Mutability with mut

Update variables with the mut keyword.

Mutability with mut

Variables whose value will change later are declared with let mut:

let mut sayac = 0;
sayac += 1;
sayac += 1;
println!("{sayac}");  // 2

The word mut tells the reader "this value will change". An unnecessary mut produces a compiler warning, nudging code toward as few mutable variables as possible.

Compound assignment

For mutable numbers the +=, -=, *=, /= and %= operators are used. Rust has no ++ or --; you write x += 1. Integer overflow panics in debug builds and wraps in release builds; for explicit behavior there are methods such as checked_add and wrapping_add.

Exercise Counter

Declare let mut toplam = 0;. Do toplam += i; in a for i in 1..=5 loop and print the result.

Show example solution

fn main() {
    let mut toplam = 0;
    for i in 1..=5 {
        toplam += i;
    }
    println!("{}", toplam);
}

Test yourself

What happens with <code>let mut x = 5; x = "beş";</code>?
- Compile error: the type can't change
- x becomes "beş"
- It warns
- A run-time error
Show answer

Compile error: the type can't change — mut only allows the value to change, not the type. For a new value of a different type, shadowing is used.

Mutable

let ____ toplam = 0;
toplam += 5;

Show answer

let mut toplam = 0;
toplam += 5;

Code and result

let x = 1; x = 2;

Compile error
let mut x = 1; x = 2;

x = 2
x++

Not in Rust
x += 1

Increments by one

Next lesson: Constants with const

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