How to make array access safer using a custom subscript

Paul Hudson    May 9th 2018    @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.

Available from iOS 8.0

Did this solution work for you? Please pass it on!

Tweet

Other people are reading…

• What is a functor?
• What is a throwing function?
• What’s the difference between a class and a struct?
• How to use local variable observers
• How to break out of multiple loop levels using labeled statements

About the Swift Knowledge Base

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