Population Count Consistency - Structs and Classes - Silver Challenge

Swift Programming (3rd Edition)
1

I’m looking to understand why the thisTown.population (Vampire.swift) count doesn’t decrease until after the 2nd terrorizeTown(). Population (Town.swift) appears to be correct whereas thisTown.population (Vampire.swift) is off by 1.

What am I missing to get these in sync?

main.swift

import Foundation
var myTown = Town()

let keseyTheVamp = Vampire()
keseyTheVamp.town = myTown

keseyTheVamp.terrorizeTown()
keseyTheVamp.town?.printDescription()

//Terrorize #1
keseyTheVamp.terrorizeTown()
keseyTheVamp.town?.printDescription()

//Terrorize #2
keseyTheVamp.terrorizeTown()
keseyTheVamp.town?.printDescription()

//Terrorize #3
keseyTheVamp.terrorizeTown()
keseyTheVamp.town?.printDescription()

Town.swift

import Foundation

struct Town {
    
    var population = 10
    var numberOfStopLights = 4
    
    func printDescription() {
        print("Population: \(population) ; # of Stop Lights: 
\(numberOfStopLights)")
    }
    
    
    mutating func changePopulation(by amount: Int) {
        population += amount
    }
    
}

Vampire.swift

import Foundation

class Vampire: Monster {
    
    var thralls = [Vampire]()
    
    override func terrorizeTown() {
        if let thisTown = town  {
            print("\(name) is terrorizing a town!")
            
            if thisTown.population >= 1 {
                thralls.append(Vampire())
                town?.changePopulation(by: -1)
                
                print("There are \(thralls.count) thralls and 
\(thisTown.population) people.")
                
            } else {
                print("There are no people left in this town!")
            }
        } else {
            print("\(name) hasn't found a town to terrorize yet...")
        }
    }
}

Console

//Terrorize #1
Monster is terrorizing a town!
There are 1 thralls and 10 people.
Population: 9 ; # of Stop Lights: 4

//Terrorize #2
Monster is terrorizing a town!
There are 2 thralls and 9 people.
Population: 8 ; # of Stop Lights: 4

//Terrorize #3
Monster is terrorizing a town!
There are 3 thralls and 8 people.
Population: 7 ; # of Stop Lights: 4
2

That’s probably because of the fundamental difference between classes and structs: classes are reference types whereas structs are value types.

See also: Instances of myTown created for each monster?

3

Thanks ibex, yeah – I assume it has something to do with struct vs. class and just want to understand it better so I have a more predictable outcome in the future. Especially in this case where both instances actually do decrease, but differently.

4

ibex10 is right, it’s because town is a struct. Go back & take a closer look at your terrorizeTown function in the Vampire class. Think about how it’s using thisTown vs town and keep in mind that those are two separate structs with separate population counts.

5

Also, don’t stress too much about fully understanding it right now. The MonsterTown program is built over three chapters that successively add a bit more to know about reference and value types, and then there’s a subsequent chapter entirely about the differences.

Take your time, and don’t hesitate to return to this chapter after you’ve finished those to help fill in the gaps!