Get an array of property values from an object array

Question

There's a class called Employee.

class Employee {

    var id: Int
    var firstName: String
    var lastName: String
    var dateOfBirth: NSDate?

    init(id: Int, firstName: String, lastName: String) {
        self.id = id
        self.firstName = firstName
        self.lastName = lastName
    }
}

And I have an array of Employee objects. What I now need is to extract the ids of all those objects in that array into a new array.

I also found this similar question. But it's in Objective-C so it's using valueForKeyPath to accomplish this.

How can I do this in Swift?

Antonio · Accepted Answer · 2015-02-08 12:19:27Z

308

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 }

edited Feb 8, 2015 at 12:19

answered Feb 8, 2015 at 11:24

Antonio

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12 Comments

Antonio Over a year ago

That's what the map does - it transforms an array of Employee to an array of Int, filled with the id field. Which is equivalent to saying "extract the id field from all instances of Employee and put them into an array"

vacawama Over a year ago

@Isuru, this answer does exactly what you want. It creates a new array called ids of all the id values from the array of Employees. Note, it leaves the original array intact.

TotoroTotoro Over a year ago

Looks like in Swift 2 beta the correct syntax would be array.map( { $0.id })

Chris Over a year ago

if you're using an optional, make sure you ! it. Took me hours.

Antonio Over a year ago

@Chris forced unwrapping is usually a bad practice, because if nil it will cause the app to crash. Use it only when strictly required, and prefer optional binding (or any other "soft" unwrapping) instead

|

Community · Accepted Answer · 2020-06-20 09:12:55Z

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]
}

huge... That's as far as I understand complete possible approaches list
Probably could've stopped at #1 (specifically the commented out line) but nice exhaustive list!

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Get an array of property values from an object array

2 Answers 2

12 Comments

1. Using `map` method

2. Using `for` loop

3. Using `while` loop

4. Using a `struct` that conforms to `IteratorProtocol` and `Sequence` protocols

5. Using `Collection` protocol extension and `AnyIterator`

6. Using KVC and `NSArray`'s `value(forKeyPath:)` method

2 Comments

Your Answer

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Collectives™ on Stack Overflow

2 Answers 2

12 Comments

1. Using map method

2. Using for loop

3. Using while loop

4. Using a struct that conforms to IteratorProtocol and Sequence protocols

5. Using Collection protocol extension and AnyIterator

6. Using KVC and NSArray's value(forKeyPath:) method

2 Comments

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Related

1. Using `map` method

2. Using `for` loop

3. Using `while` loop

4. Using a `struct` that conforms to `IteratorProtocol` and `Sequence` protocols

5. Using `Collection` protocol extension and `AnyIterator`

6. Using KVC and `NSArray`'s `value(forKeyPath:)` method