Reduce an array of objects based on object field - arrays

Question

I have a Country object and a City object
struct Country: {
let name: String
let countryCode: String
let cities: [City]
let population: Int
init(name: String, countryCode: String, cities: [City], population: Int) {
self.name = name
self.countryCode = countryCode
self.cities = cities
self.population = population
}
}
struct City {
let id: Int
let name: String
let latitude: Double
let longitude: Double
let countryCode: String
let population: Int
}
Incoming JSON data looks like this which decodes into [City] array
{
"cities":[
{
"id":1,
"name":"Paris",
"latitude":0,
"logitude":0,
"country_code":"FR",
"population":0
},
{
"id":2,
"name":"Nice",
"latitude":0,
"logitude":0,
"country_code":"FR",
"population":0
},
{
"id":3,
"name":"Berlin",
"latitude":0,
"logitude":0,
"country_code":"DE",
"population":0
},
{
"id":4,
"name":"Munich",
"latitude":0,
"logitude":0,
"country_code":"DE",
"population":0
},
{
"id":5,
"name":"Amsterdam",
"latitude":0,
"logitude":0,
"country_code":"NL",
"population":0
},
{
"id":6,
"name":"Leiden",
"latitude":0,
"logitude":0,
"country_code":"NL",
"population":0
}
]
}
How would I create [Country] array from [City] array efficiently? I've tried to use reduce:into: but not sure that's what I have to use.
I know I could go with an empty array and add/create Countries one by one then search if there is one already and add City to it. That creates awful looking code as for me. I feel like there is an elegant solution to this problem using map or reduce functions.
reduce:into: code I've tried so far
func transformArrayOf(_ cities: [City]) -> [Country] {
let empty: [Country] = []
return cities.reduce(into: empty) { countries, city in
let existing = countries.filter { $0.countryCode == city.countryCode }.first
countries[existing].cities.append(city)
}
}
EDIT:
The function only gets [City] array. So countries must be created only from that.
Dictionary(grouping:by:) with map(_:) works perfectly! Two lines instead on nested for loops and if statements :)
And Country name can be parsed from a country code

Answer 1

Use Dictionary(grouping:by:) and map(_:) combined to get the expected result.
let countries = Dictionary(grouping: cities, by: { $0.countryCode }).map { (countryCode, cities) -> Country in
return Country(name: "", countryCode: countryCode, countryName: "", cities: cities, population: cities.reduce(0) { $0 + $1.population })
}
Since the values for name and countryName are unknown, I've used empty String ("") for both.

Answer 2

This is what Dictionary(grouping:by:) is for:
let citiesByCountryCode = Dictionary(grouping: cities, by: \.countryCode)
But you'll need separate logic to create the countries, because they contain data that isn't derived from the cities like name, countryName (how are those different?), etc.