SafeAccessArrayByIndexWithOptionals_Swift_3_1.swift

extension Collection where Indices.Iterator.Element == Index {
    subscript (safe index: Index) -> Generator.Element? {
        return indices.contains(index) ? self[index] : nil
    }
}

class ArrayLookup {

    private let array = ["Hello", "World"]
    
    func valueForIndex(_ index: Int) -> String {
        
        guard let value = array[safe: index] else {
            return "n/a"
        }

        return value
    }
}

let arrayLookup = ArrayLookup()
arrayLookup.valueForIndex(0)
arrayLookup.valueForIndex(1)
arrayLookup.valueForIndex(2)