Dequeue Operation in DSA C# - Build from Scratch

Dequeue Operation

📖 Scenario: Imagine you are managing a line of customers waiting for service at a bank. The line is represented as a queue where customers join at the end and leave from the front.

🎯 Goal: You will create a queue of customers, then remove the first customer who is served using the dequeue operation, and finally display the remaining customers in the queue.

📋 What You'll Learn

Create a queue called customers with these exact entries in order: "Alice", "Bob", "Charlie", "Diana"

Create a variable called servedCustomer to store the result of the dequeue operation

Use the Dequeue() method to remove the first customer from the customers queue

Print the name of the served customer stored in servedCustomer

Print the remaining customers in the queue separated by -> and ending with null

💡 Why This Matters

🌍 Real World

Queues are used in real life to manage lines, like customers waiting for service, print jobs waiting to print, or tasks waiting to be processed.

💼 Career

Understanding queue operations like enqueue and dequeue is important for software developers working on scheduling, task management, and real-time systems.

1

Create the queue of customers

Create a queue called customers and add these exact entries in order: "Alice", "Bob", "Charlie", "Diana"

DSA C#

// Your code here

Hint

Use Queue<string> to create the queue and Enqueue() to add each customer in order.

2

Dequeue the first customer

Create a variable called servedCustomer and assign it the result of customers.Dequeue() to remove the first customer from the queue

DSA C#

using System;
using System.Collections.Generic;

class Program
{
    static void Main()
    {
        Queue customers = new Queue();
        customers.Enqueue("Alice");
        customers.Enqueue("Bob");
        customers.Enqueue("Charlie");
        customers.Enqueue("Diana");

        // Your code here
    }
}

Hint

Use string servedCustomer = customers.Dequeue(); to remove and store the first customer.

3

Print the served customer

Write a Console.WriteLine statement to print the text "Served customer: " followed by the value of servedCustomer

DSA C#

using System;
using System.Collections.Generic;

class Program
{
    static void Main()
    {
        Queue customers = new Queue();
        customers.Enqueue("Alice");
        customers.Enqueue("Bob");
        customers.Enqueue("Charlie");
        customers.Enqueue("Diana");

        string servedCustomer = customers.Dequeue();

        // Your code here
    }
}

Hint

Use Console.WriteLine("Served customer: " + servedCustomer); to print the served customer's name.

4

Print the remaining customers in the queue

Write code to print the remaining customers in customers separated by " -> " and ending with "null". Use a foreach loop with variable customer to iterate over customers and print each customer followed by " -> ". After the loop, print "null".

DSA C#

using System;
using System.Collections.Generic;

class Program
{
    static void Main()
    {
        Queue customers = new Queue();
        customers.Enqueue("Alice");
        customers.Enqueue("Bob");
        customers.Enqueue("Charlie");
        customers.Enqueue("Diana");

        string servedCustomer = customers.Dequeue();

        Console.WriteLine("Served customer: " + servedCustomer);

        // Your code here
    }
}

Hint

Use a foreach loop to print each customer followed by " -> ", then print "null" after the loop.

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