Subscripts

Subscripts are used to get and set values in collections such as arrays and dictionaries. It’s also possible to define one or more subscripts for a class, structure, or enum so values can easily be retrieved or set using an index or key, rather than using methods.

Using Subscripts to Get and Set Values

The syntax below shows how a value can be retrieved from an array using subscript notation, by passing an integer as the index in square brackets after the array name. Dictionary values are retrieved similarly, by passing the key in square brackets after the dictionary name.

array[index]
dictionary[key]

New values are set using the same notation:

array[index] = new value
dictionary[key] = new value

Getting and Setting Examples

The following example demonstrates how to set a new value in an array and then print it:

// Declare an array.
var herbGarden = ["Dill", "Lavender", "Parsley", "Rosemary", "Sage", "Thyme"]

// Set the element at position 0 to "Mint", replacing "Dill".
herbGarden[0] = "Mint"

// Print the element at position 0
print(herbGarden[0])

This example will output the following:

Mint

This next example shows how to set a new value in a dictionary and print it:

// Declare a dictionary.
var gardenInfo = ["name": "Happy Herb Garden", "type": "Herb", "length": "1m"]

// Set the element with the key "length" to "2m", replacing "1m".
gardenInfo["length"] = "2m"

// Print the element with the key "length".
print(gardenInfo["length"]!)

Note: The “!” forces the optional String? value to unwrap.

This example will output the following:

2m

Creating Subscripts

Subscripts are created using the subscript keyword followed by the input parameters (e.g. index or key) in brackets and then the return type.

The syntax below is for a subscript that can get and set values. Code for retrieving the value is contained in a get {} wrapper and code for setting the value is contained in a set {} wrapper.

subscript(input parameters) -> return type {
    get {
        return the value here
    }
    set(newValue) {
        set the new value here
    }
}

If a subscript is read-only, the code for retrieving the value can be entered directly inside the subscript {} wrapper, omitting the get {} wrapper.

subscript(input parameters) -> return type {
        return the value here
}

Creating Subscripts Example

The example below defines a read-only subscript within a struct called FlowerGarden.

struct FlowerGarden {
    let flower: String
    subscript(count: Int) -> String {
        var flowers = ""
        if count > 0 {
            for _ in 1...count {
                flowers += flower
            }
        }
        return flowers
    }
}

let sakura = FlowerGarden(flower: "🌸")
print(sakura[5])

let sunflower = FlowerGarden(flower: "🌻")
print(sunflower[3])

Two instances of FlowerGarden are declared and then subscript notation is used to access them and print out the specified number of flowers, resulting in this output:

🌸🌸🌸🌸🌸
🌻🌻🌻