Creating sub-array from list of array indicies

Question

I have an enum with a computed property that returns an array of indicies:

enum ScaleDegree {
    case tonic
    case supertonic
    case mediant
    case subdominant
    case dominant
    case submedian
    case leading

    var indexes: [Int] {
        switch self {
        case .tonic: return [0,2,4]
        case .supertonic: return [1,3,5]
        case .mediant: return [2,4,6]
        case .subdominant: return [3,5,0]
        case .dominant: return [4,6,1]
        case .submedian: return [5,0,2]
        case .leading: return [6,1,3]
        }
    }
}

I use this to extract a subarry from a larger array:

let cMajor = ["C", "D", "E", "F", "G", "A", "B"]

let cMajorTonic = [cMajor[ScaleDegree.tonic.indexes[0]], cMajor[ScaleDegree.tonic.indexes[1]], cMajor[ScaleDegree.tonic.indexes[2]]]

The cMajorTonic syntax seems cumbersome and I would expect Swift 4 would give me an easier way to extract individual elements into a new array, but my searching hasn't found a clever way to do this.

Are there any suggestions of a better way to write this?

you can also enumerate cMajor and check if your enumeration indexes contains the element offset let cMajorTonic = cMajor.enumerated().flatMap { ScaleDegree.tonic.indexes.contains($0.offset) ? $0.element : nil }. The advantage is that it won't crash your app if your array has less elements than some of your enumeration indexes — Leo Dabus
– Leo Dabus, Commented Dec 23, 2017 at 23:36
as a method of your enumeration as suggested by vacawama return scale.enumerated().flatMap{ indexes.contains($0.offset) ? $0.element : nil } — Leo Dabus
– Leo Dabus, Commented Dec 23, 2017 at 23:40

vacawama · Accepted Answer · 2017-12-23 23:19:29Z

3

This would be a good place to use map:

let cMajor = ["C", "D", "E", "F", "G", "A", "B"]
let cMajorTonic = ScaleDegree.tonic.indexes.map { cMajor[$0] }
print(cMajorTonic)

["C", "E", "G"]

You could add this function to your enum:

func appliedTo(scale: [String]) -> [String] {
    return self.indexes.map { scale[$0] }
}

And then it would become:

let cMajorTonic = ScaleDegree.tonic.appliedTo(scale: cMajor)

edited Dec 23, 2017 at 23:19

answered Dec 23, 2017 at 23:10

vacawama

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1 Comment

Eric George Over a year ago

That represents exactly the kind of way I wanted to write this. Thank you!

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