4.2 Types and numeric values

Explanation

Rust values have types. Common basic types include i32 for signed integers, usize for sizes and indices, f64 for floating-point numbers, bool for true-or-false values, and String for owned text.

Integer and floating-point literals are different. The literal 10 can be an integer type such as usize, while 0.1 is a floating-point value. Choose numeric types deliberately so that indices, counts, and physical quantities are not mixed by accident.

fn main() {
    let n: usize = 10;
    let dx: f64 = 0.1;
    let ok: bool = n > 0;

    println!("n = {n}, dx = {dx}, ok = {ok}");
}

The compiler can infer many types, but explicit annotations are useful at function boundaries and when reviewing important numerical assumptions.

Things to look up

i32
usize
f64
bool
String
Type inference

Exercise

Predict the types of small expressions such as 1 + 2, 0.5 * 2.0, 10_usize - 1, and 3.0 > 2.0. Then check your prediction by asking an AI agent or by adding type annotations in a small Rust program.

Notes for the exercise

Indices and lengths often use usize.
Physical quantities often use f64 in this course.
Do not choose a type only because a generated snippet happened to use it.