Swift 5 offers many ways to get an array of property values from an array of similar objects. According to your needs, you may choose one of the **six following Playground code examples** to solve your problem.
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## 1. Using `map` method
With Swift, types that conform to `Sequence` protocol have a [`map(_:)`][1] method. The following sample code shows how to use it:
class Employee {
let id: Int, firstName: String, lastName: String
init(id: Int, firstName: String, lastName: String) {
self.id = id
self.firstName = firstName
self.lastName = lastName
}
}
let employeeArray = [
Employee(id: 1, firstName: "Jon", lastName: "Skeet"),
Employee(id: 2, firstName: "Darin", lastName: "Dimitrov"),
Employee(id: 4, firstName: "Hans", lastName: "Passant")
]
let idArray = employeeArray.map({ (employee: Employee) -> Int in
employee.id
})
// let idArray = employeeArray.map { $0.id } // also works
print(idArray) // prints [1, 2, 4]
---
## 2. Using `for` loop
class Employee {
let id: Int, firstName: String, lastName: String
init(id: Int, firstName: String, lastName: String) {
self.id = id
self.firstName = firstName
self.lastName = lastName
}
}
let employeeArray = [
Employee(id: 1, firstName: "Jon", lastName: "Skeet"),
Employee(id: 2, firstName: "Darin", lastName: "Dimitrov"),
Employee(id: 4, firstName: "Hans", lastName: "Passant")
]
var idArray = [Int]()
for employee in employeeArray {
idArray.append(employee.id)
}
print(idArray) // prints [1, 2, 4]
---
## 3. Using `while` loop
Note that with Swift, behind the scenes, a `for` loop is just a `while` loop over a `sequence`'s iterator (see [IteratorProtocol][3] for more details).
class Employee {
let id: Int, firstName: String, lastName: String
init(id: Int, firstName: String, lastName: String) {
self.id = id
self.firstName = firstName
self.lastName = lastName
}
}
let employeeArray = [
Employee(id: 1, firstName: "Jon", lastName: "Skeet"),
Employee(id: 2, firstName: "Darin", lastName: "Dimitrov"),
Employee(id: 4, firstName: "Hans", lastName: "Passant")
]
var idArray = [Int]()
var iterator = employeeArray.makeIterator()
while let employee = iterator.next() {
idArray.append(employee.id)
}
print(idArray) // prints [1, 2, 4]
---
## 4. Using a `struct` that conforms to `IteratorProtocol` and `Sequence` protocols
class Employee {
let id: Int, firstName: String, lastName: String
init(id: Int, firstName: String, lastName: String) {
self.id = id
self.firstName = firstName
self.lastName = lastName
}
}
struct EmployeeSequence: Sequence, IteratorProtocol {
let employeeArray: [Employee]
private var index = 0
init(employeeArray: [Employee]) {
self.employeeArray = employeeArray
}
mutating func next() -> Int? {
guard index < employeeArray.count else { return nil }
defer { index += 1 }
return employeeArray[index].id
}
}
let employeeArray = [
Employee(id: 1, firstName: "Jon", lastName: "Skeet"),
Employee(id: 2, firstName: "Darin", lastName: "Dimitrov"),
Employee(id: 4, firstName: "Hans", lastName: "Passant")
]
let employeeSequence = EmployeeSequence(employeeArray: employeeArray)
let idArray = Array(employeeSequence)
print(idArray) // prints [1, 2, 4]
---
## 5. Using `Collection` protocol extension and `AnyIterator`
class Employee {
let id: Int, firstName: String, lastName: String
init(id: Int, firstName: String, lastName: String) {
self.id = id
self.firstName = firstName
self.lastName = lastName
}
}
extension Collection where Iterator.Element: Employee {
func getIDs() -> Array<Int> {
var index = startIndex
let iterator: AnyIterator<Int> = AnyIterator {
defer { index = self.index(index, offsetBy: 1) }
return index != self.endIndex ? self[index].id : nil
}
return Array(iterator)
}
}
let employeeArray = [
Employee(id: 1, firstName: "Jon", lastName: "Skeet"),
Employee(id: 2, firstName: "Darin", lastName: "Dimitrov"),
Employee(id: 4, firstName: "Hans", lastName: "Passant")
]
let idArray = employeeArray.getIDs()
print(idArray) // prints [1, 2, 4]
---
## 6. Using KVC and `NSArray`'s [`value(forKeyPath:)`][4] method
Note that this example requires `class Employee` to inherit from `NSObject`.
import Foundation
class Employee: NSObject {
@objc let id: Int, firstName: String, lastName: String
init(id: Int, firstName: String, lastName: String) {
self.id = id
self.firstName = firstName
self.lastName = lastName
}
}
let employeeArray = [
Employee(id: 1, firstName: "Jon", lastName: "Skeet"),
Employee(id: 2, firstName: "Darin", lastName: "Dimitrov"),
Employee(id: 4, firstName: "Hans", lastName: "Passant")
]
let employeeNSArray = employeeArray as NSArray
if let idArray = employeeNSArray.value(forKeyPath: #keyPath(Employee.id)) as? [Int] {
print(idArray) // prints [1, 2, 4]
}
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]
