unwrap and expect

Learn the unwrap and expect shortcuts and their risks.

unwrap and expect

unwrap() gives the value inside Ok/Some, otherwise it panics. expect("message") does the same, but you set the panic message; a message explaining why it shouldn't fail makes debugging easier:

let port: u16 = "8080".parse().expect("port sabiti geçerli olmalı");
// let x: i32 = "x".parse().unwrap();
// panic: called `Result::unwrap()` on an `Err` value: ParseIntError { kind: InvalidDigit }

They are acceptable in prototypes, tests and truly impossible cases; they shouldn't be applied to user input in production code.

Non-panicking options

Ways of continuing with a default value:

let a: i32 = "x".parse().unwrap_or(0);              // 0
let b: i32 = "".parse().unwrap_or_default();        // 0 (default for i32)
let c: i32 = "x".parse().unwrap_or_else(|_| -1);    // -1
let d: Option<i32> = "x".parse().ok();              // None

unwrap_or_else computes the default only when needed; it's preferred for an expensive default.

Exercise Defaults

Do three conversions: expect("sayı olmalı") for "12", unwrap_or(0) for "x" and unwrap_or_default() for "" (all i32). Print the results separated by spaces.

Show example solution

fn main() {
    let a: i32 = "12".parse().expect("sayı olmalı");
    let b: i32 = "x".parse().unwrap_or(0);
    let c: i32 = "".parse().unwrap_or_default();
    println!("{} {} {}", a, b, c);
}

Test yourself

Which is most suitable when converting user input to a number?
- Handling the error with match or unwrap_or
- unwrap()
- expect("hata")
- panic!
Show answer

Handling the error with match or unwrap_or — User input is an expected source of errors; instead of crashing the program, the user should be informed.

Unwrap with a message

let port: u16 = "8080".parse().____("port sabiti geçerli olmalı");

Show answer

let port: u16 = "8080".parse().expect("port sabiti geçerli olmalı");

Method and result on "x" (i32)

unwrap()

Panic
unwrap_or(0)

0
unwrap_or_else(|_| -1)

-1
ok()

None

Next lesson: Summary - Error handling

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