Enqueue Operation in DSA C# - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the enqueue operation do in a queue?

It adds an element to the end (rear) of the queue, following the First-In-First-Out (FIFO) principle.

beginner

In a queue, where is the new element added during an enqueue operation?

The new element is added at the rear of the queue.

intermediate

What happens if you try to enqueue an element when the queue is full (in a fixed-size queue)?

The enqueue operation fails or throws an error because there is no space to add a new element.

beginner

Show the state of the queue after enqueueing 5 into an empty queue.

Queue state: 5 -> null

beginner

Why is enqueue operation important in real-life scenarios like waiting lines?

Because it models adding people or items to the end of a line, ensuring fairness by serving in the order they arrived.

Where does the enqueue operation add an element in a queue?

A At the rear (end) of the queue

B At the front (start) of the queue

C In the middle of the queue

D It removes an element

Enqueue always adds an element at the rear (end) of the queue.

What is the main principle followed by the enqueue operation in a queue?

A Last-In-First-Out (LIFO)

B Priority order

C First-In-First-Out (FIFO)

D Random order

Queues follow FIFO, so enqueue adds at the rear and dequeue removes from the front.

What happens if you enqueue an element in a full fixed-size queue?

A The queue size increases automatically

B The enqueue operation fails or throws an error

C The element is added successfully

D The oldest element is removed automatically

In fixed-size queues, enqueue fails if there is no space left.

If a queue currently has elements 1 -> 2 -> 3 -> null, what will be the state after enqueueing 4?

A 4 -> 1 -> 2 -> 3 -> null

B 3 -> 4 -> null

C 1 -> 2 -> null

D 1 -> 2 -> 3 -> 4 -> null

Enqueue adds the new element at the rear, so 4 goes after 3.

Which of these is NOT true about enqueue operation?

A Removes element from the front

B Adds element at the rear

C Follows FIFO principle

D Can fail if queue is full

Removing element from the front is dequeue, not enqueue.

Explain the enqueue operation in a queue and how it affects the queue's order.

Think about how people join a line at the end.

Describe what happens step-by-step when you enqueue an element into an empty queue.

Start from no elements and add one.

Practice

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

easy

A. Adds an element to the end of the queue

B. Removes an element 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 queue behavior

   A queue works like a line where new people join at the end.

2. Step 2: Define enqueue operation

   Enqueue means adding a new element at the back of the queue.

3. Final Answer:

   Adds an element to the end of the queue -> Option A

4. Quick Check:

   Enqueue = Add at end [OK]

Hint: Enqueue means add at the back of the queue [OK]

Common Mistakes:

• Confusing enqueue with dequeue
• Thinking enqueue removes elements
• Mixing enqueue with checking size

2. Which of the following is the correct syntax to enqueue an integer value 10 into a Queue<int> named q in C#?

easy

A. q.Enqueue(10);

B. q.Add(10);

C. q.Push(10);

D. q.Insert(10);

Solution

1. Step 1: Recall C# Queue method

   The method to add elements in Queue<T> is Enqueue.

2. Step 2: Match syntax

   Calling q.Enqueue(10); adds 10 to the queue correctly.

3. Final Answer:

   q.Enqueue(10); -> Option A

4. Quick Check:

   Use Enqueue() to add in C# Queue [OK]

Hint: Use Enqueue() method to add elements in C# Queue [OK]

Common Mistakes:

• Using Add() which is not in Queue
• Using Push() which is for stacks
• Using Insert() which is not a Queue method

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

medium

A. cherry, banana, apple

B. apple, banana, cherry

C. banana, cherry, apple

D. apple, cherry, banana

Solution

1. Step 1: Trace enqueue order

   Elements are added in order: apple, then banana, then cherry.

2. Step 2: Understand queue order

   Queue keeps order of insertion, so printing shows apple, banana, cherry.

3. Final Answer:

   apple, banana, cherry -> Option B

4. Quick Check:

   Queue prints in enqueue order [OK]

Hint: Queue keeps insertion order; output matches enqueue sequence [OK]

Common Mistakes:

• Assuming reverse order output
• Mixing enqueue with stack behavior
• Confusing output order with dequeue

4. Identify the error in the following code snippet that tries to enqueue an element into a queue:

Queue<int> q = new Queue<int>();
q.Enqueue(5);
q.Enqueue(10);
q.Enqueue(15);
q.Enqueue();

medium

A. Cannot enqueue integer values

B. Queue is not initialized

C. Enqueue method does not exist

D. Missing argument in the last Enqueue call

Solution

1. Step 1: Check method calls

   The last call q.Enqueue(); has no argument, but Enqueue requires one.

2. Step 2: Confirm method signature

   Enqueue needs a value to add, so missing argument causes error.

3. Final Answer:

   Missing argument in the last Enqueue call -> Option D

4. Quick Check:

   Enqueue requires one argument [OK]

Hint: Enqueue always needs one argument to add [OK]

Common Mistakes:

• Calling Enqueue without argument
• Assuming Enqueue can be empty
• Confusing initialization errors

5. You have a queue q with elements [1, 2, 3]. You want to enqueue elements 4 and 5, but only if the queue size is less than 5. Which code snippet correctly performs this conditional enqueue operation in C#?

hard

A. if (q.Count == 5) { q.Enqueue(4); q.Enqueue(5); }

B. if (q.Count <= 5) { q.Enqueue(4); q.Enqueue(5); }

C. if (q.Count < 5) { q.Enqueue(4); q.Enqueue(5); }

D. if (q.Count > 5) { q.Enqueue(4); q.Enqueue(5); }

Solution

1. Step 1: Understand queue size condition

   We want to add only if size is less than 5, so condition is q.Count < 5.

2. Step 2: Apply enqueue calls inside condition

   If condition is true, enqueue 4 and 5 to add them safely.

3. Final Answer:

   if (q.Count < 5) { q.Enqueue(4); q.Enqueue(5); } -> Option C

4. Quick Check:

   Use < 5 to check size before enqueue [OK]

Hint: Check queue size with Count before enqueueing [OK]

Common Mistakes:

• Using <= instead of < causing overflow
• Using > or == which are wrong conditions
• Not checking size before enqueue