Queue Concept and FIFO Principle in DSA C# - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this C# Queue code?

Consider the following C# code using a Queue. What will be printed after all operations?

DSA C#

using System;
using System.Collections.Generic;

class Program {
    static void Main() {
        Queue<int> q = new Queue<int>();
        q.Enqueue(10);
        q.Enqueue(20);
        q.Enqueue(30);
        int first = q.Dequeue();
        q.Enqueue(40);
        Console.WriteLine(string.Join(", ", q));
    }
}

A 20, 30, 40

B 10, 20, 30, 40

C 30, 40

D 10, 20, 30

💡 Hint

Remember that Queue removes elements from the front (FIFO).

The first element 10 is removed by Dequeue(). Then 40 is added to the end. So the queue contains 20, 30, 40.

🧠 Conceptual

intermediate

Which statement best describes the FIFO principle in a Queue?

Choose the statement that correctly explains the FIFO principle used in Queues.

A The last element added is the first one to be removed.

B Elements are removed based on their value, highest first.

C Elements are removed randomly from anywhere in the queue.

D Elements are removed in the same order they were added.

💡 Hint

Think about a line of people waiting for service.

FIFO means First In, First Out. The first element added is the first to be removed, so elements come out in the order they went in.

🔧 Debug

advanced

What error does this C# Queue code produce?

Identify the error when running this code snippet:

DSA C#

using System;
using System.Collections.Generic;

class Program {
    static void Main() {
        Queue<int> q = new Queue<int>();
        q.Enqueue(1);
        q.Dequeue();
        q.Dequeue();
    }
}

A InvalidOperationException: Queue empty on Dequeue

B NullReferenceException

C No error, runs fine

D IndexOutOfRangeException

💡 Hint

What happens if you remove from an empty queue?

After removing the only element, the queue is empty. Calling Dequeue again causes InvalidOperationException.

🚀 Application

advanced

How many items remain in the queue after these operations?

Given this sequence of operations on a C# Queue, how many items are left?

DSA C#

Queue<string> q = new Queue<string>();
q.Enqueue("apple");
q.Enqueue("banana");
q.Enqueue("cherry");
q.Dequeue();
q.Enqueue("date");
q.Dequeue();
q.Enqueue("elderberry");

A 4

B 2

C 3

D 1

💡 Hint

Count how many Enqueue and Dequeue operations happened.

5 Enqueue and 2 Dequeue operations leave 3 items in the queue.

❓ Predict Output

expert

What is the final printed output of this C# Queue code?

Analyze the code and determine the final output printed to the console.

DSA C#

using System;
using System.Collections.Generic;

class Program {
    static void Main() {
        Queue<int> q = new Queue<int>();
        for (int i = 1; i <= 5; i++) {
            q.Enqueue(i * 10);
        }
        q.Dequeue();
        q.Enqueue(60);
        q.Dequeue();
        q.Enqueue(70);
        Console.WriteLine(string.Join(" -> ", q) + " -> null");
    }
}

A 20 -> 30 -> 40 -> 50 -> 60 -> 70 -> null

B 30 -> 40 -> 50 -> 60 -> 70 -> null

C 10 -> 20 -> 30 -> 40 -> 50 -> 60 -> 70 -> null

D 40 -> 50 -> 60 -> 70 -> null

💡 Hint

Track each enqueue and dequeue step carefully.

Initially queue: 10,20,30,40,50
After first Dequeue: 20,30,40,50
Enqueue 60: 20,30,40,50,60
Second Dequeue: 30,40,50,60
Enqueue 70: 30,40,50,60,70

Practice

1. What does the FIFO principle mean in the context of a Queue data structure?

easy

A. Last In, Last Out - the last element added is the last removed

B. First In, First Out - the first element added is the first removed

C. Last In, First Out - the last element added is the first removed

D. First In, Last Out - the first element added is the last removed

Solution

1. Step 1: Understand FIFO meaning

   FIFO stands for First In, First Out, meaning the first element added to the queue is the first one to be removed.

2. Step 2: Apply FIFO to Queue

   In a queue, elements are processed in the order they arrive, so the earliest added element leaves first.

3. Final Answer:

   First In, First Out - the first element added is the first removed -> Option B

4. Quick Check:

   FIFO = First In, First Out [OK]

Hint: Remember queue = line; first come, first served [OK]

Common Mistakes:

• Confusing FIFO with stack behavior (LIFO)
• Thinking last added element is removed first
• Mixing up terms In and Out order

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

easy

A. q.Insert(5);

B. q.Push(5);

C. q.Add(5);

D. q.Enqueue(5);

Solution

1. Step 1: Recall Queue method to add elements

   The method to add elements to a Queue in C# is Enqueue.

2. Step 2: Check each option

   Only q.Enqueue(5); is valid syntax to add 5 to the queue.

3. Final Answer:

   q.Enqueue(5); -> Option D

4. Quick Check:

   Use Enqueue to add in Queue [OK]

Hint: Use Enqueue to add, Dequeue to remove [OK]

Common Mistakes:

• Using Push which is for stacks
• Using Add or Insert which are not Queue methods
• Confusing method names with List

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();
q.Enqueue("date");
Console.WriteLine(string.Join(", ", q));

medium

A. "banana, cherry, date"

B. "apple, banana, cherry, date"

C. "date, apple, banana, cherry"

D. "cherry, date, banana"

Solution

1. Step 1: Trace Enqueue and Dequeue operations

   Initially queue: [apple, banana, cherry]. Dequeue removes "apple" (first in). Queue now: [banana, cherry]. Then Enqueue "date" adds it to the end: [banana, cherry, date].

2. Step 2: Output the queue contents

   Printing queue elements joined by comma results in "banana, cherry, date".

3. Final Answer:

   "banana, cherry, date" -> Option A

4. Quick Check:

   Dequeue removes first, Enqueue adds last [OK]

Hint: Dequeue removes front, Enqueue adds back [OK]

Common Mistakes:

• Forgetting Dequeue removes first element
• Assuming Enqueue adds to front
• Printing queue before Dequeue

4. Identify the error in the following C# code that uses a Queue<int>:

Queue<int> q = new Queue<int>();
q.Enqueue(10);
q.Enqueue(20);
int x = q.Pop();
Console.WriteLine(x);

medium

A. Pop() method does not exist for Queue

B. Enqueue should be Add

C. Queue cannot store integers

D. Missing using System.Collections.Generic;

Solution

1. Step 1: Check Queue methods

   Queue in C# does not have a Pop() method; it uses Dequeue() to remove elements.

2. Step 2: Validate other parts

   Enqueue is correct to add elements; Queue can store integers; using System.Collections.Generic; is needed but not an error here.

3. Final Answer:

   Pop() method does not exist for Queue -> Option A

4. Quick Check:

   Use Dequeue, not Pop, for Queue removal [OK]

Hint: Queue uses Dequeue, not Pop [OK]

Common Mistakes:

• Using Pop() from stack on queue
• Confusing Enqueue with Add
• Ignoring missing namespace errors

5. You have a queue of customer names: ["Anna", "Bob", "Cara"]. You want to process the first two customers and then add "Dave" and "Eva" to the queue. What will be the final queue order after these operations?

hard

A. ["Anna", "Bob", "Dave", "Eva"]

B. ["Dave", "Eva", "Cara"]

C. ["Cara", "Dave", "Eva"]

D. ["Bob", "Cara", "Dave", "Eva"]

Solution

1. Step 1: Remove first two customers using Dequeue

   Removing "Anna" and "Bob" leaves the queue with ["Cara"].

2. Step 2: Add "Dave" and "Eva" using Enqueue

   Adding "Dave" and "Eva" to the end results in ["Cara", "Dave", "Eva"].

3. Final Answer:

   ["Cara", "Dave", "Eva"] -> Option C

4. Quick Check:

   Dequeue twice, then Enqueue twice [OK]

Hint: Dequeue removes front, Enqueue adds back [OK]

Common Mistakes:

• Not removing both customers before adding
• Adding new customers to front instead of back
• Confusing order of elements after operations