~/Introduction to Python

1.1 What is Programming?

Just as we use English or Hindi to talk to people, we use programming languages to give instructions to computers.

1.2 What is Python?

Why is Python Called a "High-Level" Language?

Because Python is written in simple, English-like words that are close to how humans think – unlike low-level languages (like Assembly) which are close to machine code and hard to understand.

1.3 Features of Python

1.4 Applications of Python

1.5 How to Install & Run Python

Ways to Run Python

1. Install Python: Download from python.org and install on your computer.
2. Use IDLE: The default Python editor that comes with Python.
3. Online Python Compilers: replit.com, programiz.com/python-programming/online-compiler, onlinegdb.com – no installation needed!
4. Jupyter Notebook: Popular for AI / Data Science.
5. Code Editors: VS Code, PyCharm with Python extension.

1.6 Your First Python Program

Let's start with the classic "Hello World" program:

print("Hello, World!")

That's it! Just one line of code and your first program is ready.

1.7 Comments in Python

# This is a single-line comment
print("Hello")  # Comment at end of line

"""
This is a
multi-line comment
"""

1.8 Variables in Python

How to Create a Variable

# Syntax: variable_name = value
name = "Ravi"
age = 15
height = 5.6

print(name)
print(age)
print(height)

Rules for Naming Variables

• Must start with a letter or underscore (_).
• Cannot start with a number (e.g., 2name is invalid).
• Can contain letters, numbers, and underscores.
• Are case-sensitive: Age, age, and AGE are different.
• Cannot use reserved keywords (like print, if, for).
• Should be meaningful: use student_age instead of a.
• No spaces allowed (use _ instead).

1.9 Data Types in Python

Data types tell Python what kind of value a variable is holding. Python has these main data types:

# Examples of different data types
a = 10           # int
b = 3.14         # float
c = "Python"     # string
d = True         # boolean

print(type(a))  # shows the data type
print(type(b))
print(type(c))
print(type(d))

1.10 Type Conversion

# Type conversion examples
x = "25"            # x is a string
y = int(x)          # y is now integer 25
print(y + 5)          # prints 30

num = 3.7
print(int(num))      # prints 3 (removes decimal)

1.11 print() and input() Functions

print() Function

The print() function is used to display output on the screen.

print("Hello")                    # prints Hello
print("Name:", "Ravi")           # prints Name: Ravi
print(5 + 3)                      # prints 8
print("Age:", 15, "years")       # combines items

input() Function

The input() function is used to take input from the user. It always returns a string.

name = input("Enter your name: ")
print("Hello,", name)

age = int(input("Enter your age: "))
print("Next year you will be", age + 1)

1.12 Operators in Python

1. Arithmetic Operators

Used for mathematical calculations.

2. Assignment Operators

Used to assign values to variables.

3. Comparison Operators

Used to compare two values. They return either True or False.

4. Logical Operators

Used to combine multiple conditions.

1.13 Expressions in Python

# Examples of expressions
result = 10 + 5 * 2          # result = 20
area = length * width         # expression with variables
is_pass = marks >= 33       # boolean expression

Operator Precedence (PEMDAS / BODMAS)

1. Parentheses – ( )
2. Exponents – **
3. Multiplication, Division, Modulus – *, /, //, %
4. Addition, Subtraction – +, -

2.1 Conditional Statements (if – elif – else)

Syntax

if condition:
    # code to run if condition is True
elif another_condition:
    # code if another condition is True
else:
    # code if all above are False

Examples

num = int(input("Enter a number: "))

if num > 0:
    print("Positive number")
elif num < 0:
    print("Negative number")
else:
    print("Zero")

age = int(input("Enter your age: "))

if age >= 18:
    print("You can vote!")
else:
    print("You cannot vote yet.")

marks = int(input("Enter marks: "))

if marks >= 90:
    print("Grade: A+")
elif marks >= 75:
    print("Grade: A")
elif marks >= 60:
    print("Grade: B")
elif marks >= 33:
    print("Grade: C")
else:
    print("Grade: Fail")

2.2 Loops in Python

for Loop

The for loop is used to repeat a code a fixed number of times or iterate over a list, string, etc.

Syntax with range()

for variable in range(start, stop, step):
    # code to repeat

range() generates a sequence of numbers:

• range(5) → 0, 1, 2, 3, 4
• range(1, 6) → 1, 2, 3, 4, 5
• range(1, 10, 2) → 1, 3, 5, 7, 9

for i in range(1, 11):
    print(i)

for i in range(2, 21, 2):
    print(i)

total = 0
for i in range(1, 11):
    total += i
print("Sum =", total)

while Loop

The while loop repeats code as long as a condition is True.

while condition:
    # code to repeat

i = 1
while i <= 5:
    print(i)
    i += 1     # VERY IMPORTANT – update the variable

for vs while – When to Use

3.1 What is a List?

# Creating a list
fruits = ["apple", "banana", "mango"]
numbers = [10, 20, 30, 40, 50]
mixed = [1, "hi", 3.14, True]

Features of a List

• Can store different types of data.
• Is ordered – items have fixed positions.
• Is changeable (mutable) – can add / remove items.
• Allows duplicate values.
• Can contain another list (nested list).

3.2 List Indexing

Each item in a list has an index (position number) starting from 0.

fruits = ["apple", "banana", "mango", "orange"]
#          0        1          2        3       (positive index)
#         -4       -3         -2       -1       (negative index)

print(fruits[0])    # apple
print(fruits[2])    # mango
print(fruits[-1])   # orange (last item)

3.3 List Operations

3.4 List Slicing

Slicing means getting a part of the list. Syntax: list[start:end] (end is excluded).

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

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

4.1 PRINT Category Programs

print("Name: Rohan Sharma")
print("Father's Name: Mr. Ajay Sharma")
print("Class: IX-B")
print("School: XYZ Public School")

print("*")
print("**")
print("***")
print("****")
print("*****")

print("*****")
print("****")
print("***")
print("**")
print("*")

num = 7
square = num ** 2
print("Square of", num, "is", square)

a = 15
b = 20
total = a + b
print("Sum =", total)

km = 5
meters = km * 1000
print(km, "km =", meters, "meters")

print(5 * 1)
print(5 * 2)
print(5 * 3)
print(5 * 4)
print(5 * 5)

# OR using a loop
for i in range(1, 6):
    print("5 x", i, "=", 5*i)

principle_amount = 2000
rate_of_interest = 4.5
time = 10

SI = (principle_amount * rate_of_interest * time) / 100
print("Simple Interest =", SI)

4.2 INPUT Category Programs

length = float(input("Enter length: "))
breadth = float(input("Enter breadth: "))

area = length * breadth
perimeter = 2 * (length + breadth)

print("Area =", area)
print("Perimeter =", perimeter)

base = float(input("Enter base: "))
height = float(input("Enter height: "))

area = 0.5 * base * height
print("Area of triangle =", area)

m1 = float(input("Enter marks of subject 1: "))
m2 = float(input("Enter marks of subject 2: "))
m3 = float(input("Enter marks of subject 3: "))

average = (m1 + m2 + m3) / 3
print("Average marks =", average)

price = float(input("Enter price: "))
discount = float(input("Enter discount %: "))

discount_amount = (price * discount) / 100
final_price = price - discount_amount

print("Discount amount =", discount_amount)
print("Final price =", final_price)

l = float(input("Enter length: "))
b = float(input("Enter breadth: "))
h = float(input("Enter height: "))

surface_area = 2 * (l*b + b*h + h*l)
volume = l * b * h

print("Surface Area =", surface_area)
print("Volume =", volume)

4.3 IF / FOR / WHILE Category Programs

age = int(input("Enter your age: "))

if age >= 18:
    print("You are eligible to vote.")
else:
    print("You are not eligible to vote.")

marks = int(input("Enter marks: "))

if marks >= 90:
    print("Grade: A+")
elif marks >= 75:
    print("Grade: A")
elif marks >= 60:
    print("Grade: B")
elif marks >= 40:
    print("Grade: C")
else:
    print("Grade: Fail")

num = int(input("Enter a number: "))

if num > 0:
    print("The number is positive.")
elif num < 0:
    print("The number is negative.")
else:
    print("The number is zero.")

for i in range(1, 11):
    print(i)

for i in range(2, 21, 2):
    print(i)

n = int(input("Enter n: "))

for i in range(1, n+1, 2):
    print(i)

total = 0
for i in range(1, 11):
    total += i
print("Sum =", total)

numbers = [10, 20, 30, 40, 50]
total = 0

for num in numbers:
    total += num

print("Sum =", total)

4.4 LIST Category Programs

students = ["Arjun", "Sonakshi", "Vikram", "Sandhya", "Sonal", "Isha", "Kartik"]

# Print the whole list
print("Original list:", students)

# Delete "Vikram"
students.remove("Vikram")

# Add "Jay" at the end
students.append("Jay")

# Remove item at position 2 (second position, index 1)
students.pop(1)

print("Final list:", students)

num = [23, 12, 5, 9, 65, 44]

# Print length of list
print("Length:", len(num))

# Elements from 2nd to 4th position (positive indexing)
print(num[1:4])

# Elements from 3rd to 5th position (negative indexing)
print(num[-4:-1])

# Create list of first 10 even numbers
evens = [2, 4, 6, 8, 10, 12, 14, 16, 18, 20]

# Add 1 to each item
for i in range(len(evens)):
    evens[i] += 1

print("Final list:", evens)

List_1 = [10, 20, 30, 40]

# Extend with new elements
List_1.extend([14, 15, 12])

# Sort in ascending order
List_1.sort()

print("Sorted list:", List_1)

5.1 Common Errors in Python

5.2 Good Programming Practices

• Use meaningful variable names – student_age instead of a.
• Add comments to explain tricky parts of your code.
• Use proper indentation – always 4 spaces.
• Test your code with different inputs.
• Break big problems into smaller steps.
• Don't repeat code – use loops and functions.
• Save your work frequently.
• Learn from errors – they help you understand better.
• Practice daily – coding is a skill built by doing.