The Rust Programming Language

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)]
fn main() {
let guess: u32 = "42".parse().expect(msg: "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:

$ cargo build
   Compiling no_type_annotations v0.1.0 (file:///projects/no_type_annotations)
error[E0282]: type annotations needed
 --> src/main.rs:2:9
  |
2 |     let guess = "42".parse().expect("Not a number!");
  |         ^^^^^ consider giving `guess` a type

For more information about this error, try `rustc --explain E0282`.
error: could not compile `no_type_annotations` due to previous error

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.

Table 3-1: Integer Types in Rust

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 representation.

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 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.

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 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 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 with panic!” 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 that the standard library provides on 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 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.

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

Filename: src/main.rs

#![allow(unused)]
fn main() {
fn main() {
    let x: f64 = 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. Integer division rounds down to the nearest integer. The following code shows how you’d use each numeric operation in a let statement:

Filename: src/main.rs

#![allow(unused)]
fn main() {
fn main() {
    // addition
    let sum: i32 = 5 + 10;

    // subtraction
    let difference: f64 = 95.5 - 4.3;

    // multiplication
    let product: i32 = 4 * 30;

    // division
    let quotient: f64 = 56.7 / 32.2;
    let floored: i32 = 2 / 3; // Results in 0

    // remainder
    let remainder: i32 = 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

#![allow(unused)]
fn main() {
fn main() {
    let t: bool = 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

#![allow(unused)]
fn main() {
fn main() {
    let c: char = 'z';
    let z: char = 'ℤ';
    let heart_eyed_cat =: char '😻';
}
}

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 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:

Filename: src/main.rs

#![allow(unused)]
fn main() {
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

#![allow(unused)]
fn main() {
fn main() {
    let tup: (i32, f64, i32) = (500, 6.4, 1);

    let (x: i32, y: f64, z: i32) = 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

#![allow(unused)]
fn main() {
fn main() {
    let x: (i32, f64, u8) = (500, 6.4, 1);

    let five_hundred: i32 = x.0;

    let six_point_four: f64 = x.1;

    let one: u8 = x.2;
}
}

This program creates a tuple, x, and then makes new variables for each element by using their respective indices. As with most programming languages, the first index in a tuple is 0.

The tuple without any values, (), is a special type that has only one value, also written (). The type is called the unit type and the value is called the unit value. 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. 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

#![allow(unused)]
fn main() {
fn main() {
    let a: [i32; 5] = [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 elements:

#![allow(unused)]
fn main() {
let months: [&str; 12] = ["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)]
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 array contains five elements.

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)]
fn main() {
let a: [i32; 5] = [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:

Filename: src/main.rs

#![allow(unused)]
fn main() {
fn main() {
    let a: [i32; 5] = [1, 2, 3, 4, 5];

    let first: i32 = a[0];
    let second: i32 = 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, which uses code similar to the guessing game in Chapter 2 to get an array index from the user:

Filename: src/main.rs

#![allow(unused)]
fn main() {
use std::io;

fn main() {
    let a: [i32; 5] = [1, 2, 3, 4, 5];

    println!("Please enter an array index.");

    let mut index = String::new();

    io::stdin()
        .read_line(buf: &mut index)
        .expect(msg: "Failed to read line");

    let index: usize = index
        .trim()
        .parse()
        .expect(msg: "Index entered was not a number");

    let element: i32 = a[index];

    println!(
        "The value of the element at index {} is: {}",
        index, element
    );
}
}

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:

thread 'main' panicked at 'index out of bounds: the len is 5 but the index is 10', src/main.rs:19:19
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.