📝 Lesson 4: Control Structures

What Are Control Structures?

Imagine you're giving directions to a friend. You don't just say "go straight forever" - you say things like "IF you see the red building, TURN LEFT" or "KEEP GOING UNTIL you reach the park." Control structures let your program make similar decisions!

The IF Statement: Your Program's First Decision

The IF statement is like a bouncer at a club - it checks if a condition is true before letting code execute.

Basic IF Statement Syntax

if (condition) {
    // Code runs ONLY if condition is true
    cout << "Condition was true!" << '\n';
}

Real-World Example: Temperature Checker

int temperature = 75;

if (temperature > 70) {
    cout << "It's a warm day!" << '\n';
    cout << "Perfect for outdoor activities." << '\n';
}

if (temperature > 90) {
    cout << "It's very hot! Stay hydrated!" << '\n';
}

if (int statusCode = getStatusCode(); statusCode == 200) {
    cout << "Success!" << '\n';
} else {
    cout << "Failed with code " << statusCode << '\n';  // still visible in else
}
// statusCode is NOT visible out here

The IF-ELSE Statement: Either This OR That

IF-ELSE is like a fork in the road - you MUST go one way or the other, never both!

IF-ELSE Example: Pass/Fail Grade

int score = 75;

if (score >= 60) {
    cout << "Congratulations! You passed!" << '\n';
    cout << "Your score: " << score << '\n';
} else {
    cout << "Sorry, you didn't pass." << '\n';
    cout << "You need at least 60 to pass." << '\n';
}

The IF-ELSE-IF Ladder: Multiple Choices

Sometimes life isn't just yes or no - it's multiple choice! The IF-ELSE-IF ladder handles multiple conditions.

Complete Grade Calculator Example

int score = 85;
char grade;

if (score >= 90) {
    grade = 'A';
    cout << "Excellent work!" << '\n';
} else if (score >= 80) {
    grade = 'B';
    cout << "Good job!" << '\n';
} else if (score >= 70) {
    grade = 'C';
    cout << "Satisfactory." << '\n';
} else if (score >= 60) {
    grade = 'D';
    cout << "Needs improvement." << '\n';
} else {
    grade = 'F';
    cout << "Please see your instructor." << '\n';
}

cout << "Your grade is: " << grade << '\n';

Nested IF Statements: Decisions Within Decisions

Like Russian nesting dolls, you can put IF statements inside other IF statements!

Nested IF Example: Event Planning

bool isWeekend = true;
bool isRaining = false;
int temperature = 75;

if (isWeekend) {
    cout << "It's the weekend!" << '\n';

    if (isRaining) {
        cout << "Stay inside and watch movies." << '\n';
    } else {
        if (temperature > 70) {
            cout << "Perfect day for a picnic!" << '\n';
        } else {
            cout << "Maybe visit a museum." << '\n';
        }
    }
} else {
    cout << "It's a weekday - time for work/school!" << '\n';
}

The Switch Statement: Multiple Choice Made Easy

When you have many specific values to check, SWITCH is like a more elegant multiple-choice answer sheet.

Switch Statement Syntax

int choice = 2;

switch (choice) {
    case 1:
        cout << "You ordered a hamburger!" << '\n';
        break;
    case 2:
        cout << "You ordered pizza!" << '\n';
        break;
    case 3:
        cout << "You ordered a salad!" << '\n';
        break;
    case 4:
        cout << "You ordered pasta!" << '\n';
        break;
    default:
        cout << "Invalid choice!" << '\n';
        break;
}

Important: The Break Statement

Without 'break', switch cases "fall through" like dominoes!

switch (choice) {
    case 1:
    case 2:  // cases 1 and 2 stacked with no code between — a safe, common fall-through
        cout << "Low option" << '\n';
        break;
    case 3:
        cout << "Handling 3..." << '\n';
        [[fallthrough]];  // intentional: also run case 4's code
    case 4:
        cout << "Handling 4" << '\n';
        break;
}

Ternary Operator: The Compact IF-ELSE

The ternary operator is like IF-ELSE compressed into one line - perfect for simple decisions!

Ternary Operator Examples

// Example 1: Simple assignment
int age = 20;
string status = (age >= 18) ? "Adult" : "Minor";

// Example 2: In output
int score = 85;
cout << "You " << (score >= 60 ? "passed" : "failed") << " the test!" << '\n';

// Example 3: Calculating values
int items = 5;
double price = (items > 3) ? 9.99 : 12.99;

Practice Exercise: Adventure Game

#include <iostream>
using namespace std;

int main() {
    int choice;

    cout << "=== The Mysterious Cave ===" << '\n';
    cout << "You stand at the entrance of a dark cave." << '\n';
    cout << "What do you do?" << '\n';
    cout << "1. Enter the cave" << '\n';
    cout << "2. Walk away" << '\n';
    cout << "3. Call for help" << '\n';
    cout << "Enter your choice (1-3): ";
    cin >> choice;

    // TODO: Use switch or if-else to handle choices
    // Each choice should lead to different outcomes

    return 0;
}

switch (choice) {
    case 1:
        cout << "\nYou bravely enter the cave..." << '\n';
        cout << "You find a treasure chest!" << '\n';
        break;
    case 2:
        cout << "\nYou decide to play it safe." << '\n';
        cout << "Maybe another day..." << '\n';
        break;
    case 3:
        cout << "\nYou call out loudly." << '\n';
        cout << "A friendly explorer appears to help!" << '\n';
        break;
    default:
        cout << "\nThat's not a valid choice!" << '\n';
}

Common Patterns and Best Practices

Debugging Tips: Common Control Structure Mistakes

Challenge Exercise: Grade Report System

Real-World Application: ATM Machine

// Simplified ATM System
double balance = 1000.00;
int choice;
double amount;

cout << "=== Welcome to SimpleBank ATM ===" << '\n';
cout << "1. Check Balance" << '\n';
cout << "2. Withdraw" << '\n';
cout << "3. Deposit" << '\n';
cout << "4. Exit" << '\n';
cout << "Enter choice: ";
cin >> choice;

switch (choice) {
    case 1:
        cout << "Your balance is: $" << balance << '\n';
        break;

    case 2:
        cout << "Enter withdrawal amount: $";
        cin >> amount;
        if (amount > balance) {
            cout << "Insufficient funds!" << '\n';
        } else if (amount <= 0) {
            cout << "Invalid amount!" << '\n';
        } else {
            balance -= amount;
            cout << "Dispensing $" << amount << '\n';
            cout << "New balance: $" << balance << '\n';
        }
        break;

    case 3:
        cout << "Enter deposit amount: $";
        cin >> amount;
        if (amount > 0) {
            balance += amount;
            cout << "Deposit successful!" << '\n';
            cout << "New balance: $" << balance << '\n';
        } else {
            cout << "Invalid amount!" << '\n';
        }
        break;

    case 4:
        cout << "Thank you for using SimpleBank!" << '\n';
        break;

    default:
        cout << "Invalid choice!" << '\n';
}

🎯 Quick Quiz

Summary

📚 Additional Resources

• cppreference — if statement
• cppreference — switch statement
• LearnCpp — Conditional expressions

🚀 What's Next?

You can now make your programs decide between paths - but so far each decision has run once. Next, in Lesson 5: Loops, you'll learn how to repeat those decisions and actions automatically with for, while, and do-while loops, so your programs can process many values without you writing the same code over and over.