Classes, structures, and enumerations can define subscripts, which are shortcuts for accessing the member elements of a collection, list, or sequence. You use subscripts to set and retrieve values by index without needing separate methods for setting and retrieval. For example, you access elements in an Array instance as someArray[index] and elements in a Dictionary instance as someDictionary[key].
You can define multiple subscripts for a single type, and the appropriate subscript overload to use is selected based on the type of index value you pass to the subscript. Subscripts aren’t limited to a single dimension, and you can define subscripts with multiple input parameters to suit your custom type’s needs.
Subscripts enable you to query instances of a type by writing one or more values in square brackets after the instance name. Their syntax is similar to both instance method syntax and computed property syntax. You write subscript definitions with the subscript keyword, and specify one or more input parameters and a return type, in the same way as instance methods. Unlike instance methods, subscripts can be read-write or read-only. This behavior is communicated by a getter and setter in the same way as for computed properties:
subscript(index: Int) -> Int {
get {
// Return an appropriate subscript value here.
}
set(newValue) {
// Perform a suitable setting action here.
}
}As with read-only computed properties, you can simplify the declaration of a read-only subscript by removing the get keyword and its braces:
subscript(index: Int) -> Int {
// Return an appropriate subscript value here.
}Here’s an example of a read-only subscript implementation, which defines a TimesTable structure to represent an n-times-table of integers:
struct TimesTable {
let multiplier: Int
subscript(index: Int) -> Int {
return multiplier * index
}
}
let threeTimesTable = TimesTable(multiplier: 3)
print("six times three is \(threeTimesTable[6])")
// Prints "six times three is 18"ou can query the threeTimesTable instance by calling its subscript, as shown in the call to threeTimesTable[6]. This requests the sixth entry in the three-times-table, which returns a value of 18, or 3 times 6
The exact meaning of “subscript” depends on the context in which it’s used. Subscripts are typically used as a shortcut for accessing the member elements in a collection, list, or sequence. You are free to implement subscripts in the most appropriate way for your particular class or structure’s functionality.
For example, Swift’s Dictionary type implements a subscript to set and retrieve the values stored in a Dictionary instance. You can set a value in a dictionary by providing a key of the dictionary’s key type within subscript brackets, and assigning a value of the dictionary’s value type to the subscript:
var numberOfLegs = ["spider": 8, "ant": 6, "cat": 4]
numberOfLegs["bird"] = 2Subscripts can take any number of input parameters, and these input parameters can be of any type. Subscripts can also return a value of any type.
Like functions, subscripts can take a varying number of parameters and provide default values for their parameters, as discussed in Variadic Parameters and Default Parameter Values. However, unlike functions, subscripts can’t use in-out parameters.
A class or structure can provide as many subscript implementations as it needs, and the appropriate subscript to be used will be inferred based on the types of the value or values that are contained within the subscript brackets at the point that the subscript is used. This definition of multiple subscripts is known as subscript overloading.
While it’s most common for a subscript to take a single parameter, you can also define a subscript with multiple parameters if it’s appropriate for your type. The following example defines a Matrix structure, which represents a two-dimensional matrix of Double values. The Matrix structure’s subscript takes two integer parameters:
struct Matrix {
let rows: Int, columns: Int
var grid: [Double]
init(rows: Int, columns: Int) {
self.rows = rows
self.columns = columns
grid = Array(repeating: 0.0, count: rows * columns)
}
func indexIsValid(row: Int, column: Int) -> Bool {
return row >= 0 && row < rows && column >= 0 && column < columns
}
subscript(row: Int, column: Int) -> Double {
get {
assert(indexIsValid(row: row, column: column), "Index out of range")
return grid[(row * columns) + column]
}
set {
assert(indexIsValid(row: row, column: column), "Index out of range")
grid[(row * columns) + column] = newValue
}
}
}You can construct a new Matrix instance by passing an appropriate row and column count to its initializer:
var matrix = Matrix(rows: 2, columns: 2)
//The example above creates a new Matrix instance with two rows and two columns. The grid array for this Matrix instance is effectively a flattened version of the matrix, as read from top left to bottom right:
Values in the matrix can be set by passing row and column values into the subscript, separated by a comma:
matrix[0, 1] = 1.5
matrix[1, 0] = 3.2These two statements call the subscript’s setter to set a value of 1.5 in the top right position of the matrix (where row is 0 and column is 1), and 3.2 in the bottom left position (where row is 1 and column is 0):
The Matrix subscript’s getter and setter both contain an assertion to check that the subscript’s row and column values are valid. To assist with these assertions, Matrix includes a convenience method called indexIsValid(row:column:), which checks whether the requested row and column are inside the bounds of the matrix:
func indexIsValid(row: Int, column: Int) -> Bool {
return row >= 0 && row < rows && column >= 0 && column < columns
}An assertion is triggered if you try to access a subscript that’s outside of the matrix bounds:
let someValue = matrix[2, 2]
// This triggers an assert, because [2, 2] is outside of the matrix bounds.Instance subscripts, as described above, are subscripts that you call on an instance of a particular type. You can also define subscripts that are called on the type itself. This kind of subscript is called a type subscript. You indicate a type subscript by writing the static keyword before the subscript keyword. Classes can use the class keyword instead, to allow subclasses to override the superclass’s implementation of that subscript. The example below shows how you define and call a type subscript:
enum Planet: Int {
case mercury = 1, venus, earth, mars, jupiter, saturn, uranus, neptune
static subscript(n: Int) -> Planet {
return Planet(rawValue: n)!
}
}
let mars = Planet[4]
print(mars)