Unit 1: Python Fundamentals

3.1 What are Variables?

Variables are containers for storing data values. Unlike other programming languages, Python has no command for declaring a variable. A variable is created the moment you first assign a value to it.

# Creating variables
name = "Python"      # String variable
age = 30             # Integer variable
height = 5.9         # Float variable
is_active = True     # Boolean variable

3.2 Variable Naming Rules

• Must start with a letter (a-z, A-Z) or underscore (_)
• Cannot start with a number
• Can only contain alphanumeric characters and underscores (A-z, 0-9, _)
• Variable names are case-sensitive (age, Age, AGE are different)
• Cannot use Python keywords (if, else, for, while, etc.)

# Valid variable names
my_name = "John"
_private = 100
userName2 = "Alice"
CONSTANT_VALUE = 3.14

# Invalid variable names (will cause error)
# 2name = "Invalid"    # Cannot start with number
# my-name = "Invalid"  # Cannot use hyphen
# class = "Invalid"    # Cannot use keyword

3.3 Python Data Types in Detail

Python has the following built-in data types:

# Integer examples
x = 10
y = -50
z = 0
big_number = 1234567890123456789  # Python handles large integers automatically

print(type(x))  # Output: <class 'int'>

# Float examples
pi = 3.14159
temperature = -40.5
scientific = 2.5e4  # Scientific notation (25000.0)

print(type(pi))  # Output: <class 'float'>

# Complex examples
c1 = 3 + 4j
c2 = complex(2, 5)  # 2 + 5j

print(c1.real)  # Output: 3.0
print(c1.imag)  # Output: 4.0

# String creation methods
single_quote = 'Hello'
double_quote = "World"
multi_line = '''This is a
multi-line string'''

# String with quotes inside
quote1 = "He said 'Hello'"
quote2 = 'She said "Hi"'

# Escape characters
escaped = "Hello\nWorld"  # \n = new line
tab_string = "Hello\tWorld"  # \t = tab

text = "Python Programming"

# Indexing (starts from 0)
print(text[0])     # Output: P
print(text[-1])    # Output: g (negative index from end)

# Slicing
print(text[0:6])   # Output: Python
print(text[7:])    # Output: Programming
print(text[:6])    # Output: Python

# String methods
print(text.upper())        # PYTHON PROGRAMMING
print(text.lower())        # python programming
print(text.split())        # ['Python', 'Programming']
print(len(text))           # 18 (length)
print(text.replace("Python", "Java"))  # Java Programming

name = "Alice"
age = 25

# Method 1: f-strings (Python 3.6+) - Recommended
print(f"My name is {name} and I am {age} years old")

# Method 2: format() method
print("My name is {} and I am {} years old".format(name, age))

# Method 3: % operator (old style)
print("My name is %s and I am %d years old" % (name, age))

# Boolean examples
is_student = True
is_adult = False

# Boolean from comparisons
print(10 > 5)     # True
print(10 == 5)    # False
print(10 != 5)    # True

# Truthy and Falsy values
print(bool(1))     # True
print(bool(0))     # False
print(bool(""))    # False (empty string)
print(bool("Hi"))  # True (non-empty string)
print(bool([]))    # False (empty list)
print(bool([1,2])) # True (non-empty list)

# None examples
result = None
print(result)       # None
print(type(result)) # <class 'NoneType'>

# Common use: Function that doesn't return anything
def greet(name):
    print(f"Hello, {name}")

x = greet("Alice")  # Prints: Hello, Alice
print(x)            # None (function returned nothing)

3.4 Checking Data Types

# Using type() function
x = 10
y = 3.14
z = "Hello"
w = True

print(type(x))  # <class 'int'>
print(type(y))  # <class 'float'>
print(type(z))  # <class 'str'>
print(type(w))  # <class 'bool'>

# Using isinstance() function
print(isinstance(x, int))    # True
print(isinstance(y, float))  # True
print(isinstance(z, str))    # True

3.5 Multiple Assignment

# Assign same value to multiple variables
x = y = z = 100
print(x, y, z)  # 100 100 100

# Assign different values to multiple variables
a, b, c = 1, 2, 3
print(a, b, c)  # 1 2 3

# Swap variables
x, y = 10, 20
x, y = y, x  # Swap!
print(x, y)  # 20 10

4.1 Arithmetic Operators

Used for mathematical operations.

# Arithmetic operators in action
a, b = 17, 5

print(f"Addition: {a} + {b} = {a + b}")        # 22
print(f"Subtraction: {a} - {b} = {a - b}")     # 12
print(f"Multiplication: {a} * {b} = {a * b}")  # 85
print(f"Division: {a} / {b} = {a / b}")        # 3.4
print(f"Floor Division: {a} // {b} = {a // b}")# 3
print(f"Modulus: {a} % {b} = {a % b}")         # 2
print(f"Power: {a} ** 2 = {a ** 2}")           # 289

4.2 Comparison (Relational) Operators

Used to compare two values. Returns True or False.

# Comparison operators
x, y = 10, 20

print(x == y)   # False
print(x != y)   # True
print(x > y)    # False
print(x < y)    # True
print(x >= 10)  # True
print(y <= 20)  # True

# Chained comparisons (Python special feature!)
age = 25
print(18 <= age <= 65)  # True (age is between 18 and 65)

4.3 Logical Operators

Used to combine conditional statements.

# Logical operators
a, b = True, False

print(a and b)  # False (both must be True)
print(a or b)   # True (at least one is True)
print(not a)    # False (reverses True)
print(not b)    # True (reverses False)

# Practical example
age = 25
has_license = True

can_drive = (age >= 18) and has_license
print(f"Can drive: {can_drive}")  # True

4.4 Assignment Operators

Used to assign values to variables.

# Assignment operators
x = 10

x += 5   # x = 15
x -= 3   # x = 12
x *= 2   # x = 24
x /= 4   # x = 6.0
x //= 2  # x = 3.0
x **= 2  # x = 9.0

print(x)  # 9.0

4.5 Bitwise Operators

Used to perform bit-level operations.

# Bitwise operators
a = 5   # Binary: 0101
b = 3   # Binary: 0011

print(a & b)   # 1  (0001)
print(a | b)   # 7  (0111)
print(a ^ b)   # 6  (0110)
print(~a)      # -6 (inverts bits)
print(a << 1)  # 10 (1010) - shift left
print(a >> 1)  # 2  (0010) - shift right

4.6 Identity Operators

Used to compare if two objects are the same object (same memory location).

# Identity operators
a = [1, 2, 3]
b = [1, 2, 3]
c = a

print(a == b)      # True (same values)
print(a is b)      # False (different objects)
print(a is c)      # True (same object)
print(a is not b)  # True

4.7 Membership Operators

Used to test if a value is present in a sequence.

# Membership operators
fruits = ["apple", "banana", "cherry"]
text = "Hello World"

print("apple" in fruits)      # True
print("grape" in fruits)      # False
print("grape" not in fruits)  # True

print("Hello" in text)        # True
print("hello" in text)        # False (case-sensitive)

4.8 Operator Precedence

Order in which operators are evaluated (highest to lowest):

1. () - Parentheses
2. ** - Exponentiation
3. ~, +, - - Unary operators
4. *, /, //, % - Multiplication, Division
5. +, - - Addition, Subtraction
6. >>, << - Bitwise shifts
7. & - Bitwise AND
8. ^ - Bitwise XOR
9. | - Bitwise OR
10. ==, !=, >, <, >=, <= - Comparisons
11. not - Logical NOT
12. and - Logical AND
13. or - Logical OR

# Operator precedence examples
result = 2 + 3 * 4      # 14 (not 20)
result = (2 + 3) * 4    # 20 (parentheses first)
result = 2 ** 3 ** 2    # 512 (right to left: 3**2=9, 2**9=512)

5.1 Output with print()

The print() function outputs data to the console.

# Basic print
print("Hello, World!")

# Print multiple values
print("Name:", "John", "Age:", 25)

# Print with separator
print("A", "B", "C", sep="-")  # A-B-C

# Print without newline
print("Hello", end=" ")
print("World")  # Hello World (on same line)

# Print with variables
name = "Alice"
age = 25
print("Name:", name, "Age:", age)

# Formatted output using f-strings
print(f"My name is {name} and I am {age} years old")

# Formatting numbers
pi = 3.14159265359
print(f"Pi = {pi:.2f}")      # Pi = 3.14 (2 decimal places)
print(f"Pi = {pi:.4f}")      # Pi = 3.1416 (4 decimal places)

# Alignment
print(f"{'Left':<10}")    # Left aligned, width 10
print(f"{'Right':>10}")   # Right aligned, width 10
print(f"{'Center':^10}")  # Center aligned, width 10

5.2 Input with input()

The input() function reads user input from keyboard.

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

# Input always returns a string!
age = input("Enter your age: ")
print(type(age))  # <class 'str'>

# Convert input to number
age = int(input("Enter your age: "))
height = float(input("Enter your height: "))

# Multiple inputs on one line
x, y = input("Enter two numbers: ").split()
x, y = int(x), int(y)
print(f"Sum = {x + y}")

# Alternative: using map()
a, b = map(int, input("Enter two numbers: ").split())
print(f"Sum = {a + b}")

5.3 Practical Examples

# Example 1: Simple Calculator
num1 = float(input("Enter first number: "))
num2 = float(input("Enter second number: "))
operation = input("Enter operation (+, -, *, /): ")

if operation == "+":
    result = num1 + num2
elif operation == "-":
    result = num1 - num2
elif operation == "*":
    result = num1 * num2
elif operation == "/":
    result = num1 / num2 if num2 != 0 else "Error: Division by zero"

print(f"Result: {result}")

# Example 2: Temperature Converter
celsius = float(input("Enter temperature in Celsius: "))
fahrenheit = (celsius * 9/5) + 32
print(f"{celsius}°C = {fahrenheit}°F")

6.1 Implicit Type Conversion (Automatic)

Python automatically converts one data type to another.

# Implicit conversion
x = 10      # int
y = 3.14    # float
z = x + y   # Python converts x to float automatically

print(z)         # 13.14
print(type(z))   # <class 'float'>

6.2 Explicit Type Conversion (Manual)

Programmer manually converts data types using built-in functions.

# Convert to integer
num = int("123")      # String to int: 123
num = int(45.67)      # Float to int: 45 (truncates decimal)
num = int(True)       # Bool to int: 1

# Convert to float
num = float("3.14")   # String to float: 3.14
num = float(10)       # Int to float: 10.0

# Convert to string
text = str(123)       # Int to string: "123"
text = str(3.14)      # Float to string: "3.14"
text = str(True)      # Bool to string: "True"

# Convert to boolean
bool_val = bool(1)    # True (non-zero is True)
bool_val = bool(0)    # False
bool_val = bool("")   # False (empty string)
bool_val = bool("Hi") # True (non-empty string)

# Convert between collections
my_list = list("Hello")         # ['H', 'e', 'l', 'l', 'o']
my_tuple = tuple([1, 2, 3])     # (1, 2, 3)
my_set = set([1, 2, 2, 3, 3])   # {1, 2, 3}

📊 Comparison of Data Structures

*Dictionaries maintain insertion order since Python 3.7

8.1 Key Differences Overview

8.2 Variable Declaration

// Must declare type
int age = 25;
float height = 5.9;
char name[20] = "John";
int x, y, z;

// Constants
const int MAX = 100;

# No type declaration needed
age = 25
height = 5.9
name = "John"
x = y = z = 0

# Constants (by convention, all caps)
MAX = 100

8.3 Input/Output

#include <stdio.h>

int main() {
    int age;
    char name[50];
    
    // Output
    printf("Hello World\n");
    printf("Age: %d\n", 25);
    
    // Input
    printf("Enter name: ");
    scanf("%s", name);
    printf("Enter age: ");
    scanf("%d", &age);
    
    return 0;
}

# No imports needed for basic I/O

# Output
print("Hello World")
print("Age:", 25)
print(f"Age: {25}")

# Input (always returns string)
name = input("Enter name: ")
age = int(input("Enter age: "))

# No main function required
# No return statement needed

8.4 Conditional Statements (if-else)

int age = 20;

if (age >= 18) {
    printf("Adult\n");
} else if (age >= 13) {
    printf("Teenager\n");
} else {
    printf("Child\n");
}

// Switch statement
switch (grade) {
    case 'A':
        printf("Excellent");
        break;
    case 'B':
        printf("Good");
        break;
    default:
        printf("Invalid");
}

age = 20

if age >= 18:
    print("Adult")
elif age >= 13:
    print("Teenager")
else:
    print("Child")

# No switch, use match (Python 3.10+)
match grade:
    case 'A':
        print("Excellent")
    case 'B':
        print("Good")
    case _:
        print("Invalid")

# Or use if-elif-else / dictionary

8.5 Loops

// Print 0 to 4
for (int i = 0; i < 5; i++) {
    printf("%d\n", i);
}

// Iterate array
int arr[] = {10, 20, 30};
int size = sizeof(arr)/sizeof(arr[0]);
for (int i = 0; i < size; i++) {
    printf("%d\n", arr[i]);
}

# Print 0 to 4
for i in range(5):
    print(i)

# Iterate list (directly!)
arr = [10, 20, 30]
for item in arr:
    print(item)

# With index
for i, item in enumerate(arr):
    print(i, item)

int i = 0;
while (i < 5) {
    printf("%d\n", i);
    i++;
}

// Do-while loop
do {
    printf("%d\n", i);
    i++;
} while (i < 5);

i = 0
while i < 5:
    print(i)
    i += 1  # No ++ operator!

# No do-while in Python
# Use while True with break
while True:
    print(i)
    i += 1
    if i >= 5:
        break

8.6 Functions

// Function declaration
int add(int a, int b);

// Function definition
int add(int a, int b) {
    return a + b;
}

// Function with no return
void greet(char name[]) {
    printf("Hello %s\n", name);
}

int main() {
    int result = add(5, 3);
    greet("John");
    return 0;
}

# Function definition (no declaration)
def add(a, b):
    return a + b

# Function with no return
def greet(name):
    print(f"Hello {name}")

# Default parameters
def greet(name="World"):
    print(f"Hello {name}")

# Multiple return values!
def get_stats(nums):
    return min(nums), max(nums)

result = add(5, 3)
greet("John")
low, high = get_stats([1,2,3])

8.7 Arrays vs Lists

// Fixed size, same type
int numbers[5] = {1, 2, 3, 4, 5};
int matrix[3][3];

// Access elements
numbers[0] = 10;
int val = numbers[2];

// Size must be known
int size = sizeof(numbers)/sizeof(numbers[0]);

// Cannot resize once created

# Dynamic size, any type
numbers = [1, 2, 3, 4, 5]
matrix = [[1,2], [3,4], [5,6]]

# Access elements
numbers[0] = 10
val = numbers[2]

# Get size easily
size = len(numbers)

# Can resize anytime!
numbers.append(6)
numbers.extend([7, 8])
numbers.pop()

8.8 Strings

#include <string.h>

char str1[20] = "Hello";
char str2[20] = "World";
char result[40];

// String length
int len = strlen(str1);

// String copy
strcpy(result, str1);

// String concatenation
strcat(result, str2);

// String comparison
if (strcmp(str1, str2) == 0) {
    printf("Equal");
}

# No imports needed

str1 = "Hello"
str2 = "World"

# String length
length = len(str1)

# String copy (just assign)
result = str1

# String concatenation
result = str1 + " " + str2

# String comparison
if str1 == str2:
    print("Equal")

# Many built-in methods!
print(str1.upper())
print(str1.lower())
print(str1.replace("l", "L"))

8.9 Comments

// Single line comment

/* Multi-line
   comment in C */

int x = 5; // Inline comment

# Single line comment

"""
Multi-line comment
(using docstring)
"""

'''
Also works with
single quotes
'''

x = 5  # Inline comment

8.10 Complete Program Comparison

Example: Find the largest of three numbers

#include <stdio.h>

int main() {
    int a, b, c, largest;
    
    printf("Enter three numbers: ");
    scanf("%d %d %d", &a, &b, &c);
    
    if (a >= b && a >= c) {
        largest = a;
    } else if (b >= a && b >= c) {
        largest = b;
    } else {
        largest = c;
    }
    
    printf("Largest: %d\n", largest);
    return 0;
}

# Much simpler!

a = int(input("Enter first number: "))
b = int(input("Enter second number: "))
c = int(input("Enter third number: "))

# Method 1: Using if-elif-else
if a >= b and a >= c:
    largest = a
elif b >= a and b >= c:
    largest = b
else:
    largest = c

# Method 2: Using built-in max()
largest = max(a, b, c)

print(f"Largest: {largest}")

8.11 Quick Reference Cheat Sheet