Custom ordering in Rust

Implementing Ord decides how your type sorts everywhere it is compared.

#[derive(Debug, PartialEq, Eq)]
struct Step {
    stars: u8,
    title: String,
}

impl Ord for Step {
    fn cmp(&self, other: &Self) -> std::cmp::Ordering {
        other
            .stars
            .cmp(&self.stars)
            .then_with(|| self.title.cmp(&other.title))
    }
}

impl PartialOrd for Step {
    fn partial_cmp(&self, other: &Self) -> Option<std::cmp::Ordering> {
        Some(self.cmp(other))
    }
}

fn main() {
    let mut steps = vec![
        Step { stars: 1, title: "Slices".to_string() },
        Step { stars: 3, title: "Traits".to_string() },
        Step { stars: 3, title: "Errors".to_string() },
    ];
    steps.sort();
    for s in &steps {
        println!("{} {}", s.stars, s.title);
    }
}

How it works

1. Deriving PartialOrd compares fields in declaration order.
2. A hand-written Ord picks the rule you want.
3. sort then needs no comparator at the call site.

Keywords and builtins used here

• Option
• Ord
• PartialOrd
• Self
• Some
• Step
• String
• cmp
• fn
• for
• impl
• in
• let
• main
• mut
• partial_cmp
• self
• std
• struct
• u8

The run, in numbers

Lines

32

Characters to type

602

Tokens

198

Three-star pace

105 tpm

At the three-star pace of 105 tokens a minute, this run takes about 113 seconds.

Type this snippet

Step 4 of 4 in Standard traits, step 11 of 19 in Traits & generics.

← Previous Next →