Option instead of null

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");    // None

That'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.

Exercise 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.

Show example 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);
}

Test yourself

What does <code>Vec::new::<i32>().iter().max()</code> return?
- Some(0)
- None
- 0
- Panic
Show answer

None — An empty collection has no largest element; this is stated honestly with None.

Default value

let j: Option<usize> = None;
let n = j.____(0);   // 0

Show answer

let j: Option<usize> = None;
let n = j.unwrap_or(0);   // 0

Method and result on None

is_some()

false
unwrap_or(7)

7
map(|n| n + 1)

None
unwrap()

Panic

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