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How to add raw values to enums

Swift version: 5.1

Paul Hudson    @twostraws   

Raw values for enums are primitive values that sit behind each case. For example, you might create an enum for the planets in our solar system, and want to refer to each planet by a number as well as its name:

enum Planets: Int {
    case mercury
    case venus
    case earth
    case mars
} 

Swift will assign each case a raw integer value, starting from 0 and counting up. You can then use this to load and save the enum, or perhaps transfer it over the network.

You can provide custom raw values for any or all of your cases, and Swift will fill in the rest. For example, if we wanted mercury to be planet number 1, venus to be number 2, and so on, we could do this:

enum Planets: Int {
    case mercury = 1
    case venus
    case earth
    case mars
}

That will cause Swift to count upwards from 1.

If your raw value type is String, Swift will automatically create strings from each case name.

So, this:

enum Planets: String {
    case mercury
    case venus
    case earth
    case mars
}

Is equivalent to this:

enum Planets: String {
    case mercury = "mercury"
    case venus = "venus"
    case earth = "earth"
    case mars = "mars"
}

Finally, you can create enums from their raw value, but you get back an optional enum because your raw value might not match any of the available cases. For example, given our original Planets enum with integer raw values starting from 0, this would create an optional Planet pointing at Venus:

let venus = Planets(rawValue: 1)
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