## Vectors Store Lists of Values

The first collection type we’ll look at is `Vec<T>`, also known as a *vector*.
Vectors allow us to store more than one value in a single data structure that
puts all the values next to each other in memory. Vectors can only store values
of the same type. They are useful in situations in which you have a list of
items, such as the lines of text in a file or the prices of items in a shopping
cart.

### Creating a New Vector

To create a new, empty vector, we can call the `Vec::new` function as shown in
Listing 8-1:

```rust
let v: Vec<i32> = Vec::new();
```

Listing 8-1: Creating a new, empty vector to hold values
of type `i32`

Note that we added a type annotation here. Because we aren’t inserting any
values into this vector, Rust doesn’t know what kind of elements we intend to
store. This is an important point. Vectors are implemented using generics;
we’ll cover how to use generics with your own types in Chapter 10. For now,
know that the `Vec<T>` type provided by the standard library can hold any type,
and when a specific vector holds a specific type, the type is specified within
angle brackets. In Listing 8-1, we’ve told Rust that the `Vec<T>` in `v` will
hold elements of the `i32` type.

In more realistic code, Rust can often infer the type of value we want to store
once we insert values, so you rarely need to do this type annotation. It’s more
common to create a `Vec<T>` that has initial values, and Rust provides the
`vec!` macro for convenience. The macro will create a new vector that holds the
values we give it. Listing 8-2 creates a new `Vec<i32>` that holds the values
`1`, `2`, and `3`:

```rust
let v = vec![1, 2, 3];
```

Listing 8-2: Creating a new vector containing
values

Because we’ve given initial `i32` values, Rust can infer that the type of `v`
is `Vec<i32>`, and the type annotation isn’t necessary. Next, we’ll look at how
to modify a vector.

### Updating a Vector

To create a vector and then add elements to it, we can use the `push` method as
shown in Listing 8-3:

```rust
let mut v = Vec::new();

v.push(5);
v.push(6);
v.push(7);
v.push(8);
```

Listing 8-3: Using the `push` method to add values to a
vector

As with any variable, as discussed in Chapter 3, if we want to be able to
change its value, we need to make it mutable using the `mut` keyword. The
numbers we place inside are all of type `i32`, and Rust infers this from the
data, so we don’t need the `Vec<i32>` annotation.

### Dropping a Vector Drops Its Elements

Like any other `struct`, a vector will be freed when it goes out of scope, as
annotated in Listing 8-4:

```rust
{
    let v = vec![1, 2, 3, 4];

    // do stuff with v

} // <- v goes out of scope and is freed here
```

Listing 8-4: Showing where the vector and its elements
are dropped

When the vector gets dropped, all of its contents will also be dropped, meaning
those integers it holds will be cleaned up. This may seem like a
straightforward point but can get a bit more complicated when we start to
introduce references to the elements of the vector. Let’s tackle that next!

### Reading Elements of Vectors

Now that you know how to create, update, and destroy vectors, knowing how to
read their contents is a good next step. There are two ways to reference a
value stored in a vector. In the examples, we’ve annotated the types of the
values that are returned from these functions for extra clarity.

Listing 8-5 shows both methods of accessing a value in a vector either with
indexing syntax or the `get` method:

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

let third: &i32 = &v[2];
let third: Option<&i32> = v.get(2);
```

Listing 8-5: Using indexing syntax or the `get` method to
access an item in a vector

Note two details here. First, we use the index value of `2` to get the third
element: vectors are indexed by number, starting at zero. Second, the two
different ways to get the third element are by using `&` and `[]`, which gives
us a reference, or by using the `get` method with the index passed as an
argument, which gives us an `Option<&T>`.

The reason Rust has two ways to reference an element is so you can choose how
the program behaves when you try to use an index value that the vector doesn’t
have an element for. As an example, let’s see what a program will do if it has
a vector that holds five elements and then tries to access an element at index
100, as shown in Listing 8-6:

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

let does_not_exist = &v[100];
let does_not_exist = v.get(100);
```

Listing 8-6: Attempting to access the element at index
100 in a vector containing 5 elements

When you run this code, the first `[]` method will cause a `panic!` because it
references a nonexistent element. This method is best used when you want your
program to consider an attempt to access an element past the end of the vector
to be a fatal error that crashes the program.

When the `get` method is passed an index that is outside the vector, it returns
`None` without panicking. You would use this method if accessing an element
beyond the range of the vector happens occasionally under normal circumstances.
Your code will then have logic to handle having either `Some(&element)` or
`None`, as discussed in Chapter 6. For example, the index could be coming from
a person entering a number. If they accidentally enter a number that’s too
large and the program gets a `None` value, you could tell the user how many
items are in the current vector and give them another chance to enter a valid
value. That would be more user-friendly than crashing the program due to a typo!

#### Invalid References

When the program has a valid reference, the borrow checker enforces the
ownership and borrowing rules (covered in Chapter 4) to ensure this reference
and any other references to the contents of the vector remain valid. Recall the
rule that states we can’t have mutable and immutable references in the same
scope. That rule applies in Listing 8-7 where we hold an immutable reference to
the first element in a vector and try to add an element to the end, which won’t
work:

```rust,ignore
let mut v = vec![1, 2, 3, 4, 5];

let first = &v[0];

v.push(6);
```

Listing 8-7: Attempting to add an element to a vector
while holding a reference to an item

Compiling this code will result in this error:

```text
error[E0502]: cannot borrow `v` as mutable because it is also borrowed as immutable
 -->
  |
4 |     let first = &v[0];
  |                  - immutable borrow occurs here
5 |
6 |     v.push(6);
  |     ^ mutable borrow occurs here
7 |
8 | }
  | - immutable borrow ends here
```

The code in Listing 8-7 might look like it should work: why should a reference
to the first element care about what changes at the end of the vector? The
reason behind this error is due to the way vectors work: adding a new element
onto the end of the vector might require allocating new memory and copying the
old elements to the new space if there isn’t enough room to put all the
elements next to each other where the vector was. In that case, the reference
to the first element would be pointing to deallocated memory. The borrowing
rules prevent programs from ending up in that situation.

> Note: For more on the implementation details of the `Vec<T>` type, see “The
> Nomicon” at https://doc.rust-lang.org/stable/nomicon/vec.html.

### Iterating Over the Values in a Vector

If we want to access each element in a vector in turn, we can iterate through
all of the elements rather than use indexes to access one at a time. Listing
8-8 shows how to use a `for` loop to get immutable references to each element
in a vector of `i32` values and print them out:

```rust
let v = vec![100, 32, 57];
for i in &v {
    println!("{}", i);
}
```

Listing 8-8: Printing each element in a vector by
iterating over the elements using a `for` loop

We can also iterate over mutable references to each element in a mutable vector
in order to make changes to all the elements. The `for` loop in Listing 8-9
will add `50` to each element:

```rust
let mut v = vec![100, 32, 57];
for i in &mut v {
    *i += 50;
}
```

Listing 8-9: Iterating over mutable references to
elements in a vector

To change the value that the mutable reference refers to, we have to use the
dereference operator (`*`) to get to the value in `i` before we can use the
`+=` operator .

### Using an Enum to Store Multiple Types

At the beginning of this chapter, we said that vectors can only store values
that are the same type. This can be inconvenient; there are definitely use
cases for needing to store a list of items of different types. Fortunately, the
variants of an enum are defined under the same enum type, so when we need to
store elements of a different type in a vector, we can define and use an enum!

For example, let’s say we want to get values from a row in a spreadsheet where
some of the columns in the row contain integers, some floating-point numbers,
and some strings. We can define an enum whose variants will hold the different
value types, and then all the enum variants will be considered the same type:
that of the enum. Then we can create a vector that holds that enum and so,
ultimately, holds different types. We’ve demonstrated this in Listing 8-10:

```rust
enum SpreadsheetCell {
    Int(i32),
    Float(f64),
    Text(String),
}

let row = vec![
    SpreadsheetCell::Int(3),
    SpreadsheetCell::Text(String::from("blue")),
    SpreadsheetCell::Float(10.12),
];
```

Listing 8-10: Defining an `enum` to store values of
different types in one vector

The reason Rust needs to know what types will be in the vector at compile time
is so it knows exactly how much memory on the heap will be needed to store each
element. A secondary advantage is that we can be explicit about what types are
allowed in this vector. If Rust allowed a vector to hold any type, there would
be a chance that one or more of the types would cause errors with the
operations performed on the elements of the vector. Using an enum plus a
`match` expression means that Rust will ensure at compile time that we always
handle every possible case, as discussed in Chapter 6.

If you don’t know the exhaustive set of types the program will get at runtime
to store in a vector when you’re writing a program, the enum technique won’t
work. Instead, you can use a trait object, which we’ll cover in Chapter 17.

Now that we’ve discussed some of the most common ways to use vectors, be sure
to review the API documentation for all the many useful methods defined on
`Vec<T>` by the standard library. For example, in addition to `push`, a `pop`
method removes and returns the last element. Let’s move on to the next
collection type: `String`!
