## 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”][comparing-the-guess-to-the-secret-number]<!-- ignore --> section in
Chapter 2, we must add a type annotation, like this:

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

If we don’t add the `: u32` type annotation shown in the preceding code, Rust
will display the following error, which means the compiler needs more
information from us to know which type we want to use:

```console
{{#include ../listings/ch03-common-programming-concepts/output-only-01-no-type-annotations/output.txt}}
```

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. We can use any of these variants to declare
the type of an integer value.

<span class="caption">Table 3-1: Integer Types in Rust</span>

| Length  | Signed  | Unsigned |
| ------- | ------- | -------- |
| 8-bit   | `i8`    | `u8`     |
| 16-bit  | `i16`   | `u16`    |
| 32-bit  | `i32`   | `u32`    |
| 64-bit  | `i64`   | `u64`    |
| 128-bit | `i128`  | `u128`   |
| arch    | `isize` | `usize`  |

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][twos-complement]<!-- ignore
--> representation.

Each signed variant can store numbers from -(2<sup>n - 1</sup>) to 2<sup>n -
1</sup> - 1 inclusive, where _n_ is the number of bits that variant uses. So an
`i8` can store numbers from -(2<sup>7</sup>) to 2<sup>7</sup> - 1, which equals
-128 to 127. Unsigned variants can store numbers from 0 to 2<sup>n</sup> - 1,
so a `u8` can store numbers from 0 to 2<sup>8</sup> - 1, which equals 0 to 255.

Additionally, the `isize` and `usize` types depend on the architecture of the
computer your program is running on, which is denoted in the table as “arch”:
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 number literals that can be multiple numeric types allow a type suffix,
such as `57u8`, to designate the type. Number literals can also use `_` as a
visual separator to make the number easier to read, such as `1_000`, which will
have the same value as if you had specified `1000`.

<span class="caption">Table 3-2: Integer Literals in Rust</span>

| Number literals  | Example       |
| ---------------- | ------------- |
| Decimal          | `98_222`      |
| Hex              | `0xff`        |
| Octal            | `0o77`        |
| Binary           | `0b1111_0000` |
| Byte (`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 places to start: integer types default to `i32`.
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 that range, such as
> 256, _integer overflow_ will occur, which can result in one of two behaviors.
> 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!`”][unrecoverable-errors-with-panic]<!-- ignore --> 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`, the value 256 becomes
> 0, the value 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 provided by the standard library for 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 the `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. All floating-point types are signed.

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

<span class="filename">Filename: src/main.rs</span>

```rust
{{#rustdoc_include ../listings/ch03-common-programming-concepts/no-listing-06-floating-point/src/main.rs}}
```

Floating-point numbers are represented according to the IEEE-754 standard.

#### Numeric Operations

Rust supports the basic mathematical operations you’d expect for all the number
types: addition, subtraction, multiplication, division, and remainder. Integer
division truncates toward zero to the nearest integer. The following code shows
how you’d use each numeric operation in a `let` statement:

<span class="filename">Filename: src/main.rs</span>

```rust
{{#rustdoc_include ../listings/ch03-common-programming-concepts/no-listing-07-numeric-operations/src/main.rs}}
```

Each expression in these statements uses a mathematical operator and evaluates
to a single value, which is then bound to a variable. [Appendix
B][appendix_b]<!-- ignore --> 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:

<span class="filename">Filename: src/main.rs</span>

```rust
{{#rustdoc_include ../listings/ch03-common-programming-concepts/no-listing-08-boolean/src/main.rs}}
```

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”][control-flow]<!-- ignore --> section.

#### The Character Type

Rust’s `char` type is the language’s most primitive alphabetic type. Here are
some examples of declaring `char` values:

<span class="filename">Filename: src/main.rs</span>

```rust
{{#rustdoc_include ../listings/ch03-common-programming-concepts/no-listing-09-char/src/main.rs}}
```

Note that we specify `char` literals with single quotes, as opposed to string
literals, which use double quotes. Rust’s `char` type is four bytes in size and
represents a Unicode Scalar Value, which means it can represent a lot more than
just ASCII. Accented letters; Chinese, Japanese, and Korean characters; emoji;
and zero-width spaces are all valid `char` values in Rust. Unicode Scalar
Values range from `U+0000` to `U+D7FF` and `U+E000` to `U+10FFFF` inclusive.
However, a “character” isn’t really a concept in Unicode, so your human
intuition for what a “character” is may not match up with what a `char` is in
Rust. We’ll discuss this topic in detail in [“Storing UTF-8 Encoded Text with
Strings”][strings]<!-- ignore --> in Chapter 8.

### Compound Types

_Compound types_ can group multiple values into one type. Rust has two
primitive compound types: tuples and arrays.

#### The Tuple Type

A _tuple_ is a general way of grouping together a number of values with a
variety of types into one compound type. Tuples have a fixed length: once
declared, they cannot grow or shrink in size.

We create a tuple by writing a comma-separated list of values inside
parentheses. Each position in the tuple has a type, and the types of the
different values in the tuple don’t have to be the same. We’ve added optional
type annotations in this example:

<span class="filename">Filename: src/main.rs</span>

```rust
{{#rustdoc_include ../listings/ch03-common-programming-concepts/no-listing-10-tuples/src/main.rs}}
```

The variable `tup` binds to the entire tuple because a tuple is considered a
single compound element. To get the individual values out of a tuple, we can
use pattern matching to destructure a tuple value, like this:

<span class="filename">Filename: src/main.rs</span>

```rust
{{#rustdoc_include ../listings/ch03-common-programming-concepts/no-listing-11-destructuring-tuples/src/main.rs}}
```

This program first creates a tuple and binds it to the variable `tup`. It then
uses a pattern with `let` to take `tup` and turn it into three separate
variables, `x`, `y`, and `z`. This is called _destructuring_ because it breaks
the single tuple into three parts. Finally, the program prints the value of
`y`, which is `6.4`.

We can also access a tuple element directly by using a period (`.`) followed by
the index of the value we want to access. For example:

<span class="filename">Filename: src/main.rs</span>

```rust
{{#rustdoc_include ../listings/ch03-common-programming-concepts/no-listing-12-tuple-indexing/src/main.rs}}
```

This program creates the tuple `x` and then accesses each element of the tuple
using their respective indices. As with most programming languages, the first
index in a tuple is 0.

The tuple without any values has a special name, _unit_. This value and its
corresponding type are both written `()` and represent an empty value or an
empty return type. Expressions implicitly return the unit value if they don’t
return any other value.

#### The Array Type

Another way to have a collection of multiple values is with an _array_. Unlike
a tuple, every element of an array must have the same type. Unlike arrays in
some other languages, arrays in Rust have a fixed length.

We write the values in an array as a comma-separated list inside square
brackets:

<span class="filename">Filename: src/main.rs</span>

```rust
{{#rustdoc_include ../listings/ch03-common-programming-concepts/no-listing-13-arrays/src/main.rs}}
```

Arrays are useful when you want your data allocated on the stack, the same as
the other types we have seen so far, rather than the heap (we will discuss the
stack and the heap more in [Chapter 4][stack-and-heap]<!-- ignore -->) or when
you want to ensure you always have a fixed number of elements. An array isn’t as
flexible as the vector type, though. A _vector_ is a similar collection type
provided by the standard library that _is_ allowed to grow or shrink in size. If
you’re unsure whether to use an array or a vector, chances are you should use a
vector. [Chapter 8][vectors]<!-- ignore --> discusses vectors in more detail.

However, arrays are more useful when you know the number of elements will not
need to change. For example, if you were using the names of the month in a
program, you would probably use an array rather than a vector because you know
it will always contain 12 elements:

```rust
let months = ["January", "February", "March", "April", "May", "June", "July",
              "August", "September", "October", "November", "December"];
```

You write an array’s type using square brackets with the type of each element,
a semicolon, and then the number of elements in the array, like so:

```rust
let a: [i32; 5] = [1, 2, 3, 4, 5];
```

Here, `i32` is the type of each element. After the semicolon, the number `5`
indicates the array contains five elements.

You can also initialize an array to contain the same value for each element by
specifying the initial value, followed by a semicolon, and then the length of
the array in square brackets, as shown here:

```rust
let a = [3; 5];
```

The array named `a` will contain `5` elements that will all be set to the value
`3` initially. This is the same as writing `let a = [3, 3, 3, 3, 3];` but in a
more concise way.

##### Accessing Array Elements

An array is a single chunk of memory of a known, fixed size that can be
allocated on the stack. You can access elements of an array using indexing,
like this:

<span class="filename">Filename: src/main.rs</span>

```rust
{{#rustdoc_include ../listings/ch03-common-programming-concepts/no-listing-14-array-indexing/src/main.rs}}
```

In this example, the variable named `first` will get the value `1` because that
is the value at index `[0]` in the array. The variable named `second` will get
the value `2` from index `[1]` in the array.

##### Invalid Array Element Access

Let’s see what happens if you try to access an element of an array that is past
the end of the array. Say you run this code, similar to the guessing game in
Chapter 2, to get an array index from the user:

<span class="filename">Filename: src/main.rs</span>

```rust,ignore,panics
{{#rustdoc_include ../listings/ch03-common-programming-concepts/no-listing-15-invalid-array-access/src/main.rs}}
```

This code compiles successfully. If you run this code using `cargo run` and
enter `0`, `1`, `2`, `3`, or `4`, the program will print out the corresponding
value at that index in the array. If you instead enter a number past the end of
the array, such as `10`, you’ll see output like this:

<!-- manual-regeneration
cd listings/ch03-common-programming-concepts/no-listing-15-invalid-array-access
cargo run
10
-->

```console
thread 'main' panicked at src/main.rs:19:19:
index out of bounds: the len is 5 but the index is 10
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
```

The program resulted in a _runtime_ error at the point of using an invalid
value in the indexing operation. The program exited with an error message and
didn’t execute the final `println!` statement. When you attempt to access an
element using indexing, Rust will check that the index you’ve specified is less
than the array length. If the index is greater than or equal to the length,
Rust will panic. This check has to happen at runtime, especially in this case,
because the compiler can’t possibly know what value a user will enter when they
run the code later.

This is an example of Rust’s memory safety principles in action. In many
low-level languages, this kind of check is not done, and when you provide an
incorrect index, invalid memory can be accessed. Rust protects you against this
kind of error by immediately exiting instead of allowing the memory access and
continuing. Chapter 9 discusses more of Rust’s error handling and how you can
write readable, safe code that neither panics nor allows invalid memory access.

[comparing-the-guess-to-the-secret-number]: ch02-00-guessing-game-tutorial.html#comparing-the-guess-to-the-secret-number
[twos-complement]: https://en.wikipedia.org/wiki/Two%27s_complement
[control-flow]: ch03-05-control-flow.html#control-flow
[strings]: ch08-02-strings.html#storing-utf-8-encoded-text-with-strings
[stack-and-heap]: ch04-01-what-is-ownership.html#the-stack-and-the-heap
[vectors]: ch08-01-vectors.html
[unrecoverable-errors-with-panic]: ch09-01-unrecoverable-errors-with-panic.html
[appendix_b]: appendix-02-operators.md
