Variables with let
- Rust course
- Beginner
- Lesson 7/115
- 4 min
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 syntaxUnused variables produce a warning; a name you deliberately don't use starts with _: let _gecici = 1;.
Practice Variables
Use let to declare ad ("Ada"), yas (36) and boy: f64 (1.72) with an explicit type. Print println!("{} {} {}", ad, yas, boy);.
Starter code
fn main() {
// buraya yaz
}Sample solution
fn main() {
let ad = "Ada";
let yas = 36;
let boy: f64 = 1.72;
println!("{} {} {}", ad, yas, boy);
}Expected output
Ada 36 1.72
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 What happens with <code>let x = 5; x = 6;</code>?
- It warns but runs
- Compile error: assigning twice to an immutable variable Correct answer
- x becomes 6
- A run-time panic
Explanation: Rust rejects reassigning an immutable binding at compile time (E0384); changing it requires let mut.
2 Write the type
let fiyat____ = 12.5;let fiyat: f64 = 12.5;Explanation: The type is written after the variable name with a colon.
3 Value and inferred type
| Term | Meaning |
|---|---|
42 |
i32 |
3.14 |
f64 |
"Ada" |
&str |
true |
bool |