Protocol

Comparable

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

``protocol Comparable : Equatable``

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.

Supertypes

• `protocol Equatable`

A type that can be compared for value equality.

Requirements

• `static func < (Self, Self) -> Bool`

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

• `static func <= (Self, Self) -> Bool`

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

• `static func > (Self, Self) -> Bool`

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

• `static func >= (Self, Self) -> Bool`

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

Citizens in Swift

Members

• `static func ... (Self) -> PartialRangeFrom<Self>`

Returns a partial range extending upward from a lower bound.

• `static func ... (Self) -> PartialRangeThrough<Self>`

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

• `static func ... (Self, Self) -> ClosedRange<Self>`

Returns a closed range that contains both of its bounds.

• `static func ..< (Self) -> PartialRangeUpTo<Self>`

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

• `static func ..< (Self, Self) -> Range<Self>`

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

Subtypes

• `protocol BinaryFloatingPoint`

• `protocol BinaryInteger`

An integer type with a binary representation.

• `protocol DurationProtocol`

A type that defines a duration for a given `InstantProtocol` type.

• `protocol FixedWidthInteger`

An integer type that uses a fixed size for every instance.

• `protocol FloatingPoint`

A floating-point numeric type.

• `protocol InstantProtocol`
• `protocol SignedInteger`

An integer type that can represent both positive and negative values.

• `protocol Strideable`

A type representing continuous, one-dimensional values that can be offset and measured.

• `protocol StringProtocol`

A type that can represent a string as a collection of characters.

• `protocol UnsignedInteger`

An integer type that can represent only nonnegative values.

Extension in Atomics

Subtypes

• `protocol AtomicInteger`

A type that supports atomic integer operations through a separate atomic storage representation.

Extension in Durations

Subtypes

• `protocol QuantizedDuration<RawValue>`

Extension in RealModule

Subtypes

• `protocol Real`

A type that models the real numbers.

Extension in SemanticVersions

Subtypes

• `protocol VectorVersion`
• `protocol VectorVersionComponents`