xcode - Safe (bounds-checked) array lookup in Swift, through optional bindings?

Question

If I have an array in Swift, and try to access an index that is out of bounds, there is an unsurprising runtime error:

var str = ["Apple", "Banana", "Coconut"]

str[0] // "Apple"
str[3] // EXC_BAD_INSTRUCTION

However, I would have thought with all the optional chaining and safety that Swift brings, it would be trivial to do something like:

let theIndex = 3
if let nonexistent = str[theIndex] { // Bounds check + Lookup
    print(nonexistent)
    ...do other things with nonexistent...
}

Instead of:

let theIndex = 3
if (theIndex < str.count) {         // Bounds check
    let nonexistent = str[theIndex] // Lookup
    print(nonexistent)   
    ...do other things with nonexistent... 
}

But this is not the case - I have to use the ol' if statement to check and ensure the index is less than str.count.

I tried adding my own subscript() implementation, but I'm not sure how to pass the call to the original implementation, or to access the items (index-based) without using subscript notation:

extension Array {
    subscript(var index: Int) -> AnyObject? {
        if index >= self.count {
            NSLog("Womp!")
            return nil
        }
        return ... // What?
    }
}

Answer 1

Alex's answer has good advice and solution for the question, however, I've happened to stumble on a nicer way of implementing this functionality:

Swift 3.2 and newer

extension Collection {

    /// Returns the element at the specified index if it is within bounds, otherwise nil.
    subscript (safe index: Index) -> Element? {
        return indices.contains(index) ? self[index] : nil
    }
}

Swift 3.0 and 3.1

extension Collection where Indices.Iterator.Element == Index {

    /// Returns the element at the specified index if it is within bounds, otherwise nil.
    subscript (safe index: Index) -> Generator.Element? {
        return indices.contains(index) ? self[index] : nil
    }
}

Credit to Hamish for coming up with the solution for Swift 3.

Swift 2

extension CollectionType {

    /// Returns the element at the specified index if it is within bounds, otherwise nil.
    subscript (safe index: Index) -> Generator.Element? {
        return indices.contains(index) ? self[index] : nil
    }
}

---

Example

let array = [1, 2, 3]

for index in -20...20 {
    if let item = array[safe: index] {
        print(item)
    }
}