Enqueue Operation in DSA C++ - Build from Scratch

Implementing Enqueue Operation in a Queue

📖 Scenario: You are managing a line of customers waiting to buy tickets at a counter. The line works on a first-come, first-served basis. When a new customer arrives, they join the end of the line.

🎯 Goal: Build a simple queue using an array and implement the enqueue operation to add new customers to the end of the queue.

📋 What You'll Learn

Create an integer array called queue with size 5 initialized with zeros

Create an integer variable called rear initialized to -1 to track the last position in the queue

Write a function called enqueue that takes an integer value and adds it to the queue if there is space

Print the queue array after adding new elements

💡 Why This Matters

🌍 Real World

Queues are used in real life to manage lines, like customers waiting at a bank or tasks waiting to be processed by a computer.

💼 Career

Understanding queues and enqueue operations is important for software developers working on scheduling, resource management, and real-time systems.

1

Create the queue array and rear variable

Create an integer array called queue of size 5 initialized with zeros and an integer variable called rear set to -1.

DSA C++

// Your code here

Hint

Think of queue as the line of customers and rear as the position of the last customer.

2

Create the enqueue function header

Write the function header for enqueue that takes an integer parameter called value.

DSA C++

int queue[5] = {0, 0, 0, 0, 0};
int rear = -1;

// Your code here

Hint

Start the function with void enqueue(int value) { to accept the new customer number.

3

Implement enqueue logic to add value to queue

Inside the enqueue function, check if rear is less than 4. If yes, increment rear by 1 and assign value to queue[rear]. Close the function with a closing brace.

DSA C++

int queue[5] = {0, 0, 0, 0, 0};
int rear = -1;

void enqueue(int value) {
    // Your code here
}

Hint

Remember to increase rear before adding the new value to the queue.

4

Add values and print the queue

Call enqueue three times with values 10, 20, and 30. Then use a for loop to print all elements of queue separated by spaces.

DSA C++

int queue[5] = {0, 0, 0, 0, 0};
int rear = -1;

void enqueue(int value) {
    if (rear < 4) {
        rear++;
        queue[rear] = value;
    }
}

// Your code here

Hint

Use std::cout inside a loop from 0 to 4 to print the queue elements.

Practice

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

easy

A. Returns the size of the queue

B. Removes an element from the front of the queue

C. Checks if the queue is empty

D. Adds an element to the end of the queue

Solution

1. Step 1: Understand the queue structure

   A queue works like a line where elements are added at the back and removed from the front.

2. Step 2: Define the enqueue operation

   Enqueue means adding a new element to the end of the queue to wait its turn.

3. Final Answer:

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

4. Quick Check:

   Enqueue = Add at end [OK]

Hint: Enqueue always adds at the back of the queue [OK]

Common Mistakes:

• Confusing enqueue with dequeue (removal)
• Thinking enqueue removes elements
• Mixing enqueue with checking queue size

2. Which of the following is the correct syntax to enqueue an element 5 into a queue named q in C++ STL?

easy

A. q.push(5);

B. q.add(5);

C. q.enqueue(5);

D. q.insert(5);

Solution

1. Step 1: Recall C++ STL queue methods

   The standard queue in C++ uses push() to add elements at the back.

2. Step 2: Match the correct method

   enqueue is not a method in STL queue; push is used instead.

3. Final Answer:

   q.push(5); -> Option A

4. Quick Check:

   STL queue uses push() to enqueue [OK]

Hint: Use push() to add elements in C++ STL queue [OK]

Common Mistakes:

• Using enqueue() which is not in STL queue
• Using add() or insert() which are not queue methods
• Confusing push() with pop()

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

std::queue<int> q;
q.push(10);
q.push(20);
q.push(30);
q.pop();
std::cout << q.front();

medium

A. 10

B. Compilation error

C. 20

D. 30

Solution

1. Step 1: Trace the enqueue operations

   Elements 10, 20, 30 are added in order: queue = [10, 20, 30]

2. Step 2: Apply the dequeue operation

   pop() removes the front element 10, so queue = [20, 30]

3. Step 3: Check the front element

   front() now returns 20, the new front.

4. Final Answer:

   20 -> Option C

5. Quick Check:

   After pop, front = 20 [OK]

Hint: pop removes front; front() shows new front [OK]

Common Mistakes:

• Assuming front remains 10 after pop
• Confusing pop with removing from back
• Expecting output 30 instead of 20

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

std::queue<int> q;
q.enqueue(15);

medium

A. Missing #include <queue>

B. enqueue is not a member function of std::queue

C. Queue must be initialized with size

D. Cannot enqueue integer values

Solution

1. Step 1: Check std::queue methods

   std::queue does not have an enqueue method; it uses push to add elements.

2. Step 2: Identify the error cause

   Calling enqueue causes a compilation error because the method does not exist.

3. Final Answer:

   enqueue is not a member function of std::queue -> Option B

4. Quick Check:

   Use push() not enqueue() in std::queue [OK]

Hint: std::queue uses push(), not enqueue() [OK]

Common Mistakes:

• Assuming enqueue() exists in std::queue
• Thinking queue needs size at initialization
• Believing integers cannot be enqueued

5. You have a queue initially empty. You perform these operations:

enqueue(1)
enqueue(2)
dequeue()
enqueue(3)
dequeue()
enqueue(4)

What is the final state of the queue (front to back)?

hard

A. 3 -> 4 -> null

B. 2 -> 3 -> 4 -> null

C. 1 -> 3 -> 4 -> null

D. 4 -> null

Solution

1. Step 1: Perform operations step-by-step

   Start empty: []
   enqueue(1): [1]
   enqueue(2): [1, 2]
   dequeue(): removes 1 -> [2]
   enqueue(3): [2, 3]
   dequeue(): removes 2 -> [3]
   enqueue(4): [3, 4]

2. Step 2: Write final queue state

   Queue front is 3, then 4, then null (end).

3. Final Answer:

   3 -> 4 -> null -> Option A

4. Quick Check:

   After operations, queue = 3,4 [OK]

Hint: Track each enqueue/dequeue step carefully [OK]

Common Mistakes:

• Forgetting to remove elements on dequeue
• Mixing order of elements after operations
• Assuming dequeue removes from back