Updated primitive-types.md

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# primitive-types.md

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5.3. 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` conditionals][if].

[if]: if.html

You can find more documentation for `bool`s [in the standard library

[bool]: ../std/primitive.bool.html

# `char`

The `char` type represents a single Unicode scalar value. You can create `char`s
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 `char`s [in the standard library

[char]: ../std/primitive.char.html

# 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:

* [i8](../std/primitive.i8.html)
* [i16](../std/primitive.i16.html)
* [i32](../std/primitive.i32.html)
* [i64](../std/primitive.i64.html)
* [u8](../std/primitive.u8.html)
* [u16](../std/primitive.u16.html)
* [u32](../std/primitive.u32.html)
* [u64](../std/primitive.u64.html)
* [isize](../std/primitive.isize.html)
* [usize](../std/primitive.usize.html)
* [f32](../std/primitive.f32.html)
* [f64](../std/primitive.f64.html)

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 complement 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 has 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][generics]. The `N` is a compile-time constant, for the length of the

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

You can find more documentation for `array`s [in the standard library

[array]: ../std/primitive.array.html

# 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
let complete = &a[..]; // A slice containing all of the elements in a

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

[generics]: generics.html

You can find more documentation for slices [in the standard library

[slice]: ../std/primitive.slice.html

# `str`

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

[dst]: unsized-types.html
[strings]: strings.html

You can find more documentation for `str` [in the standard library

[str]: ../std/primitive.str.html

# 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

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.

[arity]: glossary.html#arity

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][let] 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.

[let]: variable-bindings.html

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

You can disambiguate a single-element tuple from a value in parentheses with a

(0,); // single-element tuple
(0); // zero in parentheses

## 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

[tuple]: ../std/primitive.tuple.html

# 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`