Lesson 3: Variables and Mutability

Key Concepts

• Variables are immutable by default.
• Use mut for mutability.
• Constants with const are always immutable and require types.
• Shadowing redeclares variables with let, allowing type changes.

1. Immutable vs. Mutable Variables

fn main() {
    let x = 5;
    println!("The value of x is: {x}");
    // x = 6; // Error!
}

Mutable:

fn main() {
    let mut x = 5;
    println!("The value of x is: {x}");
    x = 6;
    println!("The value of x is: {x}");
}

2. Constants

const THREE_HOURS_IN_SECONDS: u32 = 60 * 60 * 3;

3. Shadowing

fn main() {
    let x = 5;
    let x = x + 1;

    {
        let x = x * 2;
        println!("Inner: {x}"); // 12
    }

    println!("Outer: {x}"); // 6
}

4. Shadowing with Type Change

let spaces = "   ";
let spaces = spaces.len(); // Now usize

Rust's immutability enhances safety and concurrency.

You Try

Edit the shadowing example below (e.g., change values or add more shadowing) and click Run to see the outputs.

fn main() {
    let x = 5;
    let x = x + 1;

    {
        let x = x * 2;
        println!("Inner: {x}"); // 12
    }

    println!("Outer: {x}"); // 6
}