Option: when a record might be absent

Before this lesson: lessons 15, 23–24 and 29–30. If match branches are unclear, revisit lesson 30.

Familiar image and recall map

Ask an archivist for a numbered card that is not in the drawer. They either hand you the card or say that it is absent. Option<T> is Rust’s standard enum for a value that might be absent. Some(T) holds a value of type T; None means no value is present. The image has a limit: None does not explain why the card is missing. The next lesson introduces a different type when a failure needs a reason.

Request → Option<T> → Some(value) or None → handle both with match. As with Vec<String>, the angle brackets name the type of possible contents. In Option<&String>, the contents would be a reference to a String, not a copied string. Some and None need no separate import. An Option<T> is not already a T; inspect its variant before using the inner value.

Read a list safely

Save the previous src/main.rs in organizer, replace the whole file, and run cargo run beside Cargo.toml. No other files or dependencies change. Predict both output lines.

The .get(index) method on Vec asks for an element at a position. Unlike titles[index], a missing position does not stop the program: .get() returns Option<&String>. Some(title) in match names a reference to the found text; None handles absence. We build the list with Vec::new() and push from lesson 24. Position 0 exists, but position 2 does not exist in a one-element list.

fn main() {
    let mut titles: Vec<String> = Vec::new();
    titles.push(String::from("Reading"));
    for index in [0, 2] {
        match titles.get(index) {
            Some(title) => println!("Found: {title}"),
            None => println!("No record at position {index}"),
        }
    }
}

Found: Reading
No record at position 2

get neither copies the title nor transfers ownership. The found value is borrowed from titles, so the list remains available.

Absent is different from empty text

Some(String::from("")) means a value exists but its text is empty. None means no value exists at all. These are different states. With None alone, Rust sometimes cannot infer what could have been inside; let missing: Option<String> = None; states the type. For titles.get(index), the list determines it.

Assigning the result of .get() to a plain reference without inspecting the variant makes this separate example fail with E0308:

fn main() {
    let mut titles: Vec<String> = Vec::new();
    titles.push(String::from("Reading"));
    let title: &String = titles.get(0);
    println!("{title}");
}

Use match with Some and None, or another absence-handling method after it has been explained. A list position is still not a task’s permanent ID: get avoids an out-of-bounds access but does not solve the stable identifier problem in lesson 38.

Check your understanding

1. How does Option<&String> differ from &String?
2. Does None mean that an empty string was found?
3. Why does .get(2) avoid a panic for a one-element list?

Check: the reference might be absent; no, empty text is still a value; get returns None. Close the page and rebuild the recall map.

Exercise

Required. Build a list containing Reading and Walk. Request positions 1 and 2 through .get() and handle both with match. Print Found: Walk and Position 2 is absent. Do not check the length yourself or put a finished title in the message.

Boundary check. Create an empty list and explain what .get(0) returns.

Hint

For each index in [1, 2], use match titles.get(index). Print the found title in Some(title) and the index in None.

Reference answer after trying

fn main() {
    let mut titles: Vec<String> = Vec::new();
    titles.push(String::from("Reading"));
    titles.push(String::from("Walk"));
    for index in [1, 2] {
        match titles.get(index) {
            Some(title) => println!("Found: {title}"),
            None => println!("Position {index} is absent"),
        }
    }
}

Found: Walk
Position 2 is absent

The list remains the owner of its titles, and absence is handled explicitly. Official Option chapter.

Previous lesson · Contents · Next lesson