% 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:

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:

let v = vec![0; 10]; // A vector of ten zeroes.

Vectors store their contents as contiguous arrays of T on the heap. This means that they must be able to know the size of T at compile time (that is, how many bytes are needed to store a T?). The size of some things can't be known at compile time. For these you'll have to store a pointer to that thing: thankfully, the Box type works perfectly for this.

Accessing elements

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

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:

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 bound `collections::vec::Vec<_> : core::ops::Index<i32>`
is not satisfied [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.

Out-of-bounds Access

If you try to access an index that doesn’t exist:

let v = vec![1, 2, 3];
println!("Item 7 is {}", v[7]);

then the current thread will panic with a message like this:

thread 'main' panicked at 'index out of bounds: the len is 3 but the index is 7'

If you want to handle out-of-bounds errors without panicking, you can use methods like get or get_mut that return None when given an invalid index:

let v = vec![1, 2, 3];
match v.get(7) {
    Some(x) => println!("Item 7 is {}", x),
    None => println!("Sorry, this vector is too short.")
}

Iterating

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

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);
}

Note: You cannot use the vector again once you have iterated by taking ownership of the vector. You can iterate the vector multiple times by taking a reference to the vector whilst iterating. For example, the following code does not compile.

let v = vec![1, 2, 3, 4, 5];

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

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

Whereas the following works perfectly,

let v = vec![1, 2, 3, 4, 5];

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

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

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