By using functions like the following, it prevents app from crashing if you pass the wrong index (tip from @CassiusPacheco)

array.swift

public extension Array {
    public subscript(safe index: Int) -> Element? {
        return self.indices ~= index ? self[index] : nil
    }
}

// ...

let optionalResult = array[safe: 1] // returns nil instead of crashing