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iOS Swift 2.0 Enumerations and Optionals Introduction to Optionals Initializing Optional Values

Dalton Keen
Dalton Keen
8,082 Points
Posted by Dalton Keen
Dalton Keen
8,082 Points

Failable Initializers

Objective: "In the editor, you have a struct named Book which has few stored properties, two of which are optional.

Your task is to create a failable initializer that accepts a dictionary as input an initializes all the stored properties. (Hint: A failable init method is one that can return nil and is written as init?). Name the initializer parameter dict.

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

The previous videos explained how to use guard statements in functions so I applied the same logic to the initializer and it doesn't work. I've tried declaring variables using self (ex: let self.title) and no difference. I'm just not sure at all how to go about solving this one. Any help is appreciated.

Thanks

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


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

        let price = dict["price"]
        let pubDate = dict["pubDate"]

    }

}

2 Answers

jcorum
jcorum
71,830 Points

Almost! The type of the dict needs to be [String:String]. Once you've vetted the title and author you get from the dictionary you still need to set the member variables title and author.

struct Book {
    let title: String
    let author: String
    let price: String?
    let pubDate: String?

    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"]
    }
}
Dalton Keen
Dalton Keen
8,082 Points
Dalton Keen
Dalton Keen
8,082 Points

Ahhh. Got it! Thanks

Christian McMullin
Christian McMullin
13,086 Points

Could you demonstrate how do declare a new instance of that struct with using the init? I can't seem to figure it out? also why would you use it that way instead of like this.

struct Book {
    let title: String
    let author: String
    let price: String?
    let pubDate: String?

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

    let price = dict["price"]
    let pubDate = dict["pubDate"]

    return Book(title: title, author: author, price: price, pubDate: pubDate)
}
jcorum
jcorum
71,830 Points
jcorum
jcorum
71,830 Points

Christian, re your code: First, the challenge asked you to create a failable initializer, not a function. Second, initializers must be inside their structs, not outside them. And initializers don't return anything. They create objects and initialize their member variables.

Re your request:

let dict1 = [ "title" : "Pride & Prejudice", "author" : "Jane Austen", "price" : "23.95"]
let b1 = Book(dict: dict1)
b1?.title  //"Pride & Prejudice"
b1?.author  //"Jane Austen"
b1?.price  //"23.95"
b1?.pubDate  //nil

Note that the above code must be outside the struct, as it is using the struct to create a Book object (b1) from a dictionary.

If you put the struct and the above code in an Xcode playground (7.3.1.), and then alt-click on b1, you will see, in the popup, that b1 is Book?, i.e., an optional. If you try to remove the ? you will get an error message saying "Value of optional type 'Book?' not unwrapped; did you mean to use '!' or '?'?"

Christian McMullin
Christian McMullin
13,086 Points

My code works fine the playground. I know the challenge was asking to use init which i did. It just seems easier to me to use a func rather then init, and I was wondering if there is a benefit in using the init method rather then the func like I did.

//this is an instance of my code up above in my last comment.

let newBook = Book(title: "legend of Drittz", author: "r.a. salvator", price: "15.00", pubDate: nil)
newBook.title                  //returns "legend of Drittz"
newBook.author             //returns "r.a. salvator"
newBook.price               //returns "15.00"
newBook.pubDate        // returns nil

//this does not return an error and works perfectly.