Conditional Expressions

🚦 Control Flow: Conditional Expressions in Rust

The Traffic Light Story

Imagine you’re a little robot walking through a city. At every intersection, there’s a traffic light. The light tells you: “Should I go? Should I stop? Should I wait?”

That’s exactly what conditional expressions do in Rust! They help your program make decisions—just like you decide what to do at a traffic light.

🟢 What is an if Expression?

Think of if like asking a simple question:

“Is the light green?”

• Yes? → Walk!
• No? → Do nothing (or do something else)

Your First if Expression

let light = "green";

if light == "green" {
    println!("Walk!");
}

What’s happening here?

1. We have a light that’s "green"
2. We ask: “Is the light green?”
3. Yes! So we print “Walk!”

The Golden Rule 🌟

In Rust, the thing inside the if must be a true or false answer (a bool).

let age = 8;

if age >= 6 {
    println!("You can go to school!");
}

The question: Is age 6 or more? → Yes (true) → Print the message!

🔴🟢 Adding else - The Other Choice

What if the light is NOT green? You need a backup plan!

else means: “If the first thing isn’t true, do THIS instead.”

let light = "red";

if light == "green" {
    println!("Walk!");
} else {
    println!("Stop and wait!");
}

Like a conversation:

“Is the light green?”

• No, it’s red. “Then stop and wait!”

Real Example: Is It Raining?

let raining = true;

if raining {
    println!("Take an umbrella!");
} else {
    println!("Enjoy the sunshine!");
}

🟡 The else if Chain - Multiple Choices

Sometimes you have MORE than two options. Like a traffic light with red, yellow, AND green!

let light = "yellow";

if light == "green" {
    println!("Walk!");
} else if light == "yellow" {
    println!("Hurry up!");
} else {
    println!("Stop!");
}

How it works:

1. Is it green? → No, skip.
2. Is it yellow? → Yes! → “Hurry up!”
3. We stop checking. Done!

The Ice Cream Shop Example 🍦

let scoops = 3;

if scoops == 1 {
    println!("Small treat!");
} else if scoops == 2 {
    println!("Medium delight!");
} else if scoops == 3 {
    println!("Big happiness!");
} else {
    println!("WOW! Ice cream mountain!");
}

🎁 The Magic Trick: if in let Statements

Here’s where Rust does something really cool!

In Rust, if is an expression. That means it can give you a value back—like a gift!

The Simple Version

let weather = "sunny";

let activity = if weather == "sunny" {
    "Go to the park"
} else {
    "Read a book"
};

println!("Today: {}", activity);

What’s happening?

• The if expression figures out the answer
• It gives that answer to activity
• No semicolons inside the if arms (that’s how Rust knows it’s a gift!)

⚠️ Important Rule: Same Type!

Both choices MUST give the same type of thing.

// ✅ GOOD - Both are strings
let snack = if hungry {
    "apple"
} else {
    "nothing"
};

// ❌ BAD - One is a number, one is text
// let result = if happy { 5 } else { "sad" };

Number Example

let score = 85;

let grade = if score >= 90 {
    'A'
} else if score >= 80 {
    'B'
} else if score >= 70 {
    'C'
} else {
    'D'
};

println!("Your grade: {}", grade);

🧩 Putting It All Together

Let’s build something fun! A robot that decides what to wear:

let temperature = 15;
let raining = true;

let outfit = if temperature > 25 {
    "T-shirt and shorts"
} else if temperature > 15 {
    "Light jacket"
} else {
    "Warm coat"
};

let accessory = if raining {
    "umbrella"
} else {
    "sunglasses"
};

println!("Wear: {} with {}", outfit, accessory);

📊 Quick Decision Flow

graph TD
    A["Start: Check Condition"] --> B{Is it TRUE?}
    B -->|Yes| C["Do the IF block"]
    B -->|No| D{Any ELSE IF?}
    D -->|Yes| E{Is ELSE IF true?}
    E -->|Yes| F["Do ELSE IF block"]
    E -->|No| D
    D -->|No more| G{Is there ELSE?}
    G -->|Yes| H["Do ELSE block"]
    G -->|No| I["Skip everything"]
    C --> J["Done!"]
    F --> J
    H --> J
    I --> J

🌟 Key Takeaways

🎯 Remember These Rules

1. Condition must be bool - true or false only!
2. else if chains - Check one by one, stop at first match
3. if as expression - Can return values (no semicolons inside!)
4. Same types - Both arms must return same type

🚀 You Did It!

You now understand how Rust makes decisions! Just like choosing what to do at a traffic light, your programs can now:

• ✅ Check if something is true
• ✅ Have backup plans with else
• ✅ Handle many options with else if
• ✅ Get values from decisions using if in let

Next time you see a traffic light, remember: you’re thinking in Rust! 🦀