Queue Concept and FIFO Principle in DSA C# - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to add an element to the queue.

DSA C#

Queue<int> queue = new Queue<int>();
queue.[1](10);

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

AEnqueue

BPush

CAdd

DInsert

Attempts:

3 left

💡 Hint

Common Mistakes

Using Push which is for stacks, not queues.

Using Add which is not a method of Queue.

The Enqueue method adds an element to the end of the queue, following FIFO.

2fill in blank

medium

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

DSA C#

int first = queue.[1]();

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

APop

BExtract

CRemove

DDequeue

Attempts:

3 left

💡 Hint

Common Mistakes

Using Pop which is for stacks, not queues.

Using Remove which is not a method of Queue.

The Dequeue method removes and returns the element at the front of the queue, following FIFO.

3fill in blank

hard

Fix the error in the code to check the first element without removing it.

DSA C#

int peek = queue.[1]();

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

ADequeue

BFront

CPeek

DTop

Attempts:

3 left

💡 Hint

Common Mistakes

Using Dequeue which removes the element.

Using Top which is not a Queue method.

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

4fill in blank

hard

Fill both blanks to create a queue and add two elements.

DSA C#

Queue<string> queue = new [1]<string>();
queue.[2]("apple");

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

AQueue

BStack

CEnqueue

DPush

Attempts:

3 left

💡 Hint

Common Mistakes

Using Stack instead of Queue.

Using Push which is for stacks.

We create a Queue and add elements using Enqueue to follow FIFO.

5fill in blank

hard

Fill both blanks to remove an element and check the next element in the queue.

DSA C#

int removed = queue.[1]();
int next = queue.[2]();
Console.WriteLine(removed + " -> " + next);

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

APop

BPeek

CDequeue

DTop

Attempts:

3 left

💡 Hint

Common Mistakes

Using Pop to remove from queue.

Using Top which is not a queue method.

Dequeue removes the first element, Peek shows the next element without removing it.

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