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:
# #![allow(unused_variables)] #fn main() { 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:
error[E0282]: type annotations needed
--> src/main.rs:2:9
|
2 | let guess = "42".parse().expect("Not a number!");
| ^^^^^
| |
| cannot infer type for `_`
| consider giving `guess` a type
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.
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 or positive—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
.
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 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 withpanic!
” 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 au8
, 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. If you want to wrap explicitly, you can use the standard library typeWrapping
.
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.
The Character Type
So far we’ve worked only with numbers, but Rust supports letters too. Rust’s
char
type is the language’s most primitive alphabetic type, and the following
code shows one way to use it. (Note that char
literals are specified with
single quotes, as opposed to string literals, which use double quotes.)
Filename: src/main.rs
fn main() { let c = 'z'; let z = 'ℤ'; let heart_eyed_cat = '😻'; }
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”
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 some number of other 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:
Filename: src/main.rs
fn main() { let tup: (i32, f64, u8) = (500, 6.4, 1); }
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:
Filename: src/main.rs
fn main() { let tup = (500, 6.4, 1); let (x, y, z) = tup; println!("The value of y is: {}", y); }
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
.
In addition to destructuring through pattern matching, we can access a tuple
element directly by using a period (.
) followed by the index of the value we
want to access. For example:
Filename: src/main.rs
fn main() { let x: (i32, f64, u8) = (500, 6.4, 1); let five_hundred = x.0; let six_point_four = x.1; let one = x.2; }
This program creates a tuple, x
, and then makes new variables for each
element by using their index. As with most programming languages, the first
index in a tuple is 0.
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. Arrays in Rust are different from arrays in some other languages because arrays in Rust have a fixed length, like tuples.
In Rust, the values going into an array are written as a comma-separated list inside square brackets:
Filename: src/main.rs
fn main() { let a = [1, 2, 3, 4, 5]; }
Arrays are useful when you want your data allocated on the stack rather than the heap (we will discuss the stack and the heap more in Chapter 4) 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, you should probably use a vector. Chapter 8 discusses vectors in more detail.
An example of when you might want to use an array rather than a vector is in a program that needs to know the names of the months of the year. It’s very unlikely that such a program will need to add or remove months, so you can use an array because you know it will always contain 12 items:
# #![allow(unused_variables)] #fn main() { let months = ["January", "February", "March", "April", "May", "June", "July", "August", "September", "October", "November", "December"]; #}
You would write an array’s type by using square brackets, and within the brackets include the type of each element, a semicolon, and then the number of elements in the array, like so:
# #![allow(unused_variables)] #fn main() { 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 element contains five items.
Writing an array’s type this way looks similar to an alternative syntax for initializing an array: if you want to create an array that contains the same value for each element, you can specify the initial value, followed by a semicolon, and then the length of the array in square brackets, as shown here:
# #![allow(unused_variables)] #fn main() { 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 allocated on the stack. You can access elements of an array using indexing, like this:
Filename: src/main.rs
fn main() { let a = [1, 2, 3, 4, 5]; let first = a[0]; let second = a[1]; }
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
What happens if you try to access an element of an array that is past the end of the array? Say you change the example to the following code, which will compile but exit with an error when it runs:
Filename: src/main.rs
fn main() {
let a = [1, 2, 3, 4, 5];
let index = 10;
let element = a[index];
println!("The value of element is: {}", element);
}
Running this code using cargo run
produces the following result:
$ cargo run
Compiling arrays v0.1.0 (file:///projects/arrays)
Finished dev [unoptimized + debuginfo] target(s) in 0.31 secs
Running `target/debug/arrays`
thread 'main' panicked at 'index out of bounds: the len is 5 but the index is
10', src/main.rs:5:19
note: Run with `RUST_BACKTRACE=1` for a backtrace.
The compilation didn’t produce any errors, but the program resulted in a runtime error and didn’t exit successfully. 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 array length, Rust will panic.
This is the first example of Rust’s 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.