Constants with const

Define compile-time constants with const.

const constants

const is for values known at compile time that never change. Its type is always written explicitly and its name is conventionally in UPPER_CASE:

const MAKS_PUAN: u32 = 100;
const KDV: f64 = 0.20;

fn main() {
    println!("{}", MAKS_PUAN);
}

Constants can be defined outside functions (globally) and the compiler inlines their value wherever they are used.

const vs let

let can compute its value at run time (let n = read();); const only takes expressions computable at compile time (literals, arithmetic, const fn calls). There is no mut for const. Turning magic numbers (0.20, 100) into named constants makes code readable.

Exercise VAT calculation

Define the constant const KDV: f64 = 0.20;. In main, compute the price with VAT for let fiyat = 250.0; and print it with println!("{:.2}", ...).

Show example solution

const KDV: f64 = 0.20;

fn main() {
    let fiyat = 250.0;
    println!("{:.2}", fiyat * (1.0 + KDV));
}

Test yourself

Why doesn't <code>const N = 10;</code> compile?
- The name must be lower case
- 10 can't be a constant
- It compiles
- A const must have an explicit type
Show answer

A const must have an explicit type — Unlike let, const doesn't infer types; you must write const N: i32 = 10;.

Constant definition

____ SANIYE_SAAT: u32 = 60 * 60;

Show answer

const SANIYE_SAAT: u32 = 60 * 60;

Property

const

Compile time, type required
let

Run time, type inferred
SCREAMING_CASE

Constant naming convention
const fn

Callable in a constant

Next lesson: Static variables

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