03 Variables and Mutability

Goal

Learn how Rust handles variables, immutability by default, and the mut keyword.

Steps

1. Immutable by Default

fn main() {
    let x = 5;
    println!("x = {}", x);
    // x = 6; // error: cannot assign twice to immutable variable
}

2. Make Mutable with mut

fn main() {
    let mut x = 5;
    println!("x = {}", x);
    x = 6;
    println!("x = {}", x);
}

3. Constants

const MAX_POINTS: u32 = 100_000;

fn main() {
    println!("Max: {}", MAX_POINTS);
}

Rules:

• Always immutable
• Type annotation required
• Can be declared in any scope
• Naming: UPPER_SNAKE_CASE

4. Shadowing

fn main() {
    let x = 5;
    let x = x + 1;
    let x = x * 2;
    println!("x = {}", x); // 12
}

Shadowing vs mut:

• Creates new variable
• Can change type
• mut mutates in place

Checkpoint

fn main() {
    let mut counter = 0;
    counter = counter + 1;
    
    const MAX: u32 = 10;
    
    let spaces = "   ";
    let spaces = spaces.len(); // shadowing changes type
    
    println!("counter: {}, MAX: {}, spaces: {}", counter, MAX, spaces);
}

Output: counter: 1, MAX: 10, spaces: 3

Next

Continue to 04 Data Types.