1. To create an empty array or dictionary, use the initializer syntax.
let emptyArray = [String]()
let emptyDictionary = [String: Float]()
2. Use the init?(rawValue:) initializer to make an instance of an enumeration from a raw value. It returns either the enumeration case matching the raw value or nil if there is no matching Rank.
3. f let convertedRank = Rank(rawValue: 3) {
let threeDescription = convertedRank.simpleDescription()
}
4. One of the most important differences between structures and classes is that structures are always copied when they are passed around in your code, but classes are passed by reference.
5. Classes, enumerations, and structs can all adopt protocols.
6. Writing is the same as writing … where T: Equatable.
