How to get unique array in Swift3.0 - arrays

Question

Scene, I've an array of NSObject class named Comment like var comments:[Comment]?. This class has property commentBywhich is also a NSObject class named User and it has property to identify unique user which is var key:String.above description as code,
class Comment:NSObject {
var commentBy:User?
}
class User:NSObject {
var key:String?
}
//In some other class
class someClass {
var comments:[Comment]?
}
What I want
I want an unique array of Comment, suppose that userA has made 2 comments and userB has made 1 comment so there is three comments total, but in my unique array, I want to show that two users made comments.
what should I do?

Answer 1

var comments: [Comment]?
let userKeys = comments?.flatMap { $0.commentBy?.key } ?? []
let set = Set(userKeys)
If all you want is the total number of unique user that have commented, then I think mapping comments to user's key then create a set of key is faster.

Answer 2

See what i do, actually your question is How to init a NSSet in swift ?
class Comment:NSObject {
var commentBy:User?
}
class User:NSObject {
var key:String?
}
//In some other class
class someClass {
var comments:[Comment]?
}
let user1 = User()
user1.key = "1"
let user2 = User()
user1.key = "2"
let c1 = Comment()
c1.commentBy = user1
let c2 = Comment()
c2.commentBy = user1
let c3 = Comment()
c3.commentBy = user2
let set: NSSet = [user1, user2]
let set2: NSSet = [user2]
set2.isSubset(of: set as! Set<AnyHashable>) //true
let set3: NSSet = [c1, c1]
set3.count //1
let set4: NSSet = [c1, c2, c3] // your unique array
let set5 = NSSet(array: [c1, c2, c1, c3])
set5.count //3
set3 will always be [c1], because the elements in Set or NSSet will be unique.

Answer 3

First make User hashable in a way that makes them equal based on their unique key.
class User: NSObject
{
let key:String
init(key: String) { self.key = key }
override func isEqual(_ object: Any?) -> Bool
{
guard let other = object as? User else { return false }
return self.key == other.key
}
override var hashValue: Int { return key.hashValue }
}
let eq = User(key: "foo") == User(key: "foo") // true
let h1 = User(key: "foo").hashValue // some big number
let h2 = User(key: "foo").hashValue // same big number as above
I've made key immutable because, as it serves to uniquely identify a user, it should not be possible to change.
Because User is an NSObject it already has an implementation of == which uses isEqual: so you override that to compare based on the key. You also use the hashValue of key as the User hash value.
Now you can use User in sets and as keys in dictionaries. If you do the same for Comment so that they compare equal on User you can use comments in a set too. However, that is a bad idea. Two comments are not the same just because they have the same user. You should change SomeClass to use a dictionary keyed by user as follows:
class Comment: NSObject
{
let user: User
let text: String
init(user: User, text: String)
{
self.user = user
self.text = text
}
}
class SomeClass
{
var userComments: [User : [Comment]] = [:]
func addComment(newComment: Comment)
{
if let existingComments = userComments[newComment.user]
{
userComments[newComment.user] = existingComments + [newComment]
}
else
{
userComments[newComment.user] = [newComment]
}
}
var userCount: Int { return userComments.count }
var commentCount: Int { return userComments.values.reduce(0, { $0 + $1.count}) }
}
The property userCount is the number you asked for. The property commentCount is the total number of comments - although beware: the implementation runs in O(n) time. If speed is of the essence, you should maintain a counter that increments each time you add a comment.