I have solved the Day 19 challenge, but I have a question:

    var outputTemp: Double? {
        switch toUnit {
        case "Celsius": return inputTempInCelsius!
        case "Fahrenheit": return inputTempInCelsius! * 9 / 5 + 32
        case "Kelvin": return inputTempInCelsius! + 273.15
        default: return nil
        }
    }

Here, toUnit is state-bound to a picker with the three specified values. But the computed property doesn't know that and the Swift compiler complains if I don't have a default case, because "Switch must be exhaustive". So I addd the default case but I don't want to make it return zero, because, well, "zero" is not the same thing as "invalid", and although the default case will never happen, well, what if it does in the future after numerous changes to the app? So I returned nil instead and made my computed property an optional. Is this the best approach? If not, what am I missing?

Thank you in advance.

Keeping it simple, for the level of this challenge and fo rthis particular case, you can eliminate the optional, if you make the celsius calculation the default.

var outputTemp: Double {
    switch toUnit {
    case "Fahrenheit": return inputTempInCelsius! * 9 / 5 + 32
    case "Kelvin": return inputTempInCelsius! + 273.15
    default: return inputTempInCelsius!
    }
}

@Obelix and @greenamberred Thank you for the excellent responses. I feel like I should've been able to think of these myself if I were less sleep-deprived! But more importantly, this interaction, which was my first time in the forums, really drove home how supportive this community is and was really encouraging. Thank you again sincerely. I just refactored my code using enums, by the way. :) Cheers!