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Scalar & Compound Types

By · Reviewed & last updated · Part of Rust Tutorial

Rust’s type system has three families: scalars (integers, floats, bool, char), compounds (tuples, arrays), and the rest. Type inference is good but never silent — every variable has a fixed type at compile time.

A tour of the types

EXAMPLE
fn main() {
    // Integers (i8..i128, u8..u128, isize, usize)
    let small: i8   = -10;
    let big:   u64  = 1_000_000_000;
    let auto       = 42;        // defaults to i32

    // Floats (f32, f64)
    let pi: f64 = 3.14159;
    let x       = 1.0;          // defaults to f64

    // Bool + char (char is a Unicode scalar, 4 bytes)
    let t: bool = true;
    let c: char = '❤';

    // Tuples — fixed length, mixed types
    let user: (String, u32) = (String::from("Ada"), 36);
    let (name, age) = user;

    // Arrays — fixed length, single type
    let nums: [i32; 4] = [1, 2, 3, 4];
    let zeros = [0; 100];        // 100 zeros

    println!("{name} {age} {} {nums:?}", c);
}

Why it matters

Use usize for indexing, i32/u32 for most ordinary numbers, and explicit i64 only when you need the range. Rust’s default ints catch overflow in debug builds.

Tip: Tweak the snippet with Try it Yourself », then sit the quiz at the bottom of the page.

Example

Example
let n: i32 = -7;
let u: u64 = 1_000;
let f: f64 = 3.14;
let b: bool = true;
let t: (i32, &str) = (1, "one");
Try it Yourself »

Discussion

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