let guess: u32 = "42".parse().expect("Not a number!");
$ cargo build
Compiling no_type_annotations v0.1.0 (file:///projects/no_type_annotations)
error[E0282]: type annotations needed
--> src/main.rs:2:9
|
2 | let guess = "42".parse().expect("Not a number!");
| ^^^^^ consider giving `guess` a type
error: aborting due to previous errorFor more information about this error, try `rustc --explain E0282`.
error: could not compile `no_type_annotations`
To learn more, run the command again with --verbose.

Scalar Types

A scalar type represents a single value. Rust has four primary scalar types: integers, floating-point numbers, Booleans, and characters. You may recognize these from other programming languages. Let’s jump into how they work in Rust.

Integer Types

An integer is a number without a fractional component. We used one integer type in Chapter 2, the u32 type. This type declaration indicates that the value it’s associated with should be an unsigned integer (signed integer types start with i, instead of u) that takes up 32 bits of space. Table 3-1 shows the built-in integer types in Rust. Each variant in the Signed and Unsigned columns (for example, i16) can be used to declare the type of an integer value.

  • Wrap in all modes with the wrapping_* methods, such as wrapping_add
  • Return the None value if there is overflow with the checked_* methods
  • Return the value and a boolean indicating whether there was overflow with the overflowing_* methods
  • Saturate at the value’s minimum or maximum values with saturating_* methods

Floating-Point Types

Rust also has two primitive types for floating-point numbers, which are numbers with decimal points. Rust’s floating-point types are f32 and f64, which are 32 bits and 64 bits in size, respectively. The default type is f64 because on modern CPUs it’s roughly the same speed as f32 but is capable of more precision.

fn main() {
let x = 2.0; // f64
let y: f32 = 3.0; // f32
}

Numeric Operations

Rust supports the basic mathematical operations you’d expect for all of the number types: addition, subtraction, multiplication, division, and remainder. The following code shows how you’d use each one in a let statement:

fn main() {
// addition
let sum = 5 + 10;
// subtraction
let difference = 95.5 - 4.3;
// multiplication
let product = 4 * 30;
// division
let quotient = 56.7 / 32.2;
// remainder
let remainder = 43 % 5;
}

The Boolean Type

As in most other programming languages, a Boolean type in Rust has two possible values: true and false. Booleans are one byte in size. The Boolean type in Rust is specified using bool. For example:

fn main() {
let t = true;
let f: bool = false; // with explicit type annotation
}

--

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