Fetch data from an array with elements of second array - arrays

Question

I have one array "users" with all user data and and second array "userIds" having user's id. I have to fetch User from "users" array using "userIds" array
struct User {
let name: String
let id: Int
}
let users: [User] = [User(name: "a", id: 1),
User(name: "b", id: 2),
User(name: "c", id: 3),
User(name: "d", id: 4),
User(name: "d", id: 5)]
let userIds = [2,3,2,5]
result array that I want is :
[User(name: "b", id: 2),
User(name: "c", id: 3),
User(name: "b", id: 2),
User(name: "d", id: 5)]
so it can have duplicate data according to the data in "userIds".
Now I tried using Higher order function filter:
let result = users.filter { (user) -> Bool in
return userIds.contains(user.id)
}
but this removes the duplicate data and the output is :
[User(name: "b", id: 2),
User(name: "c", id: 3),
User(name: "d", id: 5)]
One approach that I tried is using for loop :
var result = [User]()
for i in userIds {
result.append(users.filter({ $0.id == i }).first!)
}
which gives the desired output but if there is a better approach please suggest.

Answer 1

You can solve this using first(where:) to search through users:
let result = userIds.compactMap { desiredDataValue in
users.first(where: { $0.id == desiredDataValue })
}
But if you're doing this a lot, it would probably speed things up if you built a datastructure that allows for fast lookup by the "id" value. You should compare the performance for yourself, and see if you do this enough/frequently enough for it to be worthwhile:
let dictsByData = Dictionary(uniqueKeysWithValues:
users
.lazy
.map { dict in
(key: dict.id, value: dict)
}
)
let result = userIds.compactMap { desiredDataValue in dictsByData[desiredDataValue]! }
result.forEach { print($0) }

Answer 2

Well after digging few more and with the help of this blog:
https://medium.com/#abhimuralidharan/higher-order-functions-in-swift-filter-map-reduce-flatmap-1837646a63e8
I tried doing like this:
let results = userIds.compactMap { (int) -> User? in
var matchedUser: User?
if users.contains(where: { (user) -> Bool in
if user.id == int {
matchedUser = user
}
return user.id == int
}) {
return matchedUser
}
return nil
}
and in playground I checked the count the code was executed :
and it seems like the count is less comparing to "for" loop.