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# 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:

    # #![allow(unused_variables)]
    #fn main() {
    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:

    # #![allow(unused_variables)]
    #fn main() {
    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:

    # #![allow(unused_variables)]
    #fn main() {
    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:

    # #![allow(unused_variables)]
    #fn main() {
    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:

    # #![allow(unused_variables)]
    #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);
    }
    #}

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,

    # #![allow(unused_variables)]
    #fn main() {
    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.