TEAM LICENSES: Save money and learn new skills through a Hacking with Swift+ team license >>

How to make array access safer using a custom subscript

Swift version: 5.6

Paul Hudson    @twostraws   

Swift likes to be safe, but one problematic area can be reading from arrays and dictionaries. In the case of dictionaries, reading a missing key will return nil rather than the value you might have expected, but in the case of arrays it’s worse: your app will crash.

Dictionaries have a special subscript method that can send back a default value if you request a missing key, but arrays don’t. Fortunately, we can fix that using Swift’s extensions:

extension Array {
    public subscript(index: Int, default defaultValue: @autoclosure () -> Element) -> Element {
        guard index >= 0, index < endIndex else {
            return defaultValue()

        return self[index]

That uses @autoclosure() so your default value can be calculated however you need without incurring a performance hit in times when you use a valid array index.

With that extension in place you can now create and use arrays as usual:

var names = ["Paul"]
let paul = names[0]

But if you want, you can now also read any index using the new subscript and be sure to get back a safe value:

let anon1 = names[-1, default: "Anonymous"]
let anon2 = names[1, default: "Anonymous"]
let anon3 = names[556, default: "Anonymous"]

Alternatively, you could write a safeIndex subscript that returns an optional value – nil if the index is out of bounds, or the value in question otherwise:

extension Array {
    public subscript(safeIndex index: Int) -> Element? {
        guard index >= 0, index < endIndex else {
            return nil

        return self[index]

Both solutions have their uses, so try experimenting and see which works best for you.

Hacking with Swift is sponsored by String Catalog.

SPONSORED Get accurate app localizations in minutes using AI. Choose your languages & receive translations for 40+ markets!

Localize My App

Sponsor Hacking with Swift and reach the world's largest Swift community!

Available from iOS 8.0

Similar solutions…

About the Swift Knowledge Base

This is part of the Swift Knowledge Base, a free, searchable collection of solutions for common iOS questions.

Buy Pro Swift Buy Pro SwiftUI Buy Swift Design Patterns Buy Testing Swift Buy Hacking with iOS Buy Swift Coding Challenges Buy Swift on Sundays Volume One Buy Server-Side Swift Buy Advanced iOS Volume One Buy Advanced iOS Volume Two Buy Advanced iOS Volume Three Buy Hacking with watchOS Buy Hacking with tvOS Buy Hacking with macOS Buy Dive Into SpriteKit Buy Swift in Sixty Seconds Buy Objective-C for Swift Developers Buy Beyond Code

Was this page useful? Let us know!

Average rating: 4.3/5

Unknown user

You are not logged in

Log in or create account

Link copied to your pasteboard.