Comparable

The Comparable protocol is used for types that have an inherent order, such as numbers and strings. Many types in the standard library already conform to the Comparable protocol. Add Comparable conformance to your own custom types when you want to be able to compare instances using relational operators or use standard library methods that are designed for Comparable types.

The most familiar use of relational operators is to compare numbers, as in the following example:

let currentTemp = 73

if currentTemp >= 90 {
    print("It's a scorcher!")
} else if currentTemp < 65 {
    print("Might need a sweater today.")
} else {
    print("Seems like picnic weather!")
}
// Prints "Seems like picnic weather!"

You can use special versions of some sequence and collection operations when working with a Comparable type. For example, if your array’s elements conform to Comparable, you can call the sort() method without using arguments to sort the elements of your array in ascending order.

var measurements = [1.1, 1.5, 2.9, 1.2, 1.5, 1.3, 1.2]
measurements.sort()
print(measurements)
// Prints "[1.1, 1.2, 1.2, 1.3, 1.5, 1.5, 2.9]"

Conforming to the Comparable Protocol

Types with Comparable conformance implement the less-than operator (<) and the equal-to operator (==). These two operations impose a strict total order on the values of a type, in which exactly one of the following must be true for any two values a and b:

• a == b

• a < b

• b < a

In addition, the following conditions must hold:

• a < a is always false (Irreflexivity)

• a < b implies !(b < a) (Asymmetry)

• a < b and b < c implies a < c (Transitivity)

To add Comparable conformance to your custom types, define the < and == operators as static methods of your types. The == operator is a requirement of the Equatable protocol, which Comparable extends—see that protocol’s documentation for more information about equality in Swift. Because default implementations of the remainder of the relational operators are provided by the standard library, you’ll be able to use !=, >, <=, and >= with instances of your type without any further code.

As an example, here’s an implementation of a Date structure that stores the year, month, and day of a date:

struct Date {
    let year: Int
    let month: Int
    let day: Int
}

To add Comparable conformance to Date, first declare conformance to Comparable and implement the < operator function.

extension Date: Comparable {
    static func < (lhs: Date, rhs: Date) -> Bool {
        if lhs.year != rhs.year {
            return lhs.year < rhs.year
        } else if lhs.month != rhs.month {
            return lhs.month < rhs.month
        } else {
            return lhs.day < rhs.day
        }
    }

This function uses the least specific nonmatching property of the date to determine the result of the comparison. For example, if the two year properties are equal but the two month properties are not, the date with the lesser value for month is the lesser of the two dates.

Next, implement the == operator function, the requirement inherited from the Equatable protocol.

    static func == (lhs: Date, rhs: Date) -> Bool {
        return lhs.year == rhs.year && lhs.month == rhs.month
            && lhs.day == rhs.day
    }
}

Two Date instances are equal if each of their corresponding properties is equal.

Now that Date conforms to Comparable, you can compare instances of the type with any of the relational operators. The following example compares the date of the first moon landing with the release of David Bowie’s song “Space Oddity”:

let spaceOddity = Date(year: 1969, month: 7, day: 11)   // July 11, 1969
let moonLanding = Date(year: 1969, month: 7, day: 20)   // July 20, 1969
if moonLanding > spaceOddity {
    print("Major Tom stepped through the door first.")
} else {
    print("David Bowie was following in Neil Armstrong's footsteps.")
}
// Prints "Major Tom stepped through the door first."

Note that the > operator provided by the standard library is used in this example, not the < operator implemented above.