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#learn-rust: retrieving optional values

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match statement #learn-rust

let fruits = vec!["banana", "apple", "coconut", "orange", "strawberry"];
for &index in [0, 2, 99].iter() {
    match fruits.get(index) {
        // note the "&" to get the Option<&&str>
        Some(&"coconut") => println!("Coconuts are awesome!!!"),
        Some(fruit_name) => println!("It's a delicious {}!", fruit_name),
        None => println!("There is no fruit! :("),

Whenever you use the match expression, keep the following rules in mind:

  • match arms are evaluated from top to bottom. Specific cases must be defined earlier than generic cases or they’ll never be matched and evaluated.
  • match arms must cover every possible value that the input type could have. You’ll get a compiler error if you try to match against a non-exhaustive pattern list.

Use pattern matching for convenience #

If we expect a specific Some value in an Option, we could use match (and drop anything else):

let a_number: Option = Some(7);
match a_number {
    Some(7) => println!("That's my lucky number!"),
    _ => {},

Or use a shorter, equivalent version:

let a_number: Option = Some(7);
if let Some(7) = a_number {
    println!("That's my lucky number!");

Use unwrap and expect carefully #

unwrap and expect can be used, but will panic if Option is None

let gift = Some("candy");
assert_eq!(gift.unwrap(), "candy");

let empty_gift: Option<&str> = None;
assert_eq!(empty_gift.unwrap(), "candy"); // This will panic!

This will panic like this:

thread 'main' panicked at 'called `Option::unwrap()` on a `None` value', src/

With expect we can set a custom panic message:

let a = Some("value");
assert_eq!(a.expect("fruits are healthy"), "value");

let b: Option<&str> = None;
b.expect("fruits are healthy"); // panics with `fruits are healthy`
thread 'main' panicked at 'fruits are healthy', src/

As both, unwrap and expect can panic it is recommended to avoid them and use either of following options instead:

  1. Use pattern matching and handle the None case explicitly.
  2. Call similar non-panicking methods, such as unwrap_or, which returns a default value if the variant is None or the inner value if the variant is Some(value).
assert_eq!(Some("dog").unwrap_or("cat"), "dog");
assert_eq!(None.unwrap_or("cat"), "cat");

Christoph Voigt
Christoph Voigt
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