📦 Lesson 2.1: Variables and Constants

# Python: just assign a value — no keyword needed
name = "Alex"
age = 25
is_student = True

print(name)       # Alex
print(age)        # 25
print(is_student) # True

// JavaScript: use let or const to declare
let name = "Alex";
let age = 25;
let isStudent = true;

console.log(name);      // Alex
console.log(age);       // 25
console.log(isStudent); // true

// C#: declare the type before the name
string name = "Alex";
int age = 25;
bool isStudent = true;

Console.WriteLine(name);      // Alex
Console.WriteLine(age);       // 25
Console.WriteLine(isStudent); // True

# Python: reassign freely — even change the type!
score = 0
print(score)   # 0

score = 100
print(score)   # 100

# Python even lets you change the type
score = "one hundred"
print(score)   # one hundred (now it's a string!)

// JavaScript with let: reassign freely
let score = 0;
console.log(score);  // 0

score = 100;
console.log(score);  // 100

// JavaScript also lets you change the type
score = "one hundred";
console.log(score);  // one hundred

// C#: reassign to same type only
int score = 0;
Console.WriteLine(score);  // 0

score = 100;
Console.WriteLine(score);  // 100

// This would cause an ERROR in C#:
// score = "one hundred";
// Cannot convert string to int!

# Python: no true constants — use ALL_CAPS by convention
# Other programmers know not to change these
MAX_SPEED = 299792458   # speed of light in m/s
PI = 3.14159
DAYS_IN_WEEK = 7

# Nothing stops you from changing it (Python trusts you)
# MAX_SPEED = 0  # technically works, but DON'T do this

print(f"Speed of light: {MAX_SPEED} m/s")

// JavaScript: use const instead of let
const MAX_SPEED = 299792458;
const PI = 3.14159;
const DAYS_IN_WEEK = 7;

// Trying to change a const causes an ERROR:
// MAX_SPEED = 0;  // TypeError: Assignment to constant variable

console.log(`Speed of light: ${MAX_SPEED} m/s`);

// C#: use the const keyword before the type
const int MaxSpeed = 299792458;
const double Pi = 3.14159;
const int DaysInWeek = 7;

// Trying to change a const causes a COMPILE ERROR:
// MaxSpeed = 0;  // Error: cannot assign to const

Console.WriteLine($"Speed of light: {MaxSpeed} m/s");

# Python uses snake_case for variables and functions
first_name = "Alex"
last_name = "Johnson"
birth_year = 1998
is_enrolled = True

# Constants use ALL_CAPS
MAX_RETRIES = 3
DATABASE_URL = "localhost:5432"

# Classes use PascalCase (you'll learn about these later)
# class StudentProfile:

// JavaScript uses camelCase for variables and functions
let firstName = "Alex";
let lastName = "Johnson";
let birthYear = 1998;
let isEnrolled = true;

// Constants often use ALL_CAPS (but camelCase is also fine)
const MAX_RETRIES = 3;
const databaseUrl = "localhost:5432"; // also acceptable

// Classes use PascalCase
// class StudentProfile {}

// C# uses camelCase for local variables
string firstName = "Alex";
string lastName = "Johnson";
int birthYear = 1998;
bool isEnrolled = true;

// Constants use PascalCase
const int MaxRetries = 3;
const string DatabaseUrl = "localhost:5432";

// Classes and public members use PascalCase
// class StudentProfile {}

# Python: check types at runtime with type()
value = 42
print(type(value))   # <class 'int'>

value = "hello"
print(type(value))   # <class 'str'>
# No error — Python is fine with this!

// JavaScript: check types at runtime with typeof
let value = 42;
console.log(typeof value);  // "number"

value = "hello";
console.log(typeof value);  // "string"
// No error — JavaScript is fine with this!

// C#: type is fixed at declaration
int value = 42;
Console.WriteLine(value.GetType());  // System.Int32

// This would NOT compile:
// value = "hello";
// Error: Cannot convert string to int

// C# also has 'var' for type inference:
var inferred = 42;  // C# figures out it's an int
// But it's still locked as int after this!