Reduce an array of objects based on object field

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

user_5716829 · Answer

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.

user_3141234 · Answer

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.

Reduce an array of objects based on object field

2 Answers