Vectors

Arrays' strength is also their weakness: their size must be known upfront, at compile-time. If you try to create an array with a size that's only known at runtime, you'll get a compilation error:

let n = 10;
let numbers: [u32; n];

error[E0435]: attempt to use a non-constant value in a constant
 --> src/main.rs:3:20
  |
2 | let n = 10;
3 | let numbers: [u32; n];
  |                    ^ non-constant value

Arrays wouldn't work for our ticket management system—we don't know how many tickets we'll need to store at compile-time. This is where Vec comes in.

Vec

Vec is a growable array type, provided by the standard library.
You can create an empty array using the Vec::new function:

let mut numbers: Vec<u32> = Vec::new();

You would then push elements into the vector using the push method:

numbers.push(1);
numbers.push(2);
numbers.push(3);

New values are added to the end of the vector.
You can also create an initialized vector using the vec! macro, if you know the values at creation time:

let numbers = vec![1, 2, 3];

Accessing elements

The syntax for accessing elements is the same as with arrays:

let numbers = vec![1, 2, 3];
let first = numbers[0];
let second = numbers[1];
let third = numbers[2];

The index must be of type usize.
You can also use the get method, which returns an Option<&T>:

let numbers = vec![1, 2, 3];
assert_eq!(numbers.get(0), Some(&1));
// You get a `None` if you try to access an out-of-bounds index
// rather than a panic.
assert_eq!(numbers.get(3), None);

Access is bounds-checked, just like element access with arrays. It has O(1) complexity.

Memory layout

Vec is a heap-allocated data structure.
When you create a Vec, it allocates memory on the heap to store the elements.

If you run the following code:

let mut numbers = Vec::with_capacity(3);
numbers.push(1);
numbers.push(2);

you'll get the following memory layout:

      +---------+--------+----------+
Stack | pointer | length | capacity | 
      |  |      |   2    |    3     |
      +--|------+--------+----------+
         |
         |
         v
       +---+---+---+
Heap:  | 1 | 2 | ? |
       +---+---+---+

Vec keeps track of three things:

• The pointer to the heap region you reserved.
• The length of the vector, i.e. how many elements are in the vector.
• The capacity of the vector, i.e. the number of elements that can fit in the space reserved on the heap.

This layout should look familiar: it's exactly the same as String!
That's not a coincidence: String is defined as a vector of bytes, Vec<u8>, under the hood:

pub struct String {
    vec: Vec<u8>,
}

Exercise

The exercise for this section is located in 06_ticket_management/02_vec