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

Adam Petritsis
Adam Petritsis
2,942 Points
Posted by Adam Petritsis
Adam Petritsis
2,942 Points

How can we avoid the "Extra argument ' ' in call" error?

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 of type [String : String] as input and initializes all the stored properties. (Hint: A failable init method is one that can return nil and is written as init?).

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

Note: Give your initializer argument the name dict

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

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

    }
}



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

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

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


}

1 Answer

Dan Lindsay
Dan Lindsay
39,611 Points
Dan Lindsay
Dan Lindsay
39,611 Points

Hey Adam,

It looks like your init method is incomplete. You didn’t need to create a result function outside your struct either, but some of the code you have in that function should be in your init method.

First, lets take that guard statement(which is the correct logic, just in the wrong place) out of the result function, and put it in our init, like so:

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
         }  
    }
}

Now we just need to reference our structs internal properties and assign them appropriately, like this:

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 refers to the original constant(let title: String), and title refers to the constant created in the guard statement

    self.title = title 
    self.author = author
    self.price = dict[“price”]
    self.pubDate = dict[“pubDate”]  
    }
}

and that will pass. You already took care if assigning the title and author to the appropriate dictionary strings in the guard statement, you just needed to take care of the price and pubDate.

Hope this helps

Dan

Adam Petritsis
Adam Petritsis
2,942 Points
Adam Petritsis
Adam Petritsis
2,942 Points

Super, that makes sense! Thanks Dan!