Enqueue Operation in DSA Python - Build from Scratch

Enqueue Operation in a Queue

📖 Scenario: Imagine you are managing a line of customers waiting to buy tickets. Each new customer joins the end of the line. This line is called a queue.

🎯 Goal: You will create a queue using a list, add new customers to the end of the queue using the enqueue operation, and then display the queue.

📋 What You'll Learn

Create a list called queue with initial customers.

Create a variable called new_customer with the name of the customer to add.

Add the new_customer to the end of the queue using the enqueue operation.

Print the final state of the queue.

💡 Why This Matters

🌍 Real World

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

💼 Career

Understanding queues and enqueue operations is important for jobs in software development, especially when working with data processing, task scheduling, and system design.

1

Create the initial queue

Create a list called queue with these exact customers in order: 'Alice', 'Bob', 'Charlie'

DSA Python

# Create the initial queue list with 'Alice', 'Bob', and 'Charlie'
# Your code here

Hint

Use square brackets to create a list and separate names with commas.

2

Add a new customer name

Create a variable called new_customer and set it to the string 'David'

DSA Python

queue = ['Alice', 'Bob', 'Charlie']
# Create a variable new_customer with the value 'David'
# Your code here

Hint

Use an equals sign to assign the string 'David' to new_customer.

3

Enqueue the new customer

Add the new_customer to the end of the queue list using the append() method

DSA Python

queue = ['Alice', 'Bob', 'Charlie']
new_customer = 'David'
# Add new_customer to the end of queue using append()
# Your code here

Hint

Use queue.append(new_customer) to add the new customer at the end.

4

Print the final queue

Print the queue list to show the final order of customers

DSA Python

queue = ['Alice', 'Bob', 'Charlie']
new_customer = 'David'
queue.append(new_customer)
# Print the queue list
# Your code here

Hint

Use print(queue) to display the queue.

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 new elements join at the back.

2. Step 2: Define enqueue operation

   Enqueue means adding an element to the end of this line.

3. Final Answer:

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

4. Quick Check:

   Enqueue = Add to 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 element 5 into a queue named q implemented as a Python list?

easy

A. q.enqueue(5)

B. q.push(5)

C. q.append(5)

D. q.insert(0, 5)

Solution

1. Step 1: Identify Python list method for adding at end

   Python lists use append() to add elements at the end.

2. Step 2: Match enqueue operation to list method

   Enqueue adds to the back, so q.append(5) is correct.

3. Final Answer:

   q.append(5) -> Option C

4. Quick Check:

   Enqueue in list = append() [OK]

Hint: Use append() to add at the end of a Python list [OK]

Common Mistakes:

• Using push() which is not a Python list method
• Using insert(0, 5) which adds at front
• Assuming enqueue is a built-in list method

3. What will be the output of the following code?

queue = []
queue.append(10)
queue.append(20)
queue.append(30)
print(queue)

medium

A. [30, 20, 10]

B. [10, 20, 30]

C. [10, 30, 20]

D. Error

Solution

1. Step 1: Trace each enqueue operation

   Appending 10, then 20, then 30 adds elements in order at the end.

2. Step 2: Check final queue state

   The list will be [10, 20, 30] after all appends.

3. Final Answer:

   [10, 20, 30] -> Option B

4. Quick Check:

   Appending in order = [10, 20, 30] [OK]

Hint: Appending adds elements in order at the end [OK]

Common Mistakes:

• Reversing the order of elements
• Thinking append adds at front
• Expecting an error due to empty list

4. Identify the error in the following enqueue implementation for a queue using a Python list:

def enqueue(queue, item):
    queue.insert(item)

q = [1, 2, 3]
enqueue(q, 4)
print(q)

medium

A. insert() requires two arguments: index and item

B. insert() cannot be used on lists

C. enqueue should use append() instead of insert()

D. queue is not defined

Solution

1. Step 1: Check insert() method usage

   Python list's insert() needs two arguments: position index and the item.

2. Step 2: Identify missing argument

   The code calls insert(item) with only one argument, causing an error.

3. Final Answer:

   insert() requires two arguments: index and item -> Option A

4. Quick Check:

   insert() needs index and item [OK]

Hint: insert() needs index and item arguments [OK]

Common Mistakes:

• Using insert() with one argument
• Confusing insert() with append()
• Ignoring Python method argument requirements

5. You have a queue implemented as a Python list. You want to enqueue multiple elements [7, 8, 9] at once while preserving order. Which code snippet correctly enqueues all elements in order?

hard

A. queue.extend([7, 8, 9])

B. for x in [7, 8, 9]:
queue.insert(0, x)

C. queue.append([7, 8, 9])

D. queue = [7, 8, 9] + queue

Solution

1. Step 1: Understand enqueue multiple elements

   Enqueue adds elements at the end preserving order.

2. Step 2: Analyze each option

   for x in [7, 8, 9]: queue.insert(0, x) inserts at front reversing order; queue.append([7, 8, 9]) appends list as single element; queue = [7, 8, 9] + queue prepends elements; queue.extend([7, 8, 9]) extends list adding elements in order at end.

3. Final Answer:

   queue.extend([7, 8, 9]) -> Option A

4. Quick Check:

   extend() adds multiple elements at end [OK]

Hint: Use extend() to add multiple elements at queue end [OK]

Common Mistakes:

• Using append() which nests list inside queue
• Using insert(0, x) which reverses order
• Prepending elements instead of appending