8 Mutable Vectors

Objective

Learn how to create and modify vectors with mutable operations.

In Rust, vectors (Vec<T>) are growable arrays. To modify a vector after creating it, the vector itself must be declared as mut.

You will often see <T> in Rust code or documentation. T is a way of saying “of any type.” Seeing Vec<T> can be read as “a vector of any type” or “generic over type T.”

Creating empty vectors

You can create an empty vector and let Rust infer the type based on usage:

fn main() {
    let mut names = Vec::new();
    names.push("Alice");
    names.push("Bob");
    println!("{:?}", names);
}

Vec::new() creates an empty vector.
.push() adds an element to the end of a vector.

Clearing vectors

You can remove all elements from a vector using .clear():

fn main() {
    let mut nums = vec![1, 2, 3];
    nums.clear();
    println!("{:?}", nums); // prints []
}

Sorting vectors

Use .sort() to sort a vector. Important to note that not all types can be sorted.

fn main() {
    let mut x = vec![11, 3, 7, 10, 1];
    println!("x before sorting: {x:?}");
    x.sort();
    println!("x after sorting:  {x:?}");
}

x before sorting: [11, 3, 7, 10, 1]
x after sorting:  [1, 3, 7, 10, 11]

Combining vectors

Use .extend() to append one vector’s contents to another:

fn main() {
    let mut a = vec![1, 2];
    let b = vec![3, 4];
    // note that we don't assign.
    // Instead, `a` is modified in place
    a.extend(b);
    println!("{:?}", a); // prints [1, 2, 3, 4]
}

.extend() adds the contents of another vector.
The original vector must be mut.
The second vector is moved into the first.
b can no longer be used.

Exercise

Create an empty vector
Append the values 1.0, 2.0, and 3.0 using .push()
Clear the vector to make it empty
Lastly, extend it with another vector e.g. [4.0, 5.0]
Sort the vector
Print the final result.

Solution

View solution

fn main() {
    let mut x = Vec::new();
    x.push(1.0);
    x.push(2.0);
    x.push(3.0);
    x.clear();
    x.extend(vec![4.0, 5.0]);
    println!("{:?}", x);
}