Arrays, Vectors, and Slices

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]; // mut m: [i32; 3]

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]

Arrays have type [T; N]. We'll talk about this T notation later, when we cover generics.

You can get the number of elements in an array a with a.len(), and use a.iter() to iterate over them with a for loop. This code will print each number in order:

let a = [1, 2, 3];

println!("a has {} elements", a.len());
for e in a.iter() {
    println!("{}", e);

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 languages.

A vector is a dynamic or "growable" array, implemented as the standard library type Vec<T> (we'll talk about what the <T> means later). Vectors always allocate their data on the heap. Vectors are to slices what String is to &str. You can create them with the vec! macro:

let v = vec![1, 2, 3]; // v: Vec<i32>

(Notice that unlike the println! macro we've used in the past, we use square brackets [] with vec!. Rust allows you to use either in either situation, this is just convention.)

You can get the length of, iterate over, and subscript vectors just like arrays. In addition, (mutable) vectors can grow automatically:

let mut nums = vec![1, 2, 3]; // mut nums: Vec<i32>


println!("The length of nums is now {}", nums.len()); // Prints 4

Vectors have many more useful methods.

A slice is a reference to (or "view" into) an array. 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

for e in middle.iter() {
    println!("{}", e); // Prints 1, 2, 3

You can also take a slice of a vector, String, or &str, because they are backed by arrays. Slices have type &[T], which we'll talk about when we cover generics.

We have now learned all of the most basic Rust concepts. We're ready to start building ourselves a guessing game, we just need to know one last thing: how to get input from the keyboard. You can't have a guessing game without the ability to guess!