Iteration

Ranges can be used to iterate a fixed number of times, for example, for i in 1...12. To print out these numbers, a loop such as the following can be used:

> for i in 1...12 {
.   print("i is \(i)")
. }

If the number is not required, then an underscore (_) can be used as a hole to act as a throwaway value. An underscore can be assigned to but not read:

> for _ in 1...12 {
.   print("Looping...")
. }

However, it is more common to iterate over a collection's contents using a for in pattern. This steps through each of the items in the collection, and the body of the for loop is executed over each one:

> var shopping = [ "Milk", "Eggs", "Coffee", "Tea", ]
> var costs = [ "Milk":1, "Eggs":2, "Coffee":3, "Tea":4, ]
> var cost = 0
> for item in shopping {
.   if let itemCost = costs[item] {
.     cost += itemCost
.   }
. }
> cost
cost: Int = 10

To iterate over a dictionary, it is possible to extract the keys or the values and process them as an array:

> Array(costs.keys)
$R0: [String] = 4 values {
  [0] = "Coffee"
  [1] = "Milk"
  [2] = "Eggs"
  [3] = "Tea"
}
> Array(costs.values)
$R1: [Int] = 4 values {
  [0] = 3
  [1] = 1
  [2] = 2
  [3] = 4
}

Note

The order of keys in a dictionary is not guaranteed; as the dictionary changes, the order may change.

Converting a dictionary's values to an array will result in a copy of the data being made, which can lead to poor performance. As the underlying keys and values are of a LazyMapCollection type, they can be iterated over directly:

> costs.keys
$R2: LazyMapCollection<[String : Int], String> = {
  _base = {
    _base = 4 key/value pairs {
      [0] = { key = "Coffee" value = 3 }
      [1] = { key = "Milk"   value = 1 }
      [2] = { key = "Eggs"   value = 2 }
      [3] = { key = "Tea"    value = 4 }
    }
  _transform =}
}

To print out all the keys in a dictionary, the keys property can be used with a for in loop:

> for item in costs.keys {
.   print(item)
. }
Coffee
Milk
Eggs
Tea

Iterating over keys and values in a dictionary

Traversing a dictionary to obtain all of the keys and then subsequently looking up values will result in searching the data structure twice. Instead, both the key and the value can be iterated at the same time, using a tuple. A tuple is like a fixed-sized array, but one that allows assigning pairs (or more) of values at a time:

> var (a,b) = (1,2)
a: Int = 1
b: Int = 2

Tuples can be used to iterate pairwise over both the keys and values of a dictionary:

> for (item,cost) in costs {
.   print("The \(item) costs \(cost)")
. }
The Coffee costs 3
The Milk costs 1
The Eggs costs 2
The Tea costs 4

Both Array and Dictionary conform to the SequenceType protocol, which allows them to be iterated with a for in loop. Collections (as well as other objects, such as Range) that implement SequenceType have a generate method, which returns a GeneratorType that allows the data to be iterated over. It is possible for custom Swift objects to implement SequenceType to allow them to be used in a for in loop.

Iteration with for loops

Although the most common use of the for operator in Swift is in a for in loop, it is also possible (in Swift 1 and 2) to use a more traditional form of the for loop. This has an initialization, a condition that is tested at the start of each loop, and a step operation that is evaluated at the end of each loop. Although the parentheses around the for loop are optional, the braces for the block of code are mandatory.

Note

It has been proposed that both the traditional for loop and the increment/decrement operators should be removed from Swift 3. It is recommended that these forms of loops be avoided where possible.

Calculating the sum of integers between 1 and 10 can be performed without using the range operator:

> var sum = 0
. for var i=0; i<=10; ++i {
.   sum += i
. }
sum: Int = 55

If multiple variables need to be updated in the for loop, Swift has an expression list that is a set of comma-separated expressions. To step through two sets of variables in a for loop, the following can be used:

> for var i = 0,j = 10; i<=10 && j >= 0; ++i,--j {
.   print("\(i), \(j)")
. } 
0, 10
1, 9
…
9, 1
10, 0

Tip

Apple recommends the use of ++i instead of i++ (and conversely, --i instead of i--) because they will return the result of i after the operation, which may be the expected value. As noted earlier, these operators may be removed in a future version of Swift.

Break and continue

The break statement leaves the innermost loop early, and control jumps to the end of the loop. The continue statement takes execution to the top of the innermost loop and the next item.

To break or continue from nested loops, a label can be used. Labels in Swift can only be applied to a loop statement, such as while or for. A label is introduced by an identifier and a colon just before the loop statement:

> var deck = [1...13, 1...13, 1...13, 1...13]
> suits: for suit in deck {
.   for card in suit {
.     if card == 3 {
.       continue // go to next card in same suit
.     }
.     if card == 5 {
.       continue suits // go to next suit
.     } 
.     if card == 7 {
.       break // leave card loop
.     }
.     if card == 13 {
.       break suits // leave suit loop
.     }
.   } 
. }