Subscripts Explained in Swift

Swift subscripts are methods that allow you to access individual elements in a collection, list or sequence.

A subscript can be defined in a class, structure or enumeration, and the syntax is like a function that operates like a computed property.

Similar to a function, a subscript can accept parameters and return a value. The arguments are supplied in square brackets at the call site.

In the same way as a computed property, a subscript call is used to reference a property — so you can fetch its values or assign new values.

Let’s see how it works!

Subscript in Action

struct DayTemp {
    
    private var temperatures: [String: Int] = [
        
        "Monday": 70,
        "Tuesday": 72,
        "Wednesday": 68,
        "Thursday": 75,
        "Friday": 71,
        "Saturday": 74,
        "Sunday": 73
    ]
    
    subscript(day: String) -> Int? {
        
        get {
            temperatures[day]
        }
        set(newTemp) {
            if let temp = newTemp {
                temperatures[day] = temp
            }
        }
    }
}


var weekTemp = DayTemp()

if let mondayTemp = weekTemp["Monday"] {
    print("Monday's temperature is \(mondayTemp)") // output: Monday's temperature is 70
}

weekTemp["Monday"] = 90

if let updateMondayTemp = weekTemp["Monday"] {
    print("Monday's updated temperature is \(updateMondayTemp)") // output: Monday's updated temperature is 90
}
        

The code begins with the DayTemp structure, and it has a dictionary named temperatures that stores temperatures for each day of the week.

subscript(day: String) -> Int?        

Here, a subscript is defined. It takes a String (day name) as an index and returns an optional Int? (temperature).

The get block allows you to read (access) the temperature for a given day.

The set block allows you to modify the temperature for a given day.

Using weekTemp["Monday"] lets you get Monday’s temperature.

To set a new temperature, you assign a new integer to the weekTemp (subscript), passing in the day of the week as an argument, like so: weekTemp[“Monday”] = 90.

Subscripts provide a way to make accessing or updating data in custom types more convenient and similar to array or dictionary syntax.