20, 25]

C. [20, 25, 30]

D. [5, 10, 15, 20, 25, 30]

Solution

1. Step 1: Initial queue and first Dequeue

   Queue: 5(front),10,15,20(rear). Dequeue removes 5. Queue now: 10,15,20.

2. Step 2: Enqueue 25

   Add 25 at rear. Queue: 10,15,20,25.

3. Step 3: Second Dequeue

   Remove front 10. Queue: 15,20,25.

4. Step 4: Enqueue 30

   Add 30 at rear. Queue: 15,20,25,30.

5. Final Answer:

   [15, 20, 25, 30] -> Option A

6. Quick Check:

   After ops queue front to rear = [15,20,25,30] [OK]

Hint: Track front and rear after each operation [OK]

Common Mistakes:

• Forgetting to remove elements on Dequeue
• Adding new elements at front instead of rear
• Mixing order of elements after operations