Variables with let

Define variables with the let keyword.

Variables with let

let binds a value to a name. In Rust variables are immutable by default; this prevents a value from changing somewhere unexpected. The type is usually inferred from the value, or written explicitly if wanted:

let yas = 36;          // inferred as i32
let boy: f64 = 1.72;   // explicit type
let ad = "Ada";        // &str
println!("{ad} {yas} {boy}");

Type inference

The compiler infers the type from the value and later use. The default for integers is i32 and for decimals f64. When inference isn't enough (e.g. "42".parse()), the type is given explicitly:

let n: i32 = "42".parse().unwrap();
let m = "7".parse::<u8>().unwrap();  // turbofish syntax

Unused variables produce a warning; a name you deliberately don't use starts with _: let _gecici = 1;.

Exercise Variables

Use let to declare ad ("Ada"), yas (36) and boy: f64 (1.72) with an explicit type. Print println!("{} {} {}", ad, yas, boy);.

Show example solution

fn main() {
    let ad = "Ada";
    let yas = 36;
    let boy: f64 = 1.72;
    println!("{} {} {}", ad, yas, boy);
}

Test yourself

What happens with <code>let x = 5; x = 6;</code>?
- It warns but runs
- Compile error: assigning twice to an immutable variable
- x becomes 6
- A run-time panic
Show answer

Compile error: assigning twice to an immutable variable — Rust rejects reassigning an immutable binding at compile time (E0384); changing it requires let mut.

Write the type

let fiyat____ = 12.5;

Show answer

let fiyat: f64 = 12.5;

Value and inferred type

42

i32
3.14

f64
"Ada"

&str
true

bool

Next lesson: Mutability with mut

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