arrays - 从数组索引列表创建子数组

Question

我有一个带有计算属性的枚举，它返回一个索引数组:

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]
        }
    }
}

我用它从一个更大的数组中提取一个子数组:

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]]]

cMajorTonic 语法看起来很麻烦，我希望 Swift 4 能给我一个更简单的方法来将单个元素提取到一个新数组中，但我的搜索没有找到一个聪明的方法来做到这一点。

有没有更好的写法建议？

Answer 1

这是使用 map 的好地方:

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

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

---

您可以将此函数添加到您的枚举中:

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

然后它会变成:

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