Example: A Rectangle

Time to build something useful with a struct! Let’s make a Rectangle — like a door, a phone screen, or a chocolate bar. Every rectangle has a width and a height, and once we know those two numbers, we can figure out its area (how much space it covers). 📐

The Big Idea The area of a rectangle is just width * height. If we bundle width and height into a struct, we can hand the whole rectangle to a function and let it do the math.

Our Rectangle kit

struct Rectangle {
    width: u32,
    height: u32,
}

Two fields, both whole numbers (u32). Now we need a helper that takes a rectangle and gives back its area. We’ll borrow the rectangle with an &, which means “let me look at it without taking it away.” 👀

Measuring the space

struct Rectangle {
    width: u32,
    height: u32,
}

fn area(rect: &Rectangle) -> u32 {
    rect.width * rect.height
}

fn main() {
    let door = Rectangle {
        width: 5,
        height: 6,
    };

    println!("Width: {}, Height: {}", door.width, door.height);
    println!("The area is {} squares.", area(&door));
}

A rectangle that is 5 wide and 6 tall covers 5 × 6 = 30 little squares. The function reached into the struct with rect.width and rect.height and multiplied them. 🎉

Think of it like this… Picture a chocolate bar with 5 chunks across and 6 rows down. Count all the chunks: 30! That's the area. 🍫

A peek inside with Debug

Sometimes you want to print the whole struct at once to peek inside it. Add #[derive(Debug)] above the struct, then print it with {:?}.

#[derive(Debug)]
struct Rectangle {
    width: u32,
    height: u32,
}

fn main() {
    let screen = Rectangle {
        width: 8,
        height: 3,
    };

    println!("Here is my rectangle: {:?}", screen);
}

The {:?} is like asking Rust to “show me everything inside this box.” 📦

Ferris says: derive(Debug) is a free feature Rust gives you so you can peek inside your structs while building. Really handy. 🦀

Try this! Change the width to 10 and the height to 4 in the first playground. What area do you get? Run it and check!

Quick quiz

A Rectangle has width: 7 and height: 2. What is its area?

Right! Area is width times height, and 7 × 2 = 14. 🎉

You learned… You can build a Rectangle struct, write a function that borrows it with & to compute the area, and use #[derive(Debug)] with {:?} to peek inside. Next up: teaching your structs cool tricks called methods! 🎓