Get an array of property values from an object array
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.
1. Using map
method
With Swift, types that conform to Sequence
protocol have a map(_:)
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 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:)
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]
}
You can use the map
method, which transform an array of a certain type to an array of another type - in your case, from array of Employee
to array of Int
:
var array = [Employee]()
array.append(Employee(id: 4, firstName: "", lastName: ""))
array.append(Employee(id: 2, firstName: "", lastName: ""))
let ids = array.map { $0.id }