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iOS Swift Collections and Control Flow Control Flow With Conditional Statements Working With Switch Statements

Posted by Juan Torres
Juan Torres
7,416 Points

I can't tell what's wrong with my code.

when i run it on xcode it comes up with "optional" and then brussels. or something like that.

operators.swift 
var europeanCapitals: [String] = []
var asianCapitals: [String] = []
var otherCapitals: [String] = []

let world = [
  "BEL": "Brussels", 
  "LIE": "Vaduz", 
  "BGR": "Sofia", 
  "USA": "Washington D.C.", 
  "MEX": "Mexico City", 
  "BRA": "Brasilia", 
  "IND": "New Delhi", 
  "VNM": "Hanoi"]

for (key, value) in world {
    // Enter your code below
    switch key
    {
    case "BEL", "LIE", "BGR": europeanCapitals.append("\(world[key])")
    case "IND", "VNM": asianCapitals.append("\(world[key])")
    default: otherCapitals.append("\(world[key])")
    }

    // End code
}

1 Answer

Alex Koumparos
seal-mask
.a{fill-rule:evenodd;}techdegree
Alex Koumparos
Python Development Techdegree Student 36,887 Points
Alex Koumparos
.a{fill-rule:evenodd;}techdegree
Alex Koumparos
Python Development Techdegree Student 36,887 Points

Hi Juan,

When you perform a dictionary lookup, there's no general guarantee that there will be a value for the specified key. This means that the compiler has to be prepared to handle the case where the result of the lookup is nil. Because a dictionary lookup can potentially return nil, and because of the way the type system works in Swift, the lookup can only return one type, the type for a lookup has to be Optional(T) (for whatever T is your dictionary value type).

In contrast, when you do for (key, value) in someDictionary, the compiler knows for certain that every key exists and thus every value exists, so it doesn't need to wrap the values in Optionals.

Your code:

.append("\(world[key])")

is performing the dictionary lookup on world and has to get back an Optional. Since your keys are coming from iterating through your for loop, you know for certain that every key has a corresponding value, so you could (but shouldn't!) force unwrap each lookup to get the underlying value:

.append("\(world[key]!)")

This will pass the challenge (because your output array will comprise Strings instead of Optional Strings) but is very dangerous code, so don't do it!

Instead, remember that In your code, you've specified for (key, value) when defining your for loop. This means that when you write your case statement, you don't need to lookup a value in the dictionary (which is going to give you value wrapped in an Optional), you already have the value in the constant value. This means that anytime you write:

.append("\(world[key])")

to access the value and insert it into the array, you could instead just have written:

.append(value)

Hope that's clear

Cheers

Alex