Arrays are a common data structure in Swift:

let l = ["alice", "bob", "charlie"]
dump(l[2])

But array subscripting is unchecked, so this will crash at runtime:

dump(l[3])

Here's an extension to add a checked subscripting operator:

extension Array {
    subscript(checked index: UInt) -> Element? {
        if index < count {
            return self[Index(index)]
        } else {
            return nil
        }
    }
}

So you can do:

dump(l[checked: 2])
dump(l[checked: 3])

Since it returns an Optional, you can do:

if let element = l[checked: 2] {
    NSLog("Value: \(element)")
}

Note that the argument is UInt instead of Int or Index so the compiler can check when you pass in a negative index like this:

dump(l[checked: -1])

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