Data Types

Every value in Rust is of a certain data type, which tells Rust what kind of data is being specified so it knows how to work with that data. We’ll look at two data type subsets: scalar and compound.

Keep in mind that Rust is a statically typed language, which means that it must know the types of all variables at compile time. The compiler can usually infer what type we want to use based on the value and how we use it. In cases when many types are possible, such as when we converted a String to a numeric type using parse in the “Comparing the Guess to the Secret Number” section in Chapter 2, we must add a type annotation, like this:

let guess: u32 = "42".parse().expect("Not a number!");

If we don’t add the type annotation here, Rust will display the following error, which means the compiler needs more information from us to know which type we want to use:

$ 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 error
For 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.

You’ll see different type annotations for other data types.

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.

Table 3–1: Integer Types in Rust

LengthSignedUnsigned8-biti8u816-biti16u1632-biti32u3264-biti64u64128-biti128u128archisizeusize

Each variant can be either signed or unsigned and has an explicit size. Signed and unsigned refer to whether it’s possible for the number to be negative — in other words, whether the number needs to have a sign with it (signed) or whether it will only ever be positive and can therefore be represented without a sign (unsigned). It’s like writing numbers on paper: when the sign matters, a number is shown with a plus sign or a minus sign; however, when it’s safe to assume the number is positive, it’s shown with no sign. Signed numbers are stored using two’s complement representation.

Each signed variant can store numbers from -(2n — 1) to 2n — 1–1 inclusive, where n is the number of bits that variant uses. So an i8 can store numbers from -(27) to 27 - 1, which equals -128 to 127. Unsigned variants can store numbers from 0 to 2n - 1, so a u8 can store numbers from 0 to 28 - 1, which equals 0 to 255.

Additionally, the isize and usize types depend on the kind of computer your program is running on: 64 bits if you’re on a 64-bit architecture and 32 bits if you’re on a 32-bit architecture.

You can write integer literals in any of the forms shown in Table 3–2. Note that all number literals except the byte literal allow a type suffix, such as 57u8, and _ as a visual separator, such as 1_000.

Table 3–2: Integer Literals in Rust

Number literalsExampleDecimal98_222Hex0xffOctal0o77Binary0b1111_0000Byte (u8 only)b'A'

So how do you know which type of integer to use? If you’re unsure, Rust’s defaults are generally good choices, and integer types default to i32: this type is generally the fastest, even on 64-bit systems. The primary situation in which you’d use isize or usize is when indexing some sort of collection.

Integer Overflow

Let’s say you have a variable of type u8 that can hold values between 0 and 255. If you try to change the variable to a value outside of that range, such as 256, integer overflow will occur. Rust has some interesting rules involving this behavior. When you’re compiling in debug mode, Rust includes checks for integer overflow that cause your program to panic at runtime if this behavior occurs. Rust uses the term panicking when a program exits with an error; we’ll discuss panics in more depth in the “Unrecoverable Errors with panic!” section in Chapter 9.

When you’re compiling in release mode with the --release flag, Rust does not include checks for integer overflow that cause panics. Instead, if overflow occurs, Rust performs two’s complement wrapping. In short, values greater than the maximum value the type can hold “wrap around” to the minimum of the values the type can hold. In the case of a u8, 256 becomes 0, 257 becomes 1, and so on. The program won’t panic, but the variable will have a value that probably isn’t what you were expecting it to have. Relying on integer overflow’s wrapping behavior is considered an error.

To explicitly handle the possibility of overflow, you can use these families of methods that the standard library provides on primitive numeric types:

• 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.

Here’s an example that shows floating-point numbers in action:

Filename: src/main.rs

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

Floating-point numbers are represented according to the IEEE-754 standard. The f32 type is a single-precision float, and f64 has double precision.

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:

Filename: src/main.rs

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;
}

Each expression in these statements uses a mathematical operator and evaluates to a single value, which is then bound to a variable. Appendix B contains a list of all operators that Rust provides.

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:

Filename: src/main.rs

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

The main way to use Boolean values is through conditionals, such as an if expression. We’ll cover how if expressions work in Rust in the “Control Flow” section.