Rust Booleans

A boolean is the simplest type there is: it can only be true or false. In Rust it’s called bool, and you’ll use it everywhere you need to make a decision in your code.

Declaring a boolean

fn main() {
    let is_active = true;
    let has_error: bool = false;
    println!("active: {is_active}, error: {has_error}");
}

▶ Try it Yourself

Rust almost always infers bool on its own from true/false, so you rarely need to write the type explicitly.

Booleans usually come from comparisons

You don’t often write true or false directly in real code — they usually come from comparing something:

fn main() {
    let age = 20;
    let can_vote = age >= 18;
    println!("Can vote: {can_vote}");
}

Output:

Can vote: true

▶ Try it Yourself

Using booleans in conditions

The main job of a bool is deciding which branch of an if to run:

fn main() {
    let is_raining = true;

    if is_raining {
        println!("Take an umbrella");
    } else {
        println!("Leave it at home");
    }
}

We cover if properly on the If…Else page — this is just a taste of why booleans matter.

Recap

bool holds exactly two values: true or false.
Comparisons (==, <, >=, …) produce booleans.
Conditions in if must be a real bool — no implicit conversions from numbers or strings.

Quick check

1. What type does a comparison like `age >= 18` produce?

2. In Rust, can you use an integer like `0` directly as a condition in an `if`, the way you might in some other languages?

Score: 0 / 2

Data Types — The Rust Book