How to represent magnitude for mass in Swift?

Question

// SI units
enum Magnitude : Measurement {
    case Milli = Measurement(-3, \"ml\")
    case Centi = Measurement(-2, \"cl\")
    case Desi = Measurement(-1, \"dl\")
         


        

Answer 1

If you want to be able to specify the unit of the mass with a String perhaps you could use String to represent the shorthand name and variables to give you more information about the unit, such as magnitude. Here's an example:

1. MassUnit

enum MassUnit: String {
    case Milligrams = "mg"
    case Grams      = "g"
    case Kilos      = "kg"
    case Tons       = "t"

    var magnitude: Int {
        let mag: Int

        switch self {
            case .Milligrams: mag = -3
            case .Grams     : mag =  0
            case .Kilos     : mag =  3
            case .Tons      : mag =  6
        }

        return mag
    }


    static func ordersOfMagnitudeFrom(unit1: MassUnit, to unit2: MassUnit) -> Int {
        return unit1.magnitude - unit2.magnitude
    }
}

extension MassUnit: Printable {
    var description: String {
        return self.rawValue
    }
}

2. Then, for storing actual masses you could use a Struct, which could also handle the conversions. For example:

struct Mass {
    var value : Double
    var unit  : MassUnit

    static func convertMass(mass: Mass, toUnit unit: MassUnit) -> Mass {
        let ordersOfMagnitude = MassUnit.ordersOfMagnitudeFrom(mass.unit, to: unit)

        let multipler = pow(10.0, Double(ordersOfMagnitude))

        return Mass(value: mass.value * multipler, unit: unit)
    }

    //  Returns an optional Mass because we can't know for sure 
    //  unitString will represent a MassUnit.
    static func convertMass(mass: Mass, toUnit unitString: String) -> Mass? {
        if let unit = MassUnit(rawValue: unitString) {
            return convertMass(mass, toUnit: unit)
        }

        return nil
    }
}

extension Mass {
    init?(value: Double, _ unitString: String) {
        if let unit = MassUnit(rawValue: unitString) {
            self = Mass(value: value, unit: unit)
        } else {
            return nil
        }
    }
}

extension Mass : Printable {
    var description: String {
        return "\(value) \(unit)"
    }
}

3. Then you can use the masses and units:

if let mass = Mass(value: 1, "kg"),
   let convertedMass = Mass.convertMass(mass, toUnit: "g") {

    println("\(mass) converted to \(MassUnit.Grams) equals \(convertedMass)")

    // Prints: 1.0 kg converted to g equals 1000.0 g
}

However if you use a unitString that isn't convertible to a MassUnit (either when creating or converting) nil will be returned. For example:

let mass = Mass(value: 1, "NotAUnit") // nil