How to iterate a loop with index and element in Swift

Question

Is there a function that I can use to iterate over an array and have both index and element, like Python's enumerate?

for index, element in enumerate(list):     ... 

Answer 1

Yes. As of Swift 3.0, if you need the index for each element along with its value, you can use the enumerated() method to iterate over the array. It returns a sequence of pairs composed of the index and the value for each item in the array. For example:

for (index, element) in list.enumerated() {   print("Item \(index): \(element)") } 

Before Swift 3.0 and after Swift 2.0, the function was called enumerate():

for (index, element) in list.enumerate() {     print("Item \(index): \(element)") } 

Prior to Swift 2.0, enumerate was a global function.

for (index, element) in enumerate(list) {     println("Item \(index): \(element)") } 

Answer 2

Swift 5 provides a method called enumerated() for Array. enumerated() has the following declaration:

func enumerated() -> EnumeratedSequence<Array<Element>> 

Returns a sequence of pairs (n, x), where n represents a consecutive integer starting at zero and x represents an element of the sequence.

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In the simplest cases, you may use enumerated() with a for loop. For example:

let list = ["Car", "Bike", "Plane", "Boat"] for (index, element) in list.enumerated() {     print(index, ":", element) }  /* prints: 0 : Car 1 : Bike 2 : Plane 3 : Boat */ 

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Note however that you're not limited to use enumerated() with a for loop. In fact, if you plan to use enumerated() with a for loop for something similar to the following code, you're doing it wrong:

let list = [Int](1...5) var arrayOfTuples = [(Int, Int)]()  for (index, element) in list.enumerated() {     arrayOfTuples += [(index, element)] }  print(arrayOfTuples) // prints [(0, 1), (1, 2), (2, 3), (3, 4), (4, 5)] 

A swiftier way to do this is:

let list = [Int](1...5) let arrayOfTuples = Array(list.enumerated()) print(arrayOfTuples) // prints [(offset: 0, element: 1), (offset: 1, element: 2), (offset: 2, element: 3), (offset: 3, element: 4), (offset: 4, element: 5)] 

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As an alternative, you may also use enumerated() with map:

let list = [Int](1...5) let arrayOfDictionaries = list.enumerated().map { (a, b) in return [a : b] } print(arrayOfDictionaries) // prints [[0: 1], [1: 2], [2: 3], [3: 4], [4: 5]] 

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Moreover, although it has some limitations, forEach can be a good replacement to a for loop:

let list = [Int](1...5) list.reversed().enumerated().forEach { print($0, ":", $1) }  /* prints: 0 : 5 1 : 4 2 : 3 3 : 2 4 : 1 */ 

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By using enumerated() and makeIterator(), you can even iterate manually on your Array. For example:

import UIKit import PlaygroundSupport  class ViewController: UIViewController {      var generator = ["Car", "Bike", "Plane", "Boat"].enumerated().makeIterator()      override func viewDidLoad() {         super.viewDidLoad()          let button = UIButton(type: .system)         button.setTitle("Tap", for: .normal)         button.frame = CGRect(x: 100, y: 100, width: 100, height: 100)         button.addTarget(self, action: #selector(iterate(_:)), for: .touchUpInside)         view.addSubview(button)     }      @objc func iterate(_ sender: UIButton) {         let tuple = generator.next()         print(String(describing: tuple))     }  }  PlaygroundPage.current.liveView = ViewController()  /*  Optional((offset: 0, element: "Car"))  Optional((offset: 1, element: "Bike"))  Optional((offset: 2, element: "Plane"))  Optional((offset: 3, element: "Boat"))  nil  nil  nil  */