primitive-types.md

% Primitive Types

The Rust language has a number of types that are considered ‘primitive’. This means that they’re built-in to the language. Rust is structured in such a way that the standard library also provides a number of useful types built on top of these ones, as well, but these are the most primitive.

Booleans

Rust has a built in boolean type, named bool. It has two values, true and false:

let x = true;

let y: bool = false;

A common use of booleans is in if statements.

You can find more documentation for bools in the standard library documentation.

char

The char type represents a single Unicode scalar value. You can create chars with a single tick: (')

let x = 'x';
let two_hearts = '💕';

Unlike some other languages, this means that Rust’s char is not a single byte, but four.

You can find more documentation for chars in the standard library documentation.

Numeric types

Rust has a variety of numeric types in a few categories: signed and unsigned, fixed and variable, floating-point and integer.

These types consist of two parts: the category, and the size. For example, u16 is an unsigned type with sixteen bits of size. More bits lets you have bigger numbers.

If a number literal has nothing to cause its type to be inferred, it defaults:

let x = 42; // x has type i32

let y = 1.0; // y has type f64

Here’s a list of the different numeric types, with links to their documentation in the standard library:

• i16
• i32
• i64
• i8
• u16
• u32
• u64
• u8
• isize
• usize
• f32
• f64

Let’s go over them by category:

Signed and Unsigned

Integer types come in two varieties: signed and unsigned. To understand the difference, let’s consider a number with four bits of size. A signed, four-bit number would let you store numbers from -8 to +7. Signed numbers use ‘two’s compliment representation’. An unsigned four bit number, since it does not need to store negatives, can store values from 0 to +15.

Unsigned types use a u for their category, and signed types use i. The i is for ‘integer’. So u8 is an eight-bit unsigned number, and i8 is an eight-bit signed number.

Fixed size types

Fixed size types have a specific number of bits in their representation. Valid bit sizes are 8, 16, 32, and 64. So, u32 is an unsigned, 32-bit integer, and i64 is a signed, 64-bit integer.

Variable sized types

Rust also provides types whose size depends on the size of a pointer of the underlying machine. These types have ‘size’ as the category, and come in signed and unsigned varieties. This makes for two types: isize and usize.

Floating-point types

Rust also two floating point types: f32 and f64. These correspond to IEEE-754 single and double precision numbers.

Arrays

Like many programming languages, Rust has list types to represent a sequence of things. The most basic is the array, a fixed-size list of elements of the same type. By default, arrays are immutable.

let a = [1, 2, 3]; // a: [i32; 3]
let mut m = [1, 2, 3]; // m: [i32; 3]

Arrays have type [T; N]. We’ll talk about this T notation in the generics section. The N is a compile-time constant, for the length of the array.

There’s a shorthand for initializing each element of an array to the same value. In this example, each element of a will be initialized to 0:

let a = [0; 20]; // a: [i32; 20]

You can get the number of elements in an array a with a.len():

let a = [1, 2, 3];

println!("a has {} elements", a.len());

You can access a particular element of an array with subscript notation:

let names = ["Graydon", "Brian", "Niko"]; // names: [&str; 3]

println!("The second name is: {}", names[1]);

Subscripts start at zero, like in most programming languages, so the first name is names[0] and the second name is names[1]. The above example prints The second name is: Brian. If you try to use a subscript that is not in the array, you will get an error: array access is bounds-checked at run-time. Such errant access is the source of many bugs in other systems programming languages.

You can find more documentation for arrays in the standard library documentation.

Slices

A ‘slice’ is a reference to (or “view” into) another data structure. They are useful for allowing safe, efficient access to a portion of an array without copying. For example, you might want to reference just one line of a file read into memory. By nature, a slice is not created directly, but from an existing variable. Slices have a length, can be mutable or not, and in many ways behave like arrays:

let a = [0, 1, 2, 3, 4];
let middle = &a[1..4]; // A slice of a: just the elements 1, 2, and 3

Slices have type &[T]. We’ll talk about that T when we cover generics.

You can find more documentation for slicess in the standard library documentation.

str

Rust’s str type is the most primitive string type. As an unsized type, it’s not very useful by itself, but becomes useful when placed behind a reference, like &str. As such, we’ll just leave it at that.

You can find more documentation for str in the standard library documentation.

Tuples

A tuple is an ordered list of fixed size. Like this:

let x = (1, "hello");

The parentheses and commas form this two-length tuple. Here’s the same code, but with the type annotated:

let x: (i32, &str) = (1, "hello");

As you can see, the type of a tuple looks just like the tuple, but with each position having a type name rather than the value. Careful readers will also note that tuples are heterogeneous: we have an i32 and a &str in this tuple. In systems programming languages, strings are a bit more complex than in other languages. For now, just read &str as a string slice, and we’ll learn more soon.

You can assign one tuple into another, if they have the same contained types and arity. Tuples have the same arity when they have the same length.

let mut x = (1, 2); // x: (i32, i32)
let y = (2, 3); // y: (i32, i32)

x = y;

You can access the fields in a tuple through a destructuring let. Here’s an example:

let (x, y, z) = (1, 2, 3);

println!("x is {}", x);

Remember before when I said the left-hand side of a let statement was more powerful than just assigning a binding? Here we are. We can put a pattern on the left-hand side of the let, and if it matches up to the right-hand side, we can assign multiple bindings at once. In this case, let "destructures," or "breaks up," the tuple, and assigns the bits to three bindings.

This pattern is very powerful, and we’ll see it repeated more later.

Tuple Indexing

You can also access fields of a tuple with indexing syntax:

let tuple = (1, 2, 3);

let x = tuple.0;
let y = tuple.1;
let z = tuple.2;

println!("x is {}", x);

Like array indexing, it starts at zero, but unlike array indexing, it uses a ., rather than []s.

You can find more documentation for tuples in the standard library documentation.

Functions

Functions also have a type! They look like this:

fn foo(x: i32) -> i32 { x }

let x: fn(i32) -> i32 = foo;

In this case, x is a ‘function pointer’ to a function that takes an i32 and returns an i32.