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...5, such that:

a -> 4.2
b -> 0
c -> 5

I know I can do the following:

let clamped = min(max(a, 0), 5)

Or something like:

let clamped = (a < 0) ? 0 : ((a > 5) ? 5 : a)

But I was wondering if there were any other ways to do this in Swift—in particular, I want to know (and document on SO, since there doesn't appear to be a question about clamping numbers in Swift) whether there is anything in the Swift standard library intended specifically for this purpose.

There may not be, and if so, that's also an answer I'll happily accept.

Answer 1

Swift 4/5

Extension of Comparable/Strideable similar to ClosedRange.clamped(to:_) -> ClosedRange from standard Swift library.

extension Comparable {
    func clamped(to limits: ClosedRange<Self>) -> Self {
        return min(max(self, limits.lowerBound), limits.upperBound)
    }
}

#if swift(<5.1)
extension Strideable where Stride: SignedInteger {
    func clamped(to limits: CountableClosedRange<Self>) -> Self {
        return min(max(self, limits.lowerBound), limits.upperBound)
    }
}
#endif

Usage:

15.clamped(to: 0...10) // returns 10
3.0.clamped(to: 0.0...10.0) // returns 3.0
"a".clamped(to: "g"..."y") // returns "g"

// this also works (thanks to Strideable extension)
let range: CountableClosedRange<Int> = 0...10
15.clamped(to: range) // returns 10

Answer 2

The ClosedInterval type already has a

func clamp(_ intervalToClamp: ClosedInterval<Bound>) -> ClosedInterval<Bound>

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<Bound : Comparable> { ... }

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
    }
}

Answer 3

Using the same syntax as Apple to do the min and max operator:

public func clamp<T>(_ value: T, minValue: T, maxValue: T) -> T where T : Comparable {
    return min(max(value, minValue), maxValue)
}

You can use as that:

let clamped = clamp(newValue, minValue: 0, maxValue: 1)

The cool thing about this approach is that any value defines the necessary type to do the operation, so the compiler handles that itself.

Answer 4

2020. The extremely simple way.

extension Comparable {
    func clamped(_ f: Self, _ t: Self)  ->  Self {
        var r = self
        if r < f { r = f }
        if r > t { r = t }
        // (use SIMPLE, EXPLICIT code here to make it utterly clear
        // whether we are inclusive, what form of equality, etc etc)
        return r
    }

While I truly love ranges in Swift, I really think the absolutely standard syntax for a clamp function ("for 50 years now in every computer language") is just simpler and better:

x = x.clamped(0.5, 5.0)

Until it is built-in to Swift, really I think that's best.

Philosophical corner:

IMO the two values in a clamp function are not really a 'range' - they're just "two values".

(Just for example: it's completely common in game code to have the two dynamic values sometimes be in the "wrong order" (i..e, the desired result is something outside) or the same (the result is just that value).)

An opinion on end-naming ...

On everything we do, we insist on explicitly stating whether inclusive or exclusive. For example if there's a call

randomIntUpTo( 13 )

in fact we will name it

randomIntUpToExclusive( 13 )

or indeed "inclusive" if that is the case. Or depending on the language something like

randomInt(fromInclusive:  upToExclusive: )

or whatever the case may be. In this way there is absolutely never ever ever a unity error, and nothing needs to be discussed. All code names should be self-documenting. So indeed, for us, the function above would be named

func clamped(fromExclusive: Self, toExclusive: Self)

or whatever describes it.

But that's just us. But it's the right thing to do :)