Vectors

A ‘vector’ is a dynamic or ‘growable’ array, implemented as the standard library type Vec<T>. The T means that we can have vectors of any type (see the chapter on generics for more). Vectors always allocate their data on the heap. You can create them with the vec! macro:

fn main() { let v = vec![1, 2, 3, 4, 5]; // v: Vec<i32> }
let v = vec![1, 2, 3, 4, 5]; // v: Vec<i32>

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

There’s an alternate form of vec! for repeating an initial value:

fn main() { let v = vec![0; 10]; // ten zeroes }
let v = vec![0; 10]; // ten zeroes

Accessing elements

To get the value at a particular index in the vector, we use []s:

fn main() { let v = vec![1, 2, 3, 4, 5]; println!("The third element of v is {}", v[2]); }
let v = vec![1, 2, 3, 4, 5];

println!("The third element of v is {}", v[2]);

The indices count from 0, so the third element is v[2].

It’s also important to note that you must index with the usize type:

fn main() { let v = vec![1, 2, 3, 4, 5]; let i: usize = 0; let j: i32 = 0; // works v[i]; // doesn’t v[j]; }
let v = vec![1, 2, 3, 4, 5];

let i: usize = 0;
let j: i32 = 0;

// works
v[i];

// doesn’t
v[j];

Indexing with a non-usize type gives an error that looks like this:

error: the trait `core::ops::Index<i32>` is not implemented for the type
`collections::vec::Vec<_>` [E0277]
v[j];
^~~~
note: the type `collections::vec::Vec<_>` cannot be indexed by `i32`
error: aborting due to previous error

There’s a lot of punctuation in that message, but the core of it makes sense: you cannot index with an i32.

Iterating

Once you have a vector, you can iterate through its elements with for. There are three versions:

fn main() { let mut v = vec![1, 2, 3, 4, 5]; for i in &v { println!("A reference to {}", i); } for i in &mut v { println!("A mutable reference to {}", i); } for i in v { println!("Take ownership of the vector and its element {}", i); } }
let mut v = vec![1, 2, 3, 4, 5];

for i in &v {
    println!("A reference to {}", i);
}

for i in &mut v {
    println!("A mutable reference to {}", i);
}

for i in v {
    println!("Take ownership of the vector and its element {}", i);
}

Vectors have many more useful methods, which you can read about in their API documentation.