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How to make array access safer using a custom subscript

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.

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