swift 下標指令碼

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下標指令碼 可以定義在類(Class)、結構體(structure)和枚舉(enumeration)這些目標中,可以讓這些類型的執行個體通過[]快速存取屬性和方法.

  subscript(index: Int) -> Int {    get {      // 返回與入參匹配的Int類型的值    }    set(newValue) {      // 執行賦值操作    }}

通常下標指令碼是用來訪問集合(collection),列表(list)或序列(sequence)中元素的捷徑

struct Matrix {    let rows: Int, columns: Int    var grid: Double[]    init(rows: Int, columns: Int) {      self.rows = rows      self.columns = columns      grid = Array(count: rows * columns, repeatedValue: 0.0)    }    func indexIsValidForRow(row: Int, column: Int) -> Bool {        return row >= 0 && row < rows && column >= 0 && column < columns    }    subscript(row: Int, column: Int) -> Double {        get {            assert(indexIsValidForRow(row, column: column), "Index out of range")            return grid[(row * columns) + column]        }        set {            assert(indexIsValidForRow(row, column: column), "Index out of range")            grid[(row * columns) + column] = newValue        }    }}var matrix = Matrix(rows: 2, columns: 2)let someValue = matrix[2, 2]
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