swift for loop: for index, element in array?
有没有一个函数可以用来遍历一个数组，并有索引和元素，就像python的枚举一样？
for index, element in enumerate(list):
...
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)")
}
Swift 3 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.
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.enumerate() {
print(index, ":", element)
}
/*
prints:
0 : Car
1 : Bike
2 : Plane
3 : Boat
*/
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)]
The correct way to do this is:
let list = [Int](1...5)
let arrayOfTuples = Array(list.enumerated())
print(arrayOfTuples) // prints [(0, 1), (1, 2), (2, 3), (3, 4), (4, 5)]
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]]
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
*/
By using enumerated() and makeIterator() , you can even iterate manually on your Array . For example:
import UIKit
class ViewController: UIViewController {
var generator = ["Car", "Bike", "Plane", "Boat"].enumerated().makeIterator()
// Link this IBAction to a UIButton in your storyboard
@IBAction func iterate(_ sender: UIButton) {
let tuple: (offset: Int, element: String)? = generator.next()
print(String(describing: tuple))
}
}
/*
Will print the following lines for 6 `touch up inside`:
Optional((0, "Car"))
Optional((1, "Bike"))
Optional((2, "Plane"))
Optional((3, "Boat"))
nil
nil
*/
从Swift 2开始，需要在集合上调用枚举函数，如下所示：
for (index, element) in list.enumerate() {
print("Item (index): (element)")
}
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