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iOS Enumerations and Optionals in Swift Introduction to Optionals Initializing Optional Values

Gokul Nishanth
Gokul Nishanth
245 Points
Posted by Gokul Nishanth
Gokul Nishanth
245 Points

Use the following keys to retrieve values from the dictionary: "title", "author", "price", "pubDate".

How to retrieve values from the dictionary? I have tried this method shown below: struct Book { let title: String let author: String let price: String? let pubDate: String?

init?(dict: [String : String]) {
  return nil
}

}

func newBook(bookdict: [String : String]) -> Book? { guard let title = bookdict["title"], let author = bookdict["author"] else { return nil }

let price = bookdict["price"] let pubDate = bookdict["pubDate"]

return Book(title: title, author: author, price: price, pubDate: pubDate) }

optionals.swift 
struct Book {
    let title: String
    let author: String
    let price: String?
    let pubDate: String?

    init?(dict: [String : String]) {
      return nil
    }
}

func newBook(bookdict: [String : String]) -> Book? {
  guard let title = bookdict["title"], let author = bookdict["author"] else {
    return nil
  }

  let price = bookdict["price"]
  let pubDate = bookdict["pubDate"]

  return Book(title: title, author: author, price: price, pubDate: pubDate)
  }

1 Answer

Nathan Tallack
Nathan Tallack
22,159 Points
Nathan Tallack
Nathan Tallack
22,159 Points

What you are wanting to do here is create an init function that will check to make sure the non optional properties are present in your dictionary and if they are not then return nil. We do that with a guard statement. It would look something like this.

    init?(dict: [String: String]) {
        guard let title = dict["title"], let author = dict["author"] else {
            return nil
        }
        self.title = title
        self.author = author
        self.price = dict["price"]
        self.pubDate = dict["pubDate"]
    }

So we have the init? to show it an init that can fail, and we use a guard let (just like we do an if let) to pull the values for those non optional properties from the dictionary.

Note that unlike an if let the guard let will escape those values from its scope allowing them to be use further down in the init function where we are assigning them to our non optional properties.

Also note that we did not need to do this for the optional properties because if they are not there we will happily set them to nil because they are optional.

Swift is so pretty! :)