Map array of objects to Dictionary in Swift

Question

I have an array of Person\'s objects:

class Person {
   let name:String
   let position:Int
}

and the array is:

Answer 1

You can use a reduce function. First I've created a designated initializer for Person class

class Person {
  var name:String
  var position:Int

  init(_ n: String,_ p: Int) {
    name = n
    position = p
  }
}

Later, I've initialized an Array of values

let myArray = [Person("Bill",1), 
               Person("Steve", 2), 
               Person("Woz", 3)]

And finally, the dictionary variable has the result:

let dictionary = myArray.reduce([Int: Person]()){
  (total, person) in
  var totalMutable = total
  totalMutable.updateValue(person, forKey: total.count)
  return totalMutable
}

Answer 2

Okay map is not a good example of this, because its just same as looping, you can use reduce instead, it took each of your object to combine and turn into single value:

let myDictionary = myArray.reduce([Int: String]()) { (dict, person) -> [Int: String] in
    var dict = dict
    dict[person.position] = person.name
    return dict
}

//[2: "b", 3: "c", 1: "a"]

In Swift 4 or higher please use the below answer for clearer syntax.

Answer 3

Since Swift 4 you can do this very easily. There are two new initializers that build a dictionary from a sequence of tuples (pairs of key and value). If the keys are guaranteed to be unique, you can do the following:

let persons = [Person(name: "Franz", position: 1),
               Person(name: "Heinz", position: 2),
               Person(name: "Hans", position: 3)]

Dictionary(uniqueKeysWithValues: persons.map { ($0.position, $0.name) })

=> [1: "Franz", 2: "Heinz", 3: "Hans"]

This will fail with a runtime error if any key is duplicated. In that case you can use this version:

let persons = [Person(name: "Franz", position: 1),
               Person(name: "Heinz", position: 2),
               Person(name: "Hans", position: 1)]

Dictionary(persons.map { ($0.position, $0.name) }) { _, last in last }

=> [1: "Hans", 2: "Heinz"]

This behaves as your for loop. If you don't want to "overwrite" values and stick to the first mapping, you can use this:

Dictionary(persons.map { ($0.position, $0.name) }) { first, _ in first }

=> [1: "Franz", 2: "Heinz"]

Swift 4.2 adds a third initializer that groups sequence elements into a dictionary. Dictionary keys are derived by a closure. Elements with the same key are put into an array in the same order as in the sequence. This allows you to achieve similar results as above. For example:

Dictionary(grouping: persons, by: { $0.position }).mapValues { $0.last! }

=> [1: Person(name: "Hans", position: 1), 2: Person(name: "Heinz", position: 2)]

Dictionary(grouping: persons, by: { $0.position }).mapValues { $0.first! }

=> [1: Person(name: "Franz", position: 1), 2: Person(name: "Heinz", position: 2)]