Swift - Subscripts


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Accessing the element members of a collection, sequence and a list in Classes, Structures and Enumerations are carried out with the help of subscripts. These subscripts are used to store and retrieve the values with the help of index. Array elements are accessed with the help of someArray[index] and its subsequent member elements in a Dictionary instance can be accessed as someDicitonary[key].

For a single type, subscripts can range from single to multiple declarations. We can use the appropriate subscript to overload the type of index value passed to the subscript. Subscripts also ranges from single dimension to multiple dimension according to the users requirements for their input data type declarations.

Subscript Declaration Syntax and its Usage

Let's have a recap to the computed properties. Subscripts too follow the same syntax as that of computed properties. For querying type instances, subscripts are written inside a square bracket followed with the instance name. Subscript syntax follows the same syntax structure as that of 'instance method' and 'computed property' syntax. 'subscript' keyword is used for defining subscripts and the user can specify single or multiple parameters with their return types. Subscripts can have read-write or read-only properties and the instances are stored and retrieved with the help of 'getter' and 'setter' properties as that of computed properties.

Syntax

subscript(index: Int) −> Int {
   get {
      // used for subscript value declarations
   }
   set(newValue) {
      // definitions are written here
   }
}

Example1

Live Demo
struct subexample {
   let decrementer: Int
   subscript(index: Int) -> Int {
      return decrementer / index
   }
}
let division = subexample(decrementer: 100)

print("The number is divisible by \(division[9]) times")
print("The number is divisible by \(division[2]) times")
print("The number is divisible by \(division[3]) times")
print("The number is divisible by \(division[5]) times")
print("The number is divisible by \(division[7]) times")

When we run the above program using playground, we get the following result −

The number is divisible by 11 times
The number is divisible by 50 times
The number is divisible by 33 times
The number is divisible by 20 times
The number is divisible by 14 times

Example2

Live Demo
class daysofaweek {
   private var days = ["Sunday", "Monday", "Tuesday", "Wednesday",
      "Thursday", "Friday", "saturday"]
   subscript(index: Int) -> String {
      get {
         return days[index]
      }
      set(newValue) {
         self.days[index] = newValue
      }
   }
}
var p = daysofaweek()

print(p[0])
print(p[1])
print(p[2])
print(p[3])

When we run the above program using playground, we get the following result −

Sunday
Monday
Tuesday
Wednesday

Options in Subscript

Subscripts takes single to multiple input parameters and these input parameters also belong to any datatype. They can also use variable and variadic parameters. Subscripts cannot provide default parameter values or use any in-out parameters.

Defining multiple subscripts are termed as 'subscript overloading' where a class or structure can provide multiple subscript definitions as required. These multiple subscripts are inferred based on the types of values that are declared within the subscript braces.

struct Matrix {
   let rows: Int, columns: Int
   var print: [Double]
   init(rows: Int, columns: Int) {
      self.rows = rows
      self.columns = columns
      print = Array(count: rows * columns, repeatedValue: 0.0)
   }
   subscript(row: Int, column: Int) -> Double {
      get {
         return print[(row * columns) + column]
      }
      set {
         print[(row * columns) + column] = newValue
      }
   }
}
var mat = Matrix(rows: 3, columns: 3)

mat[0,0] = 1.0
mat[0,1] = 2.0
mat[1,0] = 3.0
mat[1,1] = 5.0

print("\(mat[0,0])")

When we run the above program using playground, we get the following result −

1.0

Swift 4 subscript supports single parameter to multiple parameter declarations for appropriate data types. The program declares 'Matrix' structure as a 2 * 2 dimensional array matrix to store 'Double' data types. The Matrix parameter is inputted with Integer data types for declaring rows and columns.

New instance for the Matrix is created by passing row and column count to the initialize as shown below.

var mat = Matrix(rows: 3, columns: 3)

Matrix values can be defined by passing row and column values into the subscript, separated by a comma as shown below.

mat[0,0] = 1.0  
mat[0,1] = 2.0
mat[1,0] = 3.0
mat[1,1] = 5.0


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