Variables and mutability - Rust for C-Programmers

2.4 Variables: Immutability by Default

Variables are declared using the let keyword. A fundamental difference from C is that Rust variables are immutable by default.

let variable_name: OptionalType = value;

Rust requires variables to be initialized before their first use, preventing errors stemming from uninitialized data.
Rust, like C, uses = to perform assignments.

2.4.1 Immutability Example

fn main() {
    let x: i32 = 5; // x is immutable
    // x = 6; // This line would cause a compile-time error!
    println!("The value of x is: {}", x);
}

The // syntax denotes a single-line comment. Immutability helps prevent accidental modification, making code easier to reason about and enabling compiler optimizations.

2.4.2 Enabling Mutability

To allow a variable’s value to be changed, use the mut keyword.

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

The {} syntax within the println! macro string is used for string interpolation, embedding the value of variables or expressions directly into the output.

2.4.3 Comparison with C

In C, variables are mutable by default. The const keyword is used to declare variables whose values should not be changed, though the level of enforcement can vary (e.g., const pointers).

int x = 5;
x = 6; // Allowed

const int y = 5;
// y = 6; // Error: assignment of read-only variable 'y'