# 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>Run`

(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]; // ten zeroesRun`

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]);Run```

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];Run```

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]);Run```

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.")
}Run```

## 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);
}Run```

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);
}Run```

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);
}Run```

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