introduction.md

loops

for-loops

while-loops

repeat-while

control-transfer

Loops

There are three primary types of loop constructs in Swift: while loops, repeat-while loops, and for-in loops.

while loops

While loops in Swift have the following structure:

while boolean-expression {
  loop-body
}

The loops begin by first evaluating a Boolean expression. If the expression evaluates to false, then the body of the loop is skipped and execution begins on the first line following the loop. If the condition evaluates to true, then the body of the loop is executed, after which the Boolean expression is evaluated again, repeating this way until the Boolean expression evaluates to false.

var count = 3
while count > 0 {
  print("\(count)…")
  count -= 1
}
print("Liftoff!")

// prints:
// 3…
// 2…
// 1…
// Liftoff!

repeat-while loops

Repeat-while loops are similar to while loops, however, these loops differ in that the Boolean expression appears, and is evaluated, after the body of the loop is executed. As a result, these loops always execute at least once.

repeat {
  print("This runs at least once")
} while false
print("Loop done")

// prints:
// This runs at least once
// Loop done

for-in loops

For-in loops are used to iterate over a sequence of values, taking each element in turn, binding it to a variable or constant name of the developer's choosing, then executes a block of code that may refer to the element. When every element of the sequence has been iterated over, the loop exits and execution begins with the first line following the body of the loop.

let numbers = [3, 10, 7, 11]

for number in numbers {
  print("\(number) / 2 = \(number / 2)")
}
print("Done with numbers")

// prints:
// 3 / 2 = 1
// 10 / 2 = 5
// 7 / 2 = 3
// 11 / 2 = 5
// Done with numbers

Control transfer

Sometimes it is necessary to skip to the next iteration of a loop early, without completing the rest of the statements in the current iteration of the loop. The continue keyword can be used for this. When continue is executed, the loop jump to the next check to see if the next iteration of the loop can be run, i.e. the while in while and repeat-while loops or the check if there's another element in the sequence in for-in loops.

count = 1
repeat {
  count += 1
  if count == 4 { continue }
  print(count)
} while count !=4 && count < 6

prints:

2
3

rather than:

2
3
5
6