Day 4: Control Flow - Teaching Your Code to Make Decisions

Welcome back! Yesterday your code learned to compare and combine things with operators. Today we finally use all those true/false answers for something useful: making decisions. This is the day your program grows a tiny brain. 🧠

Up to now, your code has been a straight line — it runs from top to bottom like a very obedient intern. Today we teach it to fork the road.

1. The if Statement — The Bouncer at the Door

An if statement is a bouncer with a clipboard. It checks a condition (a bool expression), and if it's true, it lets the code inside the { } run. If it's false, that code gets skipped entirely.

int age = 20;

if (age >= 18)
{
    Console.WriteLine("Welcome in!");
}

Read it out loud: "IF age is greater-than-or-equal-to 18, print the welcome message."

Three things to notice:

• The condition goes in parentheses ( ).
• The code to run goes in curly braces { }.
• No semicolon after if (...) — the braces are the ending.

2. else — The Backup Plan

What if the bouncer says no? You probably want to do something else. Enter else:

int age = 15;

if (age >= 18)
{
    Console.WriteLine("Welcome in!");
}
else
{
    Console.WriteLine("Sorry, come back in a few years.");
}

Exactly one of those two blocks will run. Never both, never neither.

3. else if — The Menu of Options

When you have more than two paths, chain else if blocks:

int score = 73;

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");
}

C# checks the conditions top-to-bottom and runs the first one that's true. Everything after that is skipped. That's why you put the strictest condition (>= 90) first — if you reversed the order, everyone would get an A.

4. Nesting (Use Sparingly!)

You can put an if inside another if:

if (isLoggedIn)
{
    if (isAdmin)
    {
        Console.WriteLine("Welcome, boss!");
    }
    else
    {
        Console.WriteLine("Welcome, user!");
    }
}

But don't go crazy. Three levels deep and your code starts to look like a staircase. Usually you can flatten it by combining conditions with &&:

if (isLoggedIn && isAdmin)
{
    Console.WriteLine("Welcome, boss!");
}
else if (isLoggedIn)
{
    Console.WriteLine("Welcome, user!");
}

Much friendlier to read.

5. The switch Statement — A Cleaner Menu

When you're comparing one variable against many specific values, a long else if chain gets ugly fast. That's what switch is for:

string day = "Wednesday";

switch (day)
{
    case "Monday":
        Console.WriteLine("Fresh start!");
        break;
    case "Wednesday":
        Console.WriteLine("Hump day.");
        break;
    case "Friday":
        Console.WriteLine("Almost weekend!");
        break;
    default:
        Console.WriteLine("Just another day.");
        break;
}

Rules of the road:

• Each case lists one specific value to match.
• break; is mandatory — it tells C# to jump out of the switch. Forget it and the compiler yells at you.
• default: runs if no case matched. Think of it as the else of switches.

You can also group cases that share the same code:

switch (day)
{
    case "Saturday":
    case "Sunday":
        Console.WriteLine("Weekend!");
        break;
    default:
        Console.WriteLine("Weekday.");
        break;
}

6. Switch Expressions — The Modern, Shiny Version

C# 8 (and beyond) gave us a much tighter syntax called a switch expression. It returns a value instead of running statements:

string day = "Wednesday";

string mood = day switch
{
    "Monday" => "Fresh start!",
    "Wednesday" => "Hump day.",
    "Friday" => "Almost weekend!",
    "Saturday" or "Sunday" => "Weekend!",
    _ => "Just another day."
};

Console.WriteLine(mood);

Notice:

• No case, no break, no { } around each arm.
• => ("goes to") replaces the :.
• _ (underscore) is the discard — it's the default of switch expressions.
• Arms are separated by commas.

When you're picking a value based on input, prefer switch expressions. They're shorter, safer, and the compiler will warn you if you forget a case. See switch expression docs for the full story.

7. if vs switch — Which Do I Use?

Rule of thumb: ranges and combos → if. Specific values → switch.

8. Your Homework: A Traffic Light

Ask the user what color the traffic light is, then tell them what to do. Use a switch expression.

Console.Write("Traffic light color (red/yellow/green): ");
string color = Console.ReadLine()?.ToLower() ?? "";

string action = color switch
{
    "red" => "Stop!",
    "yellow" => "Slow down.",
    "green" => "Go!",
    _ => "That's not a traffic light color..."
};

Console.WriteLine(action);

Two tiny new things hiding in there:

• .ToLower() converts the user's input to lowercase so "Red", "RED", and "red" all match.
• ?. is the null-conditional operator — if Console.ReadLine() returns null, the whole thing short-circuits safely. The ?? "" says "if we did get null, use an empty string instead." We'll cover null properly in a later day, but get used to seeing these two.

Bonus challenge: also handle "flashing" as a special case that prints "Proceed with caution."

Summary of Day 4

• if / else / else if let your code pick different paths based on bool conditions.
• Conditions go in ( ), code bodies go in { }.
• else if chains are checked top-to-bottom — first match wins.
• switch statements are cleaner when matching one variable against many values. Every case needs a break.
• Switch expressions (x switch { ... }) are the modern way to produce a value from a match. Use _ for the default.
• Rule of thumb: ranges/combos → if. Specific values → switch.

Tomorrow: we'll talk about Loops — for, while, and foreach. Because typing Console.WriteLine 100 times is no way to live. 🔁

See you on Day 5!