Comparable

protocol Comparable

A type that can be compared using the relational operators <, <=, >=, and >.

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.

Note: A conforming type may contain a subset of values which are treated as exceptional---that is, values that are outside the domain of meaningful arguments for the purposes of the Comparable protocol. For example, the special "not a number" value for floating-point types (FloatingPoint.nan) compares as neither less than, greater than, nor equal to any normal floating-point value. Exceptional values need not take part in the strict total order.

Inheritance Equatable View Protocol Hierarchy →
Import import Swift

Instance Methods

func <(_:rhs:) Required

Returns a Boolean value indicating whether the value of the first argument is less than that of the second argument.

This function is the only requirement of the Comparable protocol. The remainder of the relational operator functions are implemented by the standard library for any type that conforms to Comparable.

Parameters: lhs: A value to compare. rhs: Another value to compare.

Declaration

func <(lhs: Self, rhs: Self) -> Bool
func <=(_:rhs:)

Returns a Boolean value indicating whether the value of the first argument is less than or equal to that of the second argument.

Parameters: lhs: A value to compare. rhs: Another value to compare.

Declaration

func <=(lhs: Self, rhs: Self) -> Bool
func ==(_:rhs:) Required

Returns a Boolean value indicating whether two values are equal.

Equality is the inverse of inequality. For any values a and b, a == b implies that a != b is false.

Parameters: lhs: A value to compare. rhs: Another value to compare.

Declaration

func ==(lhs: Self, rhs: Self) -> Bool

Declared In

Equatable
func >(_:rhs:)

Returns a Boolean value indicating whether the value of the first argument is greater than that of the second argument.

Parameters: lhs: A value to compare. rhs: Another value to compare.

Declaration

func >(lhs: Self, rhs: Self) -> Bool
func >=(_:rhs:)

Returns a Boolean value indicating whether the value of the first argument is greater than or equal to that of the second argument.

Parameters: lhs: A value to compare. rhs: Another value to compare.

Declaration

func >=(lhs: Self, rhs: Self) -> Bool

Default Implementations

func !=(_:rhs:)

Returns a Boolean value indicating whether two values are not equal.

Inequality is the inverse of equality. For any values a and b, a != b implies that a == b is false.

This is the default implementation of the not-equal-to operator (!=) for any type that conforms to Equatable.

Parameters: lhs: A value to compare. rhs: Another value to compare.

Declaration

func !=(lhs: Self, rhs: Self) -> Bool

Declared In

Equatable
prefix func ...(_: Self)

Returns a partial range up to, and including, its upper bound.

Use the prefix closed range operator (prefix ...) to create a partial range of any type that conforms to the Comparable protocol. This example creates a PartialRangeThrough<Double> instance that includes any value less than or equal to 5.0.

let throughFive = ...5.0

throughFive.contains(4.0)     // true
throughFive.contains(5.0)     // true
throughFive.contains(6.0)     // false

You can use this type of partial range of a collection's indices to represent the range from the start of the collection up to, and including, the partial range's upper bound.

let numbers = [10, 20, 30, 40, 50, 60, 70]
print(numbers[...3])
// Prints "[10, 20, 30, 40]"

maximum: The upper bound for the range.

Declaration

prefix func ...(maximum: Self) -> PartialRangeThrough<Self>
postfix func ...(_: Self)

Returns a partial range extending upward from a lower bound.

Use the postfix range operator (postfix ...) to create a partial range of any type that conforms to the Comparable protocol. This example creates a PartialRangeFrom<Double> instance that includes any value greater than or equal to 5.0.

let atLeastFive = 5.0...

atLeastFive.contains(4.0)     // false
atLeastFive.contains(5.0)     // true
atLeastFive.contains(6.0)     // true

You can use this type of partial range of a collection's indices to represent the range from the partial range's lower bound up to the end of the collection.

let numbers = [10, 20, 30, 40, 50, 60, 70]
print(numbers[3...])
// Prints "[40, 50, 60, 70]"

minimum: The lower bound for the range.

Declaration

postfix func ...(minimum: Self) -> PartialRangeFrom<Self>
func ...(_:maximum:)

Returns a closed range that contains both of its bounds.

Use the closed range operator (...) to create a closed range of any type that conforms to the Comparable protocol. This example creates a ClosedRange<Character> from "a" up to, and including, "z".

let lowercase = "a"..."z"
print(lowercase.contains("z"))
// Prints "true"

Parameters: minimum: The lower bound for the range. maximum: The upper bound for the range.

Declaration

func ...(minimum: Self, maximum: Self) -> ClosedRange<Self>
prefix func ..<(_:)

Returns a partial range up to, but not including, its upper bound.

Use the prefix half-open range operator (prefix ..<) to create a partial range of any type that conforms to the Comparable protocol. This example creates a PartialRangeUpTo<Double> instance that includes any value less than 5.0.

let upToFive = ..<5.0

upToFive.contains(3.14)       // true
upToFive.contains(6.28)       // false
upToFive.contains(5.0)        // false

You can use this type of partial range of a collection's indices to represent the range from the start of the collection up to, but not including, the partial range's upper bound.

let numbers = [10, 20, 30, 40, 50, 60, 70]
print(numbers[..<3])
// Prints "[10, 20, 30]"

maximum: The upper bound for the range.

Declaration

prefix func ..<(maximum: Self) -> PartialRangeUpTo<Self>
func ..<(_:maximum:)

Returns a half-open range that contains its lower bound but not its upper bound.

Use the half-open range operator (..<) to create a range of any type that conforms to the Comparable protocol. This example creates a Range<Double> from zero up to, but not including, 5.0.

let lessThanFive = 0.0..<5.0
print(lessThanFive.contains(3.14))  // Prints "true"
print(lessThanFive.contains(5.0))   // Prints "false"

Parameters: minimum: The lower bound for the range. maximum: The upper bound for the range.

Declaration

func ..<(minimum: Self, maximum: Self) -> Range<Self>
func <=(_:rhs:)

Returns a Boolean value indicating whether the value of the first argument is less than or equal to that of the second argument.

This is the default implementation of the less-than-or-equal-to operator (<=) for any type that conforms to Comparable.

Parameters: lhs: A value to compare. rhs: Another value to compare.

Declaration

func <=(lhs: Self, rhs: Self) -> Bool
func >(_:rhs:)

Returns a Boolean value indicating whether the value of the first argument is greater than that of the second argument.

This is the default implementation of the greater-than operator (>) for any type that conforms to Comparable.

Parameters: lhs: A value to compare. rhs: Another value to compare.

Declaration

func >(lhs: Self, rhs: Self) -> Bool
func >=(_:rhs:)

Returns a Boolean value indicating whether the value of the first argument is greater than or equal to that of the second argument.

This is the default implementation of the greater-than-or-equal-to operator (>=) for any type that conforms to Comparable.

Parameters: lhs: A value to compare. rhs: Another value to compare. Returns: true if lhs is greater than or equal to rhs; otherwise, false.

Declaration

func >=(lhs: Self, rhs: Self) -> Bool