Data Types

Every value in Rust is of a certain type, which tells Rust what kind of data is being specified so it knows how to work with that data. In this section, we’ll look at a number of types that are built into the language. We split the types into two subsets: scalar and compound.

Throughout this section, 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 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 possible type we want to use:

error[E0282]: unable to infer enough type information about `_`
 --> src/main.rs:2:9
  |
2 |     let guess = "42".parse().expect("Not a number!");
  |         ^^^^^ cannot infer type for `_`
  |
  = note: type annotations or generic parameter binding required

You’ll see different type annotations as we discuss the various 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’ll likely recognize these from other programming languages, but 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 earlier in this chapter, the i32 type. This type declaration indicates that the value it’s associated with should be a signed integer (hence the i, as opposed to a u for unsigned) 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, i32) can be used to declare the type of an integer value.

Table 3-1: Integer Types in Rust

Each variant can be either signed or unsigned and has an explicit size. Signed and unsigned refers 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 (if you’re unsure what this is, you can search for it online; an explanation is outside the scope of this book).

Each signed variant can store numbers from -(2^{n - 1}) to 2^{n - 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

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: it’s 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.

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 it’s roughly the same speed as f32 but is capable of more precision. It’s possible to use an f64 type on 32-bit systems, but it will be slower than using an f32 type on those systems. Most of the time, trading potential worse performance for better precision is a reasonable initial choice, and you should benchmark your code if you suspect floating-point size is a problem in your situation.

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 usual basic mathematic 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. 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 consume boolean values is through conditionals, such as an if statement. We’ll cover how if statements work in Rust in the “Control Flow” section.

The Character Type

So far we’ve only worked 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:

Filename: src/main.rs

fn main() {
   let c = 'z';
   let z = 'ℤ';
   let heart_eyed_cat = '😻';
}

Rust’s char type represents a Unicode Scalar Value, which means it can represent a lot more than just ASCII. Accented letters, Chinese/Japanese/Korean ideographs, emoji, and zero width spaces are all valid char types 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 the “Strings” section in Chapter 8.

Compound Types

Compound types can group multiple values of other types into one type. Rust has two primitive compound types: tuples and arrays.

Grouping Values into Tuples

A tuple is a general way of grouping together some number of other values with a variety of types into one compound type.

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, since 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 also 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.

Arrays

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 than arrays in some other languages because arrays in Rust have a fixed length: once declared, they cannot grow or shrink in size.

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. They aren’t as flexible as the vector type, though. The vector type 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:

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

Accessing Array Elements

An array is a single chunk of memory allocated on the stack. We 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 we try to access an element of an array that is past the end of the array? Say we change the example to the following:

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)
     Running `target/debug/arrays`
thread '<main>' panicked at 'index out of bounds: the len is 5 but the index is
 10', src/main.rs:6
note: Run with `RUST_BACKTRACE=1` for a backtrace.

The compilation didn’t produce any errors, but the program results 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 the length, Rust will panic, which is the term Rust uses when a program exits with an error.

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.