Level 1 · Sprout

Control Flow 🚦

So far your programs run top to bottom, every line, once. Real programs make choices and repeat things. Time to steer.

Making decisions: if / else

fn main() {
    let temperature = 33;

    if temperature > 30 {
        println!("Beach day! 🏖️");
    } else if temperature > 15 {
        println!("Nice walking weather.");
    } else {
        println!("Hot cocoa time. ☕");
    }
}

The condition must be a real bool. Rust refuses the "0 means false" trick from C and JavaScript, which quietly causes bugs there. And notice: no parentheses needed around the condition. Clean.

Plot twist: if is an expression

Remember the expression secret from Lesson 4? An if produces a value, so you can assign it directly, with no awkward "declare then maybe assign" dance:

let temperature = 33;
let outfit = if temperature > 30 { "swimsuit" } else { "jacket" };
println!("Wear the {outfit}");

Both branches must produce the same type; the compiler checks, of course.

Repeating: three loops

1. loop: forever, until break

let mut countdown = 3;
loop {
    println!("{countdown}...");
    countdown -= 1;
    if countdown == 0 {
        break;           // the only way out
    }
}
println!("Liftoff! 🚀");

2. while: as long as something is true

let mut fuel = 10;
while fuel > 0 {
    println!("Flying... fuel: {fuel}");
    fuel -= 3;
}
println!("Time to land.");

3. for: once per item (the one you'll use most)

// Over a range: 1, 2, 3, 4 (the end is excluded!)
for i in 1..5 {
    println!("bottle #{i}");
}

// ..= includes the end: 1, 2, 3, 4, 5
for i in 1..=5 {
    println!("crab #{i}");
}

// Over a collection: no index bookkeeping, no off-by-one bugs
let snacks = ["kelp", "algae", "plankton"];
for snack in snacks {
    println!("Rusty eats {snack}");
}
💡 Which loop when? Know the items? for. Know the stop condition? while. Waiting for something unpredictable (like user input)? loop + break. When in doubt, for.

Sneak peek: match 🔮

Rust has a fourth way to steer: a supercharged decision-maker you'll properly meet in Lesson 9. A taste:

let dice = 4;
match dice {
    1 => println!("Ouch, a one."),
    2 | 3 => println!("Meh."),
    4..=5 => println!("Nice roll!"),
    _ => println!("Six! Maximum crab energy! 🦀"),
}
⚠️ Common stumbles
  • Expecting 1..5 to include 5: it doesn't; use 1..=5.
  • A while whose condition never becomes false: infinite loop. (Press Ctrl+C in the terminal to rescue yourself.)
  • Using if x = 5 instead of if x == 5: one = assigns, two compare. Rust catches this; many languages don't.
Exercise 1

FizzBuzz, the classic

Print numbers 1 to 20, but for multiples of 3 print Fizz, for multiples of 5 print Buzz, and for both print FizzBuzz. (Hint: n % 3 == 0 means "divisible by 3"; % gives the division remainder. Check the "both" case first!)

Reveal solution 
fn main() {
    for n in 1..=20 {
        if n % 15 == 0 {
            println!("FizzBuzz");
        } else if n % 3 == 0 {
            println!("Fizz");
        } else if n % 5 == 0 {
            println!("Buzz");
        } else {
            println!("{n}");
        }
    }
}

This tiny program is a real (in)famous interview question. You just did it in lesson five. 🎉

Exercise 2

Sum of a range

Use a for loop and a mutable total to add up every number from 1 to 100, then print it. (Legend says young Gauss did this in his head.)

Reveal solution 
fn main() {
    let mut total = 0;
    for n in 1..=100 {
        total += n;
    }
    println!("{total}"); // 5050
}
Exercise 3

The bouncing countdown

Rewrite Lesson 2's level-up counter properly: start at level 1 and loop until level 5, printing "Level up! Now level N" each time. Which loop fits best?

Reveal solution 
fn main() {
    let mut level = 1;
    while level < 5 {
        level += 1;
        println!("Level up! Now level {level}");
    }
}

// or, arguably cleaner:
fn main() {
    for level in 2..=5 {
        println!("Level up! Now level {level}");
    }
}
🎓 Level 1 complete! You can now write real programs: variables, types, functions, decisions, loops. Lock it in with the Sprout Quiz, then rest up. Next level, we meet the idea that made Rust famous.
← PreviousLesson 4: Functions