Iteration

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.

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.

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

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
.     }
.   } 
. }