Variables, Mutability & Variable Shadowing

let vs let mut, typed constants, and the power of shadowing

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In Rust, variables are strictly IMMUTABLE by default. Mutability must be explicitly opted into using the mut keyword.

Mutability vs Shadowing

let x = 5;: Immutable. Attempting x = 6 produces a compile error.

let mut x = 5;: Explicitly mutable.

Variable Shadowing: Declaring a new variable with the same name (let spaces = " "; let spaces = spaces.len();) transforms types and values safely while keeping the resulting variable immutable.

Mutability and Shadowing in Rust

Try It Yourself »

fn main() {
    let mut score = 50;
    score += 25; // Allowed because of 'mut'
    println!("Score: {}", score);

    // Shadowing: Re-declaring 'input' with a new type!
    let input = "  42  ";
    let input: u32 = input.trim().parse().expect("Failed to parse");
    println!("Parsed number: {}", input);
}

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Test Yourself With Exercise

+15 XP

What keyword must be added to a variable declaration in Rust to allow its value to be reassigned?

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