📝 Lesson 3.1: Making Decisions with if and switch

Programs That Branch

Until now, your programs have run straight through, top to bottom. But decisions let a program take different paths depending on the situation — this is called control flow.

The condition is always something that evaluates to a bool — exactly the true/false results you built with comparison and logical operators in Lesson 2.2.

The if Statement

An if statement runs a block of code only when its condition is true. The shape is: the keyword if, a condition in parentheses, and a block in braces:

int age = 20;

if (age >= 18)
{
    Console.WriteLine("You are an adult.");
}

If the condition age >= 18 is true, the line inside the braces runs. If it's false, C# skips the whole block and moves on. With age set to 20, this prints:

You are an adult.

Change age to 15 and nothing prints — the condition is false, so the block is skipped.

💡 Read it aloud: "If age is at least 18, then print 'You are an adult.'" C# code often maps directly onto plain-English sentences like this.

else and else if

An else block runs when the if condition is false — the "otherwise" path:

int age = 15;

if (age >= 18)
{
    Console.WriteLine("You are an adult.");
}
else
{
    Console.WriteLine("You are a minor.");
}

Exactly one of the two blocks runs — never both, never neither.

Choosing among several options with else if

To test several conditions in order, chain them with else if. C# checks each condition top to bottom and runs the first one that's true, then stops checking the rest:

int score = 82;

if (score >= 90)
{
    Console.WriteLine("Grade: A");
}
else if (score >= 80)
{
    Console.WriteLine("Grade: B");
}
else if (score >= 70)
{
    Console.WriteLine("Grade: C");
}
else
{
    Console.WriteLine("Grade: F");
}

Grade: B

if (score >= 80 && score < 90)
{
    Console.WriteLine("Solid B work!");
}

Blocks, Braces, and Style

The braces { } group statements into a block — all the code that belongs to the if. You can put many statements inside one block:

if (age >= 18)
{
    Console.WriteLine("You are an adult.");
    Console.WriteLine("You can vote.");
    Console.WriteLine("Welcome!");
}

Nested if

You can place an if inside another if to check a follow-up condition only when the first is true:

if (hasTicket)
{
    if (age >= 18)
    {
        Console.WriteLine("Enjoy the show!");
    }
    else
    {
        Console.WriteLine("Ticket holders must be 18+.");
    }
}

Nesting is useful, but too much of it gets hard to read. Often you can flatten it by combining conditions with && instead.

The switch Statement

When you're comparing one value against many fixed possibilities, a long else if chain gets repetitive. A switch is cleaner for this:

int day = 3;

switch (day)
{
    case 1:
        Console.WriteLine("Monday");
        break;
    case 2:
        Console.WriteLine("Tuesday");
        break;
    case 3:
        Console.WriteLine("Wednesday");
        break;
    default:
        Console.WriteLine("Another day");
        break;
}

Wednesday

How it works:

• switch (day) — the value being tested.
• Each case lists a value to match. When one matches, its statements run.
• break; ends that case and exits the switch.
• default: is the catch-all, running when no case matches (like a final else).

Modern Shortcuts

You'll encounter two compact forms in modern C#. You don't have to use them yet, but it helps to recognize them.

The switch expression

A newer, tidier form of switch that produces a value. The => means "results in," and _ is the catch-all:

int day = 3;

string name = day switch
{
    1 => "Monday",
    2 => "Tuesday",
    3 => "Wednesday",
    _ => "Another day"
};

Console.WriteLine(name);   // Wednesday

The ternary operator

For a simple either/or choice, the ternary operator ?: picks between two values in one line. Read it as "condition ? value-if-true : value-if-false":

int age = 20;
string status = (age >= 18) ? "adult" : "minor";
Console.WriteLine(status);   // adult

Exercise & Quiz

Console.Write("Enter your score (0-100): ");
int score = int.Parse(Console.ReadLine());

// TODO: if / else if chain to print a letter grade

Console.Write("Choose a menu option (1-3): ");
int choice = int.Parse(Console.ReadLine());

// TODO: switch on choice to print Play / Settings / Quit

Console.Write("Enter your score (0-100): ");
int score = int.Parse(Console.ReadLine());

if (score >= 90)
{
    Console.WriteLine("Grade: A");
}
else if (score >= 80)
{
    Console.WriteLine("Grade: B");
}
else if (score >= 70)
{
    Console.WriteLine("Grade: C");
}
else if (score >= 60)
{
    Console.WriteLine("Grade: D");
}
else
{
    Console.WriteLine("Grade: F");
}

Console.Write("Choose a menu option (1-3): ");
int choice = int.Parse(Console.ReadLine());

switch (choice)
{
    case 1:
        Console.WriteLine("Play");
        break;
    case 2:
        Console.WriteLine("Settings");
        break;
    case 3:
        Console.WriteLine("Quit");
        break;
    default:
        Console.WriteLine("Unknown option");
        break;
}

Enter your score (0-100): 84
Grade: B
Choose a menu option (1-3): 3
Quit

Summary

📚 Additional Resources

• Selection statements (if, switch) — reference
• The conditional (ternary) operator — reference
• The switch expression — reference

🚀 What's Next?

Your programs can now choose between paths. Next, they'll learn to repeat work. In Lesson 3.2: Loops, you'll run code many times with for, while, and foreach — no copy-pasting required.