How can I convert from degrees to radians?

This is not identically what you asked, but in Swift 3 / iOS 10, you can use the Measurement type and do the conversion without knowing the formula!

let result = Measurement(value: 45, unit: UnitAngle.degrees)
    .converted(to: .radians).value

Xcode 11 • Swift 5.1 or later

extension BinaryInteger {
    var degreesToRadians: CGFloat { CGFloat(self) * .pi / 180 }
}

extension FloatingPoint {
    var degreesToRadians: Self { self * .pi / 180 }
    var radiansToDegrees: Self { self * 180 / .pi }
}

---

Playground

45.degreesToRadians         // 0.7853981633974483

Int(45).degreesToRadians    // 0.7853981633974483
Int8(45).degreesToRadians   // 0.7853981633974483
Int16(45).degreesToRadians  // 0.7853981633974483
Int32(45).degreesToRadians  // 0.7853981633974483
Int64(45).degreesToRadians  // 0.7853981633974483

UInt(45).degreesToRadians   // 0.7853981633974483
UInt8(45).degreesToRadians  // 0.7853981633974483
UInt16(45).degreesToRadians // 0.7853981633974483
UInt32(45).degreesToRadians // 0.7853981633974483
UInt64(45).degreesToRadians // 0.7853981633974483

Double(45).degreesToRadians    // 0.7853981633974483
CGFloat(45).degreesToRadians   // 0.7853981633974483
Float(45).degreesToRadians     // 0.7853981
Float80(45).degreesToRadians   // 0.78539816339744830963

If you would like to make the binary integer return a floating point type instead of always returning a CGFloat you can make a generic method instead of a computed property:

extension BinaryInteger {
    func degreesToRadians<F: FloatingPoint>() -> F {  F(self) * .pi / 180 }
}

---

let radiansDouble: Double = 45.degreesToRadians()   // 0.7853981633974483
let radiansCGFloat: CGFloat = 45.degreesToRadians() // 0.7853981633974483
let radiansFloat: Float = 45.degreesToRadians()     // 0.7853981
let radiansFloat80: Float80 = 45.degreesToRadians() // 0.78539816339744830963

Apple provides these GLKit functions for conversion:

func GLKMathDegreesToRadians(_ degrees: Float) -> Float
func GLKMathRadiansToDegrees(_ radians: Float) -> Float