Option instead of null
- Rust course
- Beginner
- Lesson 81/115
- 4 min
Learn the advantages of Option over null.
Option instead of null
In other languages "not found" is often shown with null, -1 or an empty string, and bugs appear when the check is forgotten. In Rust every result that might be missing returns an Option, and the check becomes part of the type:
let meyveler = ["armut", "elma", "kiraz"];
let i = meyveler.iter().position(|m| *m == "elma"); // Some(1)
let j = meyveler.iter().position(|m| *m == "muz"); // NoneThat's why many methods such as Vec::get, str::find, HashMap::get and iter().max() return Option.
Chaining helpers
Methods that work on Option without writing a match shorten the code:
let sira = i.map(|n| n + 1); // Some(2): position counting from 1
let varsay = j.unwrap_or(0); // 0
let var_mi = i.is_some(); // true
let metin = j.map_or("yok".to_string(), |n| n.to_string()); // "yok"map only transforms the value inside Some and leaves None as it is.
Practice Fruit position
Write fn bul(liste: &[&str], ad: &str) -> Option<usize> (iter().position). For ["armut", "elma", "kiraz"], compute bul(.., "elma").map(|i| i + 1).unwrap_or(0) and the same for "muz", and print them separated by a space.
Starter code
fn main() {
// buraya yaz
}Sample solution
fn bul(liste: &[&str], ad: &str) -> Option<usize> {
liste.iter().position(|m| *m == ad)
}
fn main() {
let meyveler = ["armut", "elma", "kiraz"];
let a = bul(&meyveler, "elma").map(|i| i + 1).unwrap_or(0);
let b = bul(&meyveler, "muz").map(|i| i + 1).unwrap_or(0);
println!("{} {}", a, b);
}Expected output
2 0
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 What does <code>Vec::new::<i32>().iter().max()</code> return?
- Some(0)
- None Correct answer
- 0
- Panic
Explanation: An empty collection has no largest element; this is stated honestly with None.
2 Default value
let j: Option<usize> = None;
let n = j.____(0); // 0let j: Option<usize> = None;
let n = j.unwrap_or(0); // 0Explanation: unwrap_or returns the given default when it is None.
3 Method and result on None
| Term | Meaning |
|---|---|
is_some() |
false |
unwrap_or(7) |
7 |
map(|n| n + 1) |
None |
unwrap() |
Panic |