Help with Optionals in Structs

Using Swift
1

Hello, all. First post here. I am a rank beginner at Swift, so please pardon the (probably) simplistic question. I have begun working through the SwiftUI for Masterminds book (3rd Ed.) and am stumped about something. In the introduction to structs, they offer the following example (Listing 3-31, pg. 44):

struct Price {
     var USD = 0.0
     var CAD = 0.0
}
struct Item {
     var name: String = "Not defined"
     var price: Price?
}
var purchase = Item()
purchase.name = "Lamps"
purchase.price?.USD = 10.50

I am confused on a couple points. First, in the results window, it shows that the value after the last assignment statement is still nil rather than 10.50. Secondly, when I tried to print this value with the command print(purchase.price!.USD), I get an execution error: " Fatal error: Unexpectedly found nil while unwrapping an Optional value". So it appears the assignment is not working? If I don't reference one struct from another and just combine the members in one struct, I can get it to work as I think it should, so seems to have something to do with the dot notation between the two? I don't know.... any help appreciated. :slight_smile:

2

If you break your code down into smaller pieces, the last bit ends up being this:

Before:

purchase.price?.USD = 10.50

After:

var price = purchase.price
if price != nil { //this bit is what the ? does
  price!.USD = 10.50
}
purchase.price = price

Hopefully it's clearer in the expanded version why it would remain nil? Somewhere in your code you'll need to actually make a Price value using Price(…).

1 Like
3

Ok, very interesting. Thanks so much for your reply and breaking it down that way. I think I get the gist of what you are saying. I am just trying to follow the book's examples and understand what's taking place. You mentioned making a Price value using Price somewhere in the code. Not sure I know how or where to do that at this early stage of my learning. Can you tell me what that reference would look like? Thanks again, David.

4

You see this bit in the code you already have? That’s how you make an Item. You need to make a Price the same way.