Create dictionary from an array of objects using property of object as key for the dictionary?

Question

With Swift is it possible to create a dictionary of [String:[Object]] from an array of objects [Object] using a property of those objects as the String key for the dictionary using swift's "map"?

class Contact:NSObject {

   var id:String = ""
   var name:String = ""
   var phone:String = ""

   init(id:String, name:String, phone:String){
      self.id = id
      self.name = name
      self.phone = phone
   }

}

var contactsArray:[Contact]
var contactsDict:[String:Contact]

contactsDict = (contactsArray as Array).map { ...WHAT GOES HERE... }

Answer 1

Let's say you want to use id as the key for the dictionary:

var contactsArray = [Contact]()
// add to contactsArray

var contactsDict = [String: Contact]()
contactsArray.forEach {
    contactsDict[$0.id] = $0
}

The difference between map and forEach is that map returns an array. forEach doesn't return anything.

Answer 2

You can achieve this via reduce in a one-line functional-style code:

let contactsDict = contactsArray.reduce([String:Contact]()) { var d = $0; d[$1.id] = $1; return d; }

This also keeps contactsDict immutable, which is the preferred way to handle variables in Swift.

Or, if you want to get fancy, you can overload the + operator for dictionaries, and make use of that:

func +<K,V>(lhs: [K:V], rhs: Contact) -> [K:V] {
    var result = lhs
    result[rhs.0] = rhs.1
    return result
}

let contactsDict = contacts.reduce([String:Contact]()) { $0 + ($1.id, $1) }

Answer 3

Swift 4

There's now a direct way to do this:

https://developer.apple.com/documentation/swift/dictionary/3127163-init

It's an initializer for Dictionary that lets you return a string key for each element in a Collection that specifies how it should be grouped in the resulting Dictionary.