🚀 C++ Fundamentals - Complete Notes 📚

// Your first C++ program
#include <iostream>
using namespace std;

int main() {
    cout << "Hello, World!";
    return 0;
}

int age = 25;              // Integer literal
float pi = 3.14159;        // Floating literal
char grade = 'A';          // Character literal
string name = "John";      // String literal
bool isStudent = true;     // Boolean literal

// This is a single-line comment

/* This is a
   multi-line comment
   spanning multiple lines */

int x = 5;  // You can add comments after code too

// ❌ BAD: This adds 2 numbers
int sum = a + b;

// ✅ GOOD: Calculate total price including 10% tax
int total = price + (price * 0.10);

// Declaration (creating the box)
int age;

// Initialization (putting value in the box)
age = 20;

// Declaration + Initialization (combined)
int score = 100;

// Multiple variables of same type
int x = 5, y = 10, z = 15;

// Good names (descriptive)
int studentAge;
float accountBalance;
string userName;

// Bad names (unclear)
int x;
float f;
string s;

int age = 25;              // Whole numbers (age, count, quantity)
float temperature = 36.5;  // Precision doesn't matter much
double bankBalance = 1234567.89;  // Need high precision
char initial = 'J';        // Single character
bool isPassed = true;      // Yes/No, True/False situations
string message = "Hello";  // Text, names, sentences

#include <iostream>
using namespace std;

int main() {
    cout << "Hello World";
    return 0;
}

int age = 25;
string name = "John";

// Multiple items in one line
cout << "Name: " << name << ", Age: " << age;

Name: John, Age: 25

// New line method 1
cout << "Line 1" << endl;
cout << "Line 2" << endl;

// New line method 2 (faster)
cout << "Line 1\n";
cout << "Line 2\n";

// Formatting
cout << "Name:\t\tJohn\n";
cout << "Age:\t\t25\n";
cout << "City:\t\tNew York\n";

int a = 10, b = 3;

cout << a + b;  // 13
cout << a - b;  // 7
cout << a * b;  // 30
cout << a / b;  // 3 (integer division!)
cout << a % b;  // 1 (remainder)

int result1 = 7 / 2;      // 3 (not 3.5!)
float result2 = 7.0 / 2;  // 3.5 (correct)

int x = 5;

x++;    // x = 6 (post-increment)
++x;    // x = 7 (pre-increment)
x--;    // x = 6 (post-decrement)
--x;    // x = 5 (pre-decrement)

// Difference between pre and post
int a = 5;
int b = a++;  // b = 5, a = 6 (use then increment)
int c = ++a;  // c = 7, a = 7 (increment then use)

Code: int a = 5; int b = a++; int c = ++a;

Step 1: int a = 5;          → a = 5
Step 2: int b = a++;        → b = 5 (copy a first), then a = 6
Step 3: int c = ++a;        → a = 7 (increment first), then c = 7

Final values: a = 7, b = 5, c = 7

int age = 18;

bool isAdult = (age >= 18);      // true
bool isTeenager = (age < 20);    // true
bool isExact = (age == 18);      // true

true  && true  = true
true  && false = false
false && true  = false
false && false = false

true  || true  = true
true  || false = true
false || true  = true
false || false = false

!true  = false
!false = true

int age = 20;
bool hasLicense = true;

// Can drive if age >= 18 AND has license
bool canDrive = (age >= 18) && hasLicense;  // true

// Weekend if Saturday OR Sunday
bool isSaturday = true;
bool isSunday = false;
bool isWeekend = isSaturday || isSunday;  // true

// Not a weekday
bool isWeekday = !isWeekend;  // false

Code: int age = 20; bool hasLicense = true;
      bool canDrive = (age >= 18) && hasLicense;

Step 1: age = 20, hasLicense = true
Step 2: Evaluate (age >= 18) → (20 >= 18) = true
Step 3: Evaluate true && true = true
Step 4: canDrive = true

Code: bool isSaturday = true; bool isSunday = false;
      bool isWeekend = isSaturday || isSunday;

Step 1: isSaturday = true, isSunday = false
Step 2: Evaluate true || false = true
Step 3: isWeekend = true

Code: bool isWeekday = !isWeekend;

Step 1: isWeekend = true
Step 2: Evaluate !true = false
Step 3: isWeekday = false

1. !           (NOT)
2. *, /, %     (Multiply, Divide, Modulus)
3. +, -        (Add, Subtract)
4. <, >, <=, >= (Comparison)
5. ==, !=      (Equality)
6. &&          (AND)
7. ||          (OR)

bool isRaining = true;
bool isSunny = false;

1 (true = 1)
0 (false = 0)

int score = 85;

bool passed = (score >= 60);        // true
bool perfect = (score == 100);      // false
bool failed = (score < 60);         // false

// Login system
bool isLoggedIn = false;
bool hasPermission = true;

// Game state
bool isGameOver = false;
bool playerAlive = true;

// Form validation
bool isEmailValid = true;
bool isPasswordStrong = false;

if (condition) {
    // Code runs if condition is TRUE
} else {
    // Code runs if condition is FALSE
}

// Example 1: Simple check
int age = 20;

if (age >= 18) {
    cout << "You are an adult";
} else {
    cout << "You are a minor";
}

// Example 2: Multiple conditions
int score = 85;

if (score >= 90) {
    cout << "Grade: A";
} else {
    cout << "Grade: Not A";
}

// Example 3: Without else (optional)
bool isRaining = true;

if (isRaining) {
    cout << "Take an umbrella!";
}
// If false, nothing happens

// Normal if-else
int age = 20;
string status;
if (age >= 18) {
    status = "Adult";
} else {
    status = "Minor";
}

// Shorthand (same thing!)
string status = (age >= 18) ? "Adult" : "Minor";
//              ^condition^   ^true^   ^false^

if (condition1) {
    // Runs if condition1 is true
} else if (condition2) {
    // Runs if condition2 is true
} else if (condition3) {
    // Runs if condition3 is true
} else {
    // Runs if ALL above are false
}

// Example 1: Grade System
int marks = 85;

if (marks >= 90) {
    cout << "Grade: A+";
} else if (marks >= 80) {
    cout << "Grade: A";
} else if (marks >= 70) {
    cout << "Grade: B";
} else if (marks >= 60) {
    cout << "Grade: C";
} else {
    cout << "Grade: F";
}

Code: int marks = 85;
      if (marks >= 90) { ... }
      else if (marks >= 80) { ... }
      else if (marks >= 70) { ... }
      else if (marks >= 60) { ... }
      else { ... }

Step 1: marks = 85
Step 2: Check marks >= 90? → 85 >= 90 = false → Skip this block
Step 3: Check marks >= 80? → 85 >= 80 = true → Execute this block
Step 4: Print "Grade: A"
Step 5: Exit (remaining conditions are skipped)

Output: Grade: A

// Example 2: Traffic Light
string light = "yellow";

if (light == "red") {
    cout << "STOP!";
} else if (light == "yellow") {
    cout << "SLOW DOWN";
} else if (light == "green") {
    cout << "GO";
} else {
    cout << "Light broken!";
}

// Example 3: Age Categories
int age = 25;

if (age < 13) {
    cout << "Child";
} else if (age < 20) {
    cout << "Teenager";
} else if (age < 60) {
    cout << "Adult";
} else {
    cout << "Senior";
}

while (condition) {
    // Code to repeat
    // Update condition (important!)
}

// Example 1: Count from 1 to 5
int i = 1;
while (i <= 5) {
    cout << i << " ";
    i++;  // MUST update!
}

1 2 3 4 5
int n = 10;
int sum = 0;
int i = 1;
while (i <= n) {
sum = sum + i;
i++;
}
cout << "Sum = " << sum;   55

Code: int n = 10; int sum = 0; int i = 1;
      while (i <= n) { sum = sum + i; i++; }

Initial: n = 10, sum = 0, i = 1

Iteration 1: i = 1 ≤ 10? YES → sum = 0 + 1 = 1,  i = 2
Iteration 2: i = 2 ≤ 10? YES → sum = 1 + 2 = 3,  i = 3
Iteration 3: i = 3 ≤ 10? YES → sum = 3 + 3 = 6,  i = 4
Iteration 4: i = 4 ≤ 10? YES → sum = 6 + 4 = 10, i = 5
Iteration 5: i = 5 ≤ 10? YES → sum = 10 + 5 = 15, i = 6
Iteration 6: i = 6 ≤ 10? YES → sum = 15 + 6 = 21, i = 7
Iteration 7: i = 7 ≤ 10? YES → sum = 21 + 7 = 28, i = 8
Iteration 8: i = 8 ≤ 10? YES → sum = 28 + 8 = 36, i = 9
Iteration 9: i = 9 ≤ 10? YES → sum = 36 + 9 = 45, i = 10
Iteration 10: i = 10 ≤ 10? YES → sum = 45 + 10 = 55, i = 11
Iteration 11: i = 11 ≤ 10? NO → Loop exits

Final: sum = 55

// Example 3: User input validation
int password;
while (password != 1234) {
    cout << "Enter password: ";
    cin >> password;
}
cout << "Access granted!";

// DANGER! This never stops
int i = 1;
while (i <= 5) {
    cout << i;
    // Forgot i++; 
    // Loop runs FOREVER!
}

for (initialization; condition; update) {
    // Code to repeat
}

for (int i = 0; i < 5; i++) {
//       ^         ^      ^
//       |         |      |___ 3. Update (after each loop)
//       |         |__________ 2. Condition (check before each loop)
//       |____________________ 1. Initialization (runs once)
}

// Example 1: Print 1 to 5
for (int i = 1; i <= 5; i++) {
    cout << i << " ";
}

1 2 3 4 5

Code: for (int i = 1; i <= 5; i++) { cout << i << " "; }

Step 1: Initialize i = 1
Step 2: Check condition 1 ≤ 5? YES → Print 1, i++ → i = 2
Step 3: Check condition 2 ≤ 5? YES → Print 2, i++ → i = 3
Step 4: Check condition 3 ≤ 5? YES → Print 3, i++ → i = 4
Step 5: Check condition 4 ≤ 5? YES → Print 4, i++ → i = 5
Step 6: Check condition 5 ≤ 5? YES → Print 5, i++ → i = 6
Step 7: Check condition 6 ≤ 5? NO → Loop exits

Output: 1 2 3 4 5

// Example 2: Countdown
for (int i = 10; i >= 1; i--) {
    cout << i << " ";
}
cout << "Blast off!";

10 9 8 7 6 5 4 3 2 1 Blast off!
for (int i = 0; i <= 10; i += 2) {
cout << i << " ";
}
0 2 4 6 8 10
int n = 5;
for (int i = 1; i <= 10; i++) {
cout << n << " x " << i << " = " << n * i << endl;
}

// Pattern: 0 to N-1 (most common!)
for (int i = 0; i < n; i++) { }

// Pattern: 1 to N
for (int i = 1; i <= n; i++) { }

// Pattern: Reverse
for (int i = n; i >= 1; i--) { }

// Sum from 1 to n
int n = 10;
int sum = 0;

for (int i = 1; i <= n; i++) {
    sum += i;
}
cout << "Sum = " << sum;  // 55

// 5! = 5 × 4 × 3 × 2 × 1 = 120
int n = 5;
int factorial = 1;

for (int i = 1; i <= n; i++) {
    factorial *= i;
}
cout << "Factorial = " << factorial;  // 120

Code: int n = 5; int factorial = 1;
      for (int i = 1; i <= n; i++) { factorial *= i; }

Initial: n = 5, factorial = 1

Iteration 1: i = 1 → factorial = 1 × 1 = 1
Iteration 2: i = 2 → factorial = 1 × 2 = 2
Iteration 3: i = 3 → factorial = 2 × 3 = 6
Iteration 4: i = 4 → factorial = 6 × 4 = 24
Iteration 5: i = 5 → factorial = 24 × 5 = 120

Final: factorial = 120

// Pattern 1: Square
// * * * *
// * * * *
// * * * *
// * * * *

for (int i = 1; i <= 4; i++) {
    for (int j = 1; j <= 4; j++) {
        cout << "* ";
    }
    cout << endl;
}

// Pattern 2: Right Triangle
// *
// * *
// * * *
// * * * *

for (int i = 1; i <= 4; i++) {
    for (int j = 1; j <= i; j++) {
        cout << "* ";
    }
    cout << endl;
}

// Pattern 3: Numbers
// 1
// 1 2
// 1 2 3
// 1 2 3 4

for (int i = 1; i <= 4; i++) {
    for (int j = 1; j <= i; j++) {
        cout << j << " ";
    }
    cout << endl;
}

int num = 1234;
int reversed = 0;

while (num > 0) {
    int digit = num % 10;      // Get last digit
    reversed = reversed * 10 + digit;
    num = num / 10;            // Remove last digit
}
cout << reversed;  // 4321

Code: int num = 1234; int reversed = 0;
      while (num > 0) {
          int digit = num % 10;
          reversed = reversed * 10 + digit;
          num = num / 10;
      }

Initial: num = 1234, reversed = 0

Iteration 1:
  digit = 1234 % 10 = 4
  reversed = 0 × 10 + 4 = 4
  num = 1234 / 10 = 123

Iteration 2:
  digit = 123 % 10 = 3
  reversed = 4 × 10 + 3 = 43
  num = 123 / 10 = 12

Iteration 3:
  digit = 12 % 10 = 2
  reversed = 43 × 10 + 2 = 432
  num = 12 / 10 = 1

Iteration 4:
  digit = 1 % 10 = 1
  reversed = 432 × 10 + 1 = 4321
  num = 1 / 10 = 0

Iteration 5: num = 0 → Loop exits

Final: reversed = 4321

int num = 12345;
int count = 0;

while (num > 0) {
    count++;
    num = num / 10;
}
cout << "Digits: " << count;  // 5

int n = 17;
bool isPrime = true;

for (int i = 2; i < n; i++) {
    if (n % i == 0) {
        isPrime = false;
        break;
    }
}

if (isPrime && n > 1) {
    cout << n << " is prime";
}

// 0, 1, 1, 2, 3, 5, 8, 13...
int n = 10;
int a = 0, b = 1;

cout << a << " " << b << " ";

for (int i = 2; i < n; i++) {
    int next = a + b;
    cout << next << " ";
    a = b;
    b = next;
}

Code: int n = 7; int a = 0, b = 1;
      cout << a << " " << b << " ";
      for (int i = 2; i < n; i++) {
          int next = a + b;
          cout << next << " ";
          a = b; b = next;
      }

Initial: n = 7, a = 0, b = 1
Print: 0 1

Iteration 1 (i=2):
  next = 0 + 1 = 1 → Print: 1
  a = 1, b = 1

Iteration 2 (i=3):
  next = 1 + 1 = 2 → Print: 2
  a = 1, b = 2

Iteration 3 (i=4):
  next = 1 + 2 = 3 → Print: 3
  a = 2, b = 3

Iteration 4 (i=5):
  next = 2 + 3 = 5 → Print: 5
  a = 3, b = 5

Iteration 5 (i=6):
  next = 3 + 5 = 8 → Print: 8
  a = 5, b = 8

Iteration 6: i = 7 → Loop exits

Output: 0 1 1 2 3 5 8

// Example 1: Exit when found
for (int i = 1; i <= 10; i++) {
    if (i == 5) {
        break;  // Stop here!
    }
    cout << i << " ";
}

1 2 3 4

Code: for (int i = 1; i <= 10; i++) {
          if (i == 5) { break; }
          cout << i << " ";
      }

Iteration 1: i = 1 → Check i == 5? NO → Print 1
Iteration 2: i = 2 → Check i == 5? NO → Print 2
Iteration 3: i = 3 → Check i == 5? NO → Print 3
Iteration 4: i = 4 → Check i == 5? NO → Print 4
Iteration 5: i = 5 → Check i == 5? YES → break → Exit loop

Output: 1 2 3 4

// Example 2: Search in array
int arr[] = {10, 20, 30, 40, 50};
int target = 30;
bool found = false;

for (int i = 0; i < 5; i++) {
    if (arr[i] == target) {
        found = true;
        break;  // No need to check further
    }
}

// Example 1: Skip even numbers
for (int i = 1; i <= 10; i++) {
    if (i % 2 == 0) {
        continue;  // Skip rest of code
    }
    cout << i << " ";
}

1 3 5 7 9

Code: for (int i = 1; i <= 6; i++) {
          if (i % 2 == 0) { continue; }
          cout << i << " ";
      }

Iteration 1: i = 1 → Check 1 % 2 == 0? NO → Print 1
Iteration 2: i = 2 → Check 2 % 2 == 0? YES → continue → Skip print
Iteration 3: i = 3 → Check 3 % 2 == 0? NO → Print 3
Iteration 4: i = 4 → Check 4 % 2 == 0? YES → continue → Skip print
Iteration 5: i = 5 → Check 5 % 2 == 0? NO → Print 5
Iteration 6: i = 6 → Check 6 % 2 == 0? YES → continue → Skip print

Output: 1 3 5

// Example 2: Skip negative numbers
int arr[] = {5, -2, 8, -1, 10};

for (int i = 0; i < 5; i++) {
    if (arr[i] < 0) {
        continue;  // Skip negative
    }
    cout << arr[i] << " ";
}

5 8 10

// Login attempts (break after success)
int attempts = 0;
while (attempts < 3) {
    int password;
    cout << "Enter password: ";
    cin >> password;

    if (password == 1234) {
        cout << "Access granted!";
        break;  // Stop asking
    }
    attempts++;
}

// Process only valid data (continue for invalid)
for (int i = 0; i < 10; i++) {
    if (data[i] == 0) {
        continue;  // Skip zero values
    }
    processData(data[i]);
}

switch (variable) {
    case value1:
        // Code
        break;
    case value2:
        // Code
        break;
    default:
        // Code if no match
}

// Example 1: Days of week
int day = 3;

switch (day) {
    case 1:
        cout << "Monday";
        break;
    case 2:
        cout << "Tuesday";
        break;
    case 3:
        cout << "Wednesday";
        break;
    case 4:
        cout << "Thursday";
        break;
    case 5:
        cout << "Friday";
        break;
    case 6:
        cout << "Saturday";
        break;
    case 7:
        cout << "Sunday";
        break;
    default:
        cout << "Invalid day";
}

// Example 2: Calculator
char op = '+';
int a = 10, b = 5;

switch (op) {
    case '+':
        cout << a + b;
        break;
    case '-':
        cout << a - b;
        break;
    case '*':
        cout << a * b;
        break;
    case '/':
        cout << a / b;
        break;
    default:
        cout << "Invalid operator";
}

// Example 3: Grade evaluation
char grade = 'B';

switch (grade) {
    case 'A':
        cout << "Excellent!";
        break;
    case 'B':
        cout << "Good job!";
        break;
    case 'C':
        cout << "Well done!";
        break;
    case 'D':
        cout << "You passed!";
        break;
    case 'F':
        cout << "Better luck next time";
        break;
    default:
        cout << "Invalid grade";
}

// WITHOUT break (fall-through)
int x = 2;

switch (x) {
    case 1:
        cout << "One ";
    case 2:
        cout << "Two ";   // Starts here
    case 3:
        cout << "Three ";  // Also runs!
}

Two Three
switch (x) {
case 1:
cout << "One";
break;
case 2:
cout << "Two";     Only this runs
break;
case 3:
cout << "Three";
break;
}
Two

returnType functionName(parameters) {
    // Code to execute
    return value;  // if not void
}

// Function structure:
int addNumbers(int a, int b) {
//  ^      ^          ^
//  |      |          |__ Parameters (input)
//  |      |_____________ Function name
//  |____________________ Return type (output)

    int sum = a + b;
    return sum;        // Send result back
}

// Example 1: Simple greeting
void greet() {
    cout << "Hello, World!";
}

// Calling the function

Hello, World!
void greetUser(string name) {
cout << "Hello, " << name << "!";
}
greetUser("John");    Hello, John!
greetUser("Sarah");   Hello, Sarah!
int square(int num) {
return num * num;
}
int result = square(5);   result = 25
cout << square(7);        49
int multiply(int a, int b) {
return a * b;
}
cout << multiply(4, 5);   20

// Without functions (repetitive!)
cout << "Welcome to our app!";
cout << "Please login";
// ... 50 more lines
cout << "Welcome to our app!";  // Repeated!
cout << "Please login";

// With functions (clean!)
void showWelcome() {
    cout << "Welcome to our app!";
    cout << "Please login";
}

showWelcome();  // Use anywhere!
// ... other code
showWelcome();  // Easy to reuse!

void greet(string name) {  // 'name' is a PARAMETER
    cout << "Hello " << name;
}

greet("John");  // "John" is an ARGUMENT

// Example 1: Single parameter
void printSquare(int num) {
    cout << num * num;
}

printSquare(5);   // 25
printSquare(10);  // 100

// Example 2: Multiple parameters
int add(int a, int b) {
    return a + b;
}

cout << add(5, 3);      // 8
cout << add(10, 20);    // 30

// Example 3: Different types
void displayInfo(string name, int age, float height) {
    cout << "Name: " << name << endl;
    cout << "Age: " << age << endl;
    cout << "Height: " << height << endl;
}

displayInfo("John", 25, 5.9);
// Name: John
// Age: 25
// Height: 5.9

// Example 4: Default parameters
void greet(string name = "Guest") {
    cout << "Hello " << name;
}

greet("John");  // Hello John
greet();        // Hello Guest (uses default)

// Pass by VALUE (copy)
void changeValue(int x) {
    x = 100;  // Only changes the copy
}

int num = 5;
changeValue(num);
cout << num;  // Still 5 (original unchanged)

// Pass by REFERENCE (original)
void changeReference(int &x) {
    x = 100;  // Changes the original!
}

int num = 5;
changeReference(num);
cout << num;  // Now 100 (original changed)

// Use pass by value for:
// - Small data (int, float, char)
// - When you don't want to change original

int calculate(int x) { }  // ✅

// Use pass by reference for:
// - Large data (arrays, objects)
// - When you want to modify original

void swap(int &a, int &b) {  // ✅
    int temp = a;
    a = b;
    b = temp;
}

int x = 5, y = 10;
swap(x, y);
// Now x = 10, y = 5

returnType functionName() {
    // Code
    return value;  // Must match returnType
}

// Example 1: Return integer
int getAge() {
    return 25;
}

int age = getAge();  // age = 25

// Example 2: Return calculation
int add(int a, int b) {
    return a + b;
}

int sum = add(5, 3);  // sum = 8

// Example 3: Return boolean
bool isEven(int num) {
    if (num % 2 == 0) {
        return true;
    } else {
        return false;
    }
}

// Shorter version:
bool isEven(int num) {
    return (num % 2 == 0);
}

// Example 4: Return string
string getGreeting(int hour) {
    if (hour < 12) {
        return "Good morning";
    } else if (hour < 18) {
        return "Good afternoon";
    } else {
        return "Good evening";
    }
}

cout << getGreeting(10);  // Good morning

void printMessage() {
    cout << "Hello!";
    // No return needed
}

// Can use return to exit early
void checkAge(int age) {
    if (age < 0) {
        cout << "Invalid age";
        return;  // Exit function early
    }
    cout << "Valid age";
}

// Function exits at first return
string checkGrade(int marks) {
    if (marks >= 90) {
        return "A";  // Exits here if true
    }
    if (marks >= 80) {
        return "B";  // Exits here if true
    }
    if (marks >= 70) {
        return "C";
    }
    return "F";  // Default return
}

// ❌ WRONG: No return in else
int getMax(int a, int b) {
    if (a > b) {
        return a;
    }
    // Missing return! Error!
}

// ✅ CORRECT
int getMax(int a, int b) {
    if (a > b) {
        return a;
    }
    return b;  // All paths covered
}

void myFunction() {
    int x = 10;  // Local to myFunction
    cout << x;   // ✅ Works
}

int main() {
    myFunction();
    cout << x;   // ❌ ERROR! x doesn't exist here
}

// Example 1: Block scope
void example() {
    int x = 5;  // Local to function

    if (x > 0) {
        int y = 10;  // Local to if block
        cout << x;   // ✅ Can use x
        cout << y;   // ✅ Can use y
    }

    cout << x;  // ✅ Can use x
    cout << y;  // ❌ ERROR! y doesn't exist here
}

// Example 2: Loop scope
void loopExample() {
    for (int i = 0; i < 5; i++) {
        int temp = i * 2;  // Created each iteration
        cout << temp;
    }
    cout << i;     // ❌ ERROR! i is local to loop
    cout << temp;  // ❌ ERROR! temp is local to loop
}

// Example 3: Same names, different scopes
void function1() {
    int num = 10;
    cout << num;  // 10
}

void function2() {
    int num = 20;  // Different variable!
    cout << num;  // 20
}

int globalVar = 100;  // Global

void myFunction() {
    int localVar = 50;  // Local

    cout << globalVar;  // ✅ Can use global
    cout << localVar;   // ✅ Can use local
}

int main() {
    cout << globalVar;  // ✅ Can use global
    cout << localVar;   // ❌ ERROR! Can't use local
}

void example() {
    int x = 5;     // Created here
    cout << x;     // Exists
    // x destroyed here
}

// Call function multiple times
example();  // x created and destroyed
example();  // NEW x created and destroyed
example();  // Another NEW x created and destroyed

// ❌ Hard-coded, not reusable
void addNumbers() {
    int a = 5;
    int b = 10;
    int sum = a + b;
    cout << sum;
}

addNumbers();  // Always adds 5 + 10
// Can't add different numbers!

// ✅ Flexible and reusable
int addNumbers(int a, int b) {
    return a + b;
}

cout << addNumbers(5, 10);    // 15
cout << addNumbers(20, 30);   // 50
cout << addNumbers(100, 200); // 300
// Same function, different values!

// One function, many uses
int square(int n) {
    return n * n;
}

int area1 = square(5);   // 25
int area2 = square(10);  // 100
int area3 = square(15);  // 225

// Adapt to different situations
double calculatePrice(double price, double tax) {
    return price + (price * tax);
}

double item1 = calculatePrice(100, 0.10);  // $110
double item2 = calculatePrice(50, 0.05);   // $52.50

// Combine functions like LEGO
int add(int a, int b) { return a + b; }
int multiply(int a, int b) { return a * b; }

// Build complex operations
int result = multiply(add(2, 3), 4);  // (2+3) * 4 = 20

int getAge() { return 25; }

// Use in conditions
if (getAge() >= 18) {
    cout << "Adult";
}

// Use in calculations
int years = 2024 - getAge();

// Use in other functions
checkAge(getAge());

// Easy to test with different inputs
bool isPrime(int n) {
    // Check if prime
    return result;
}

// Test multiple cases
cout << isPrime(2);   // true
cout << isPrime(4);   // false
cout << isPrime(17);  // true

// Like a vending machine
item getProduct(int code, double money) {
    // Input: product code and money

your product
return selectedItem;
}

#include <cmath>

// Power and roots
pow(2, 3);     // 2^3 = 8
sqrt(16);      // 4
cbrt(27);      // 3 (cube root)

// Rounding
ceil(4.3);     // 5 (round up)
floor(4.7);    // 4 (round down)
round(4.5);    // 5 (round nearest)

// Absolute value
abs(-5);       // 5
fabs(-3.14);   // 3.14 (float absolute)

// Trigonometry
sin(0);        // 0
cos(0);        // 1
tan(0);        // 0

// Min/Max
max(5, 10);    // 10
min(5, 10);    // 5

#include <iostream>
#include <cmath>
using namespace std;

int main() {
    // Calculate area of circle
    double radius = 5;
    double area = M_PI * pow(radius, 2);
    cout << "Area: " << area << endl;

    // Distance between two points
    int x1 = 0, y1 = 0;
    int x2 = 3, y2 = 4;
    double distance = sqrt(pow(x2-x1, 2) + pow(y2-y1, 2));
    cout << "Distance: " << distance << endl;  // 5

    return 0;
}

#include <string>

string str = "Hello World";

// Length
str.length();      // 11
str.size();        // 11 (same as length)

// Access characters
str[0];            // 'H'
str.at(1);         // 'e' (safer, checks bounds)

// Modify
str.append("!");   // "Hello World!"
str.insert(5, " Beautiful");  // "Hello Beautiful World"
str.erase(0, 6);   // "World"
str.replace(0, 5, "Earth");  // "Earth"

// Search
str.find("World"); // 6 (position)
str.find("xyz");   // string::npos (not found)

// Substrings
str.substr(0, 5);  // "Hello" (from 0, length 5)

// Compare
str.compare("Hello");  // 0 if equal

#include <algorithm>

// Min/Max
max(5, 10);        // 10
min(5, 10);        // 5

// Swap
int a = 5, b = 10;
swap(a, b);        // a=10, b=5

// Sort arrays
int arr[] = {5, 2, 8, 1, 9};
sort(arr, arr + 5);  // {1, 2, 5, 8, 9}

// Reverse
reverse(arr, arr + 5);  // {9, 8, 5, 2, 1}

#include <iostream>
#include <cmath>
#include <string>
#include <algorithm>
using namespace std;

int main() {
    // Math
    cout << sqrt(16);        // 4
    cout << pow(2, 3);       // 8

    // String
    string name = "John";
    cout << name.length();   // 4

    // Algorithm
    cout << max(5, 10);      // 10

    return 0;
}

// Function A calls Function B
int functionB() {
    return 10;
}

int functionA() {
    int result = functionB();  // Calling B from A
    return result * 2;
}

int main() {
    cout << functionA();  // 20
}

// Example 1: Mathematical operations
int square(int n) {
    return n * n;
}

int sumOfSquares(int a, int b) {
    return square(a) + square(b);  // Calling square() twice
}

cout << sumOfSquares(3, 4);  // 3² + 4² = 9 + 16 = 25

// Example 2: Validation
bool isPositive(int n) {
    return n > 0;
}

bool isEven(int n) {
    return n % 2 == 0;
}

bool isPositiveEven(int n) {
    return isPositive(n) && isEven(n);  // Both functions!
}

cout << isPositiveEven(4);   // true
cout << isPositiveEven(-4);  // false
cout << isPositiveEven(5);   // false

// Example 3: Complex calculation
int add(int a, int b) {
    return a + b;
}

int multiply(int a, int b) {
    return a * b;
}

int calculate(int a, int b, int c) {
    return multiply(add(a, b), c);  // (a + b) * c
}

cout << calculate(2, 3, 4);  // (2+3)*4 = 20

// Factorial using recursion
int factorial(int n) {
    if (n <= 1) {
        return 1;  // Base case
    }
    return n * factorial(n - 1);  // Calls itself!
}

cout << factorial(5);
// 5 * factorial(4)
//     4 * factorial(3)
//         3 * factorial(2)
//             2 * factorial(1)
//                 returns 1
// = 5 * 4 * 3 * 2 * 1 = 120

// Area of circle
double square(double n) {
    return n * n;
}

double circleArea(double radius) {
    return 3.14159 * square(radius);  // Uses square()
}

// Volume of cylinder
double cylinderVolume(double radius, double height) {
    return circleArea(radius) * height;  // Uses circleArea()
}

cout << cylinderVolume(5, 10);
// circleArea(5) = 78.54
// 78.54 * 10 = 785.4

// ❌ WRONG: Can't define inside
void outer() {
    void inner() {  // ERROR!
        cout << "Hi";
    }
}

// ✅ CORRECT: Define separately, call from inside
void inner() {
    cout << "Hi";
}

void outer() {
    inner();  // ✅ This works!
}

Index:   0   1   2   3   4
Array: [10][20][30][40][50]

// Method 1: Declare then assign
int numbers[5];          // Array of 5 integers
numbers[0] = 10;
numbers[1] = 20;

// Method 2: Declare with values
int numbers[5] = {10, 20, 30, 40, 50};

// Method 3: Auto size
int numbers[] = {10, 20, 30, 40, 50};  // Size = 5 automatically

// Method 4: Partial initialization
int numbers[5] = {10, 20};  // {10, 20, 0, 0, 0} (rest are 0)

// Method 5: All zeros
int numbers[5] = {0};  // {0, 0, 0, 0, 0}

int numbers[5] = {10, 20, 30, 40, 50};

// Access by index (0-based)
cout << numbers[0];  // 10 (first element)
cout << numbers[4];  // 50 (last element)

// Modify elements
numbers[2] = 100;  // {10, 20, 100, 40, 50}

// Use in expressions
int sum = numbers[0] + numbers[1];  // 30

Array: numbers[5] = {10, 20, 30, 40, 50}

Index:     0    1    2    3    4
Value:    10   20   30   40   50
         ↑                      ↑
       First                 Last
   numbers[0]           numbers[4]

// Example 1: Student scores
int scores[5] = {85, 90, 78, 92, 88};
cout << "First score: " << scores[0];  // 85
cout << "Last score: " << scores[4];   // 88

// Example 2: Days in months
int daysInMonth[12] = {31, 28, 31, 30, 31, 30, 
                       31, 31, 30, 31, 30, 31};
cout << "Days in January: " << daysInMonth[0];   // 31
cout << "Days in February: " << daysInMonth[1];  // 28

// Example 3: Character array
char vowels[5] = {'a', 'e', 'i', 'o', 'u'};
cout << vowels[2];  // 'i'