Standard way to “clamp” a number between two values in Swift

Question

Given:

let a = 4.2
let b = -1.3
let c = 6.4

I want to know the simplest, Swiftiest way to clamp these values to a given range, say 0...

Answer 1

The ClosedInterval type already has a

func clamp(_ intervalToClamp: ClosedInterval) -> ClosedInterval

method which takes another interval as an argument. There is a proposal on the Swift evolution mailing list

• Add clamp(value: Bound) -> Bound to ClosedInterval

to add another method which clamps a single value to the given interval:

/// Returns `value` clamped to `self`.
func clamp(value: Bound) -> Bound

and that is exactly what you need.

Using the implementation of the existing clamp() method at

• https://github.com/apple/swift/blob/master/stdlib/public/core/Interval.swift.gyb

as an example, this additional clamp() method can be implemented as

extension ClosedInterval {
    func clamp(value : Bound) -> Bound {
        return self.start > value ? self.start
            : self.end < value ? self.end
            : value
    }
}

Example:

(0.0 ... 5.0).clamp(4.2)    // 4.2
(0.0 ... 5.0).clamp(-1.3)   // 0.0
(0.0 ... 5.0).clamp(6.4)    // 5.0

ClosedInterval is a generic type

public struct ClosedInterval { ... }

therefore this works not only for Double but for all types which are Comparable (like Int, CGFloat, String, ...):

(1 ... 3).clamp(10)      // 3
("a" ... "z").clamp("ä") // "ä"

---

Update for Swift 3 (Xcode 8): ClosedInterval has been renamed to ClosedRange, and its properties are lower/upperBound now:

extension ClosedRange {
    func clamp(_ value : Bound) -> Bound {
        return self.lowerBound > value ? self.lowerBound
            : self.upperBound < value ? self.upperBound
            : value
    }
}