Storing Lists of Values with Vectors

The first collection type we’ll look at is Vec<T>, also known as a vector. Vectors allow you 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 when 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 call the Vec::new function, as shown in Listing 8-1.

fn main() {
    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. When we create a vector to hold a specific type, we can specify the type within angle brackets. In Listing 8-1, we’ve told Rust that the Vec<T> in v will hold elements of the i32 type.

More often, you’ll create a Vec<T> with initial values and Rust will infer the type of value you want to store, so you rarely need to do this type annotation. Rust conveniently provides the vec! macro, which will create a new vector that holds the values you give it. Listing 8-2 creates a new Vec<i32> that holds the values 1, 2, and 3. The integer type is i32 because that’s the default integer type, as we discussed in the “Data Types” section of Chapter 3.

fn main() {
    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.

fn main() {
    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, if we want to be able to change its value, we need to make it mutable using the mut keyword, as discussed in Chapter 3. 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.

Reading Elements of Vectors

There are two ways to reference a value stored in a vector: via indexing or using the get method. In the following examples, we’ve annotated the types of the values that are returned from these functions for extra clarity.

Listing 8-4 shows both methods of accessing a value in a vector, with indexing syntax and the get method.

fn main() {
    let v = vec![1, 2, 3, 4, 5];

    let third: &i32 = &v[2];
    println!("The third element is {}", third);

    let third: Option<&i32> = v.get(2);
    match third {
        Some(third) => println!("The third element is {}", third),
        None => println!("There is no third element."),
    }
}

Listing 8-4: Using indexing syntax or the get method to access an item in a vector

Note a few details here. We use the index value of 2 to get the third element because vectors are indexed by number, starting at zero. Using & and [] gives us a reference to the element at the index value. When we use the get method with the index passed as an argument, we get an Option<&T> that we can use with match.

The reason Rust provides these two ways to reference an element is so you can choose how the program behaves when you try to use an index value outside the range of existing elements. As an example, let’s see what happens when we have a vector of five elements and then we try to access an element at index 100 with each technique, as shown in Listing 8-5.

fn main() {
    let v = vec![1, 2, 3, 4, 5];

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

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

When we run this code, the first [] method will cause the program to panic because it references a nonexistent element. This method is best used when you want your program to crash if there’s an attempt to access an element past the end of the vector.

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 may happen 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!

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 you can’t have mutable and immutable references in the same scope. That rule applies in Listing 8-6, where we hold an immutable reference to the first element in a vector and try to add an element to the end. This program won’t work if we also try to refer to that element later in the function:

fn main() {
    let mut v = vec![1, 2, 3, 4, 5];

    let first = &v[0];

    v.push(6);

    println!("The first element is: {}", first);
}

Listing 8-6: Attempting to add an element to a vector while holding a reference to an item

Compiling this code will result in this error:

$ cargo run
   Compiling collections v0.1.0 (file:///projects/collections)
error[E0502]: cannot borrow `v` as mutable because it is also borrowed as immutable
 --> src/main.rs:6:5
  |
4 |     let first = &v[0];
  |                  - immutable borrow occurs here
5 | 
6 |     v.push(6);
  |     ^^^^^^^^^ mutable borrow occurs here
7 | 
8 |     println!("The first element is: {}", first);
  |                                          ----- immutable borrow later used here

For more information about this error, try `rustc --explain E0502`.
error: could not compile `collections` due to previous error

The code in Listing 8-6 might look like it should work: why should a reference to the first element care about changes at the end of the vector? This error is due to the way vectors work: because vectors put the values next to each other in memory, 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 is currently stored. 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 Rustonomicon”.

Iterating over the Values in a Vector

To access each element in a vector in turn, we would iterate through all of the elements rather than use indices to access one at a time. Listing 8-7 shows how to use a for loop to get immutable references to each element in a vector of i32 values and print them.

fn main() {
    let v = vec![100, 32, 57];
    for n_ref in &v {
        // n_ref has type &i32
        let n_plus_one: i32 = *n_ref + 1;
        println!("{}", n_plus_one);
    }
}

Listing 8-7: Accessing each element in a vector by iterating over the elements using a for loop

To read the number that n_ref refers to, we have to use the * dereference operator to get to the value in n_ref before we can add 1 to it. We’ll talk more about the dereference operator in the “Following the Pointer to the Value with the Dereference Operator” section of Chapter 15.

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 8-8 will add 50 to each element.

fn main() {
    let mut v = vec![100, 32, 57];
    for n_ref in &mut v {
        // n_ref has type &mut i32
        *n_ref += 50;
    }
}

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

To change the value that the mutable reference refers to, we again use the * dereference operator to get to the value in n_ref before we can use the += operator.

Iterating over a vector, whether immutably or mutably, is safe because of the borrow checker's rules. If we attempted to insert or remove items in the for loop bodies in Listing 8-7 and Listing 8-8, we would get a compiler error similar to the one we got with the code in Listing 8-6. The reference to the vector that the for loop holds prevents simultaneous modification of the whole vector.

Using an Enum to Store Multiple Types

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 one type to represent elements of different types, we can define and use an enum!

For example, say we want to get values from a row in a spreadsheet in which 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 all the enum variants will be considered the same type: that of the enum. Then we can create a vector to hold that enum and so, ultimately, holds different types. We’ve demonstrated this in Listing 8-9.

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

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

Listing 8-9: Defining an enum to store values of different types in one vector

Rust needs to know what types will be in the vector at compile time so it knows exactly how much memory on the heap will be needed to store each element. We must also 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 every possible case is handled, as discussed in Chapter 6.

If you don’t know the exhaustive set of types a 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 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.

Dropping a Vector Drops Its Elements

Like any other struct, a vector is freed when it goes out of scope, as annotated in Listing 8-10.

fn main() {
    {
        let v = vec![1, 2, 3, 4];

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

Listing 8-10: Showing where the vector and its elements are dropped

When the vector gets dropped, all of its contents are also dropped, meaning the integers it holds will be cleaned up. The borrow checker ensures that any references to contents of a vector are only used while the vector itself is valid.

Let’s move on to the next collection type: String!