Introduction To Python (Chapter: 3)

1. What is a List in Python?

A list in Python is a collection of ordered items.

It stores multiple values in one variable.

It allows duplicate values.

It is mutable (we can change it).

It keeps insertion order.

Syntax

list_name = [item1, item2, item3]

Example

students = ["Ravi", "Aman", "Neha"]
marks = [78, 85, 90]

Here:

students is a list of strings.

marks is a list of integers.

2. Why Lists Are Important (Real-Life Thinking)

Imagine you are:

Managing student marks in school
Storing products in an online shop
Keeping track of daily expenses
Maintaining attendance records

In all these cases, we need to store multiple related values.

That’s where lists are used.

3. Creating Lists

1. Empty List

my_list = []

2. List with Values

numbers = [10, 20, 30, 40]

3. Mixed Data Type List

data = ["Rajat", 25, True]

(Allowed, but in real applications we usually keep similar types together.)

4. Accessing List Elements

Lists use index numbers.

Index starts from 0.

Negative index starts from -1 (from end).

fruits = ["Apple", "Banana", "Mango", "Orange"]

print(fruits[0])    # Apple
print(fruits[-1])   # Orange

5. List Slicing

Used to access multiple elements.

Syntax

list[start:stop:step]

numbers = [10, 20, 30, 40, 50]

print(numbers[1:4])   # [20, 30, 40]
print(numbers[:3])    # [10, 20, 30]
print(numbers[::2])   # [10, 30, 50]

6. Modifying Lists (Because Lists Are Mutable)

students = ["Ravi", "Aman", "Neha"]
students[1] = "Sita"

print(students)

Output:

['Ravi', 'Sita', 'Neha']

7. Important List Methods (Very Important for Exams)

1. append()

Adds element at end.

numbers = [1, 2, 3]
numbers.append(4)

2. insert()

Insert at specific position.

numbers.insert(1, 100)

3. remove()

Removes by value.

numbers.remove(100)

4. pop()

Removes by index.

numbers.pop(2)

5. sort()

numbers.sort()

6. reverse()

numbers.reverse()

7. len()

len(numbers)

8. Real Life Example 1 – Student Marks System

Problem: Store marks of 5 students and calculate average.

Solution

marks = [80, 75, 90, 60, 85]

total = sum(marks)
average = total / len(marks)

print("Total Marks:", total)
print("Average:", average)

Explanation:

sum() adds all values.

len() gives number of students.

Average = total / count

This is exactly how school result systems work.

9. Real Life Example 2 – Grocery Bill Calculator

Problem: Store prices of items and calculate total bill.

prices = [50, 120, 30, 200]

total_bill = sum(prices)

print("Total Bill:", total_bill)

Practical Use: Billing software in shops works like this internally.

10. Real Life Example 3 – Attendance System

Problem: Count how many students are present.

attendance = ["P", "A", "P", "P", "A", "P"]

present_count = attendance.count("P")

print("Total Present:", present_count)

count() method counts occurrences.

11. Real Life Example 4 – Remove Duplicate Phone Numbers

numbers = [9876, 1234, 9876, 4567, 1234]

unique_numbers = []

for num in numbers:
    if num not in unique_numbers:
        unique_numbers.append(num)

print(unique_numbers)

Used in:

Contact management
Spam filtering
Data cleaning

12. Academic Exam Based Questions (With Solution)

Q1. Write a program to find largest number in a list.

numbers = [45, 12, 89, 34, 67]

largest = numbers[0]

for num in numbers:
    if num > largest:
        largest = num

print("Largest number:", largest)

Logic:

Assume first element is largest.

Compare with others.

Update if bigger found.

Q2. Write a program to count even and odd numbers.

numbers = [10, 15, 22, 33, 40]

even = 0
odd = 0

for num in numbers:
    if num % 2 == 0:
        even += 1
    else:
        odd += 1

print("Even:", even)
print("Odd:", odd)

Q3. Reverse a List Without Using reverse()

numbers = [1, 2, 3, 4, 5]

reversed_list = numbers[::-1]

print(reversed_list)

Q4. Input 5 numbers from user and store in list

numbers = []

for i in range(5):
    num = int(input("Enter number: "))
    numbers.append(num)

print(numbers)

Q5. Search Element in List

numbers = [10, 20, 30, 40]
search = 30

if search in numbers:
    print("Found")
else:
    print("Not Found")

13. Nested Lists (Very Important for Advanced Questions)

A list inside another list.

matrix = [
    [1, 2, 3],
    [4, 5, 6],
    [7, 8, 9]
]

print(matrix[1][2])   # 6