Trait std::cmp::Ord

1.0.0 · source · [−]
pub trait Ord: Eq + PartialOrd<Self> {
    fn cmp(&self, other: &Self) -> Ordering;

    fn max(self, other: Self) -> Self { ... }
    fn min(self, other: Self) -> Self { ... }
    fn clamp(self, min: Self, max: Self) -> Self { ... }
}
Expand description

Trait for types that form a total order.

Implementations must be consistent with the PartialOrd implementation, and ensure max, min, and clamp are consistent with cmp:

  • partial_cmp(a, b) == Some(cmp(a, b)).
  • max(a, b) == max_by(a, b, cmp) (ensured by the default implementation).
  • min(a, b) == min_by(a, b, cmp) (ensured by the default implementation).
  • For a.clamp(min, max), see the method docs (ensured by the default implementation).

It’s easy to accidentally make cmp and partial_cmp disagree by deriving some of the traits and manually implementing others.

Corollaries

From the above and the requirements of PartialOrd, it follows that < defines a strict total order. This means that for all a, b and c:

  • exactly one of a < b, a == b or a > b is true; and
  • < is transitive: a < b and b < c implies a < c. The same must hold for both == and >.

Derivable

This trait can be used with #[derive].

When derived on structs, it will produce a lexicographic ordering based on the top-to-bottom declaration order of the struct’s members.

When derived on enums, variants are ordered by their discriminants. By default, the discriminant is smallest for variants at the top, and largest for variants at the bottom. Here’s an example:

#[derive(PartialEq, Eq, PartialOrd, Ord)]
enum E {
    Top,
    Bottom,
}

assert!(E::Top < E::Bottom);
Run

However, manually setting the discriminants can override this default behavior:

#[derive(PartialEq, Eq, PartialOrd, Ord)]
enum E {
    Top = 2,
    Bottom = 1,
}

assert!(E::Bottom < E::Top);
Run

Lexicographical comparison

Lexicographical comparison is an operation with the following properties:

  • Two sequences are compared element by element.
  • The first mismatching element defines which sequence is lexicographically less or greater than the other.
  • If one sequence is a prefix of another, the shorter sequence is lexicographically less than the other.
  • If two sequence have equivalent elements and are of the same length, then the sequences are lexicographically equal.
  • An empty sequence is lexicographically less than any non-empty sequence.
  • Two empty sequences are lexicographically equal.

How can I implement Ord?

Ord requires that the type also be PartialOrd and Eq (which requires PartialEq).

Then you must define an implementation for cmp. You may find it useful to use cmp on your type’s fields.

Here’s an example where you want to sort people by height only, disregarding id and name:

use std::cmp::Ordering;

#[derive(Eq)]
struct Person {
    id: u32,
    name: String,
    height: u32,
}

impl Ord for Person {
    fn cmp(&self, other: &Self) -> Ordering {
        self.height.cmp(&other.height)
    }
}

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

impl PartialEq for Person {
    fn eq(&self, other: &Self) -> bool {
        self.height == other.height
    }
}
Run

Required Methods

This method returns an Ordering between self and other.

By convention, self.cmp(&other) returns the ordering matching the expression self <operator> other if true.

Examples
use std::cmp::Ordering;

assert_eq!(5.cmp(&10), Ordering::Less);
assert_eq!(10.cmp(&5), Ordering::Greater);
assert_eq!(5.cmp(&5), Ordering::Equal);
Run

Provided Methods

Compares and returns the maximum of two values.

Returns the second argument if the comparison determines them to be equal.

Examples
assert_eq!(2, 1.max(2));
assert_eq!(2, 2.max(2));
Run

Compares and returns the minimum of two values.

Returns the first argument if the comparison determines them to be equal.

Examples
assert_eq!(1, 1.min(2));
assert_eq!(2, 2.min(2));
Run

Restrict a value to a certain interval.

Returns max if self is greater than max, and min if self is less than min. Otherwise this returns self.

Panics

Panics if min > max.

Examples
assert!((-3).clamp(-2, 1) == -2);
assert!(0.clamp(-2, 1) == 0);
assert!(2.clamp(-2, 1) == 1);
Run

Implementations on Foreign Types

Implementors

Implements ordering of strings.

Strings are ordered lexicographically by their byte values. This orders Unicode code points based on their positions in the code charts. This is not necessarily the same as “alphabetical” order, which varies by language and locale. Sorting strings according to culturally-accepted standards requires locale-specific data that is outside the scope of the str type.

Implements comparison of vectors lexicographically.

Implements ordering of vectors, lexicographically.

Implements comparison of arrays lexicographically.