Vectors

The first 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 where 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:

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

Note that we added a type annotation here. Since 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 homogeneous: they may store many values, but those values must all be the same type. Vectors are implemented using generics, which Chapter 10 will cover how to use in your own types. For now, all you need to know is that the Vec type provided by the standard library can hold any type, and when a specific Vec holds a specific type, the type goes within angle brackets. We’ve told Rust that the Vec in v will hold elements of the i32 type.

In real code, Rust can 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 that has initial values, and Rust provides the vec! macro for convenience. The macro will create a new Vec that holds the values we give it. This will create a new Vec<i32> that holds the values 1, 2, and 3:

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

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. Let’s look at how to modify a vector next.

Updating a Vector

To create a vector then add elements to it, we can use the push method:

let mut v = Vec::new();

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

As with any variable as we discussed in Chapter 3, if we want to be able to change its value, we need to make it mutable with 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:

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

    // do stuff with v

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

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 little more complicated once 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.

This example shows both methods of accessing a value in a vector either with indexing syntax or the get method:

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

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

There are a few things to note here. First, that 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: using & and [], which gives us a reference, or 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 that 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, what should a program do if it has a vector that holds five elements then tries to access an element at index 100 like this:

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

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

When you run this, you will find that with the first [] method, Rust will cause a panic! when a non-existent element is referenced. This method would be preferable if you want your program to consider an attempt to access an element past the end of the vector to be a fatal error that should crash the program.

When the get method is passed an index that is outside the array, it will return None without panicking. You would use this if accessing an element beyond the range of the vector will happen occasionally under normal circumstances. Your code can then have logic to handle having either Some(&element) or None, as we 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 your program gets a None value, you could tell the user how many items are in the current Vec and give them another chance to enter a valid value. That would be more user-friendly than crashing the program for a typo!

Invalid References

Once the program has a valid reference, the borrow checker will enforce the ownership and borrowing rules covered in Chapter 4 to ensure this reference and any other references to the contents of the vector stay valid. Recall the rule that says we can’t have mutable and immutable references in the same scope. That rule applies in this example, where we hold an immutable reference to the first element in a vector and try to add an element to the end:

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

let first = &v[0];

v.push(6);

Compiling this will give us this error:

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 | }
  | - immutable borrow ends here

This code might look like it should work: why should a reference to the first element care about what changes about the end of the vector? The reason why this code isn’t allowed 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 over to the new space, in the circumstance that 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 this, see The Nomicon at https://doc.rust-lang.org/stable/nomicon/vec.html.

Using an Enum to Store Multiple Types

At the beginning of this chapter, we said that vectors can only store values that are all the same type. This can be inconvenient; there are definitely use cases for needing to store a list of things of different types. Luckily, the variants of an enum are all 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 of 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:

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

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

Listing 8-1: Defining an enum to be able to hold different types of data in a vector

The reason Rust needs to know exactly what types will be in the vector at compile time is so that it knows exactly how much memory on the heap will be needed to store each element. A secondary advantage to this 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 means that Rust will ensure at compile time that we always handle every possible case, as we discussed in Chapter 6.

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

Now that we’ve gone over some of the most common ways to use vectors, be sure to take a look at the API documentation for all of the many useful methods defined on Vec by the standard library. For example, in addition to push there’s a pop method that will remove and return the last element. Let’s move on to the next collection type: String!