Control Flow

• loop

  • while
  • for-in
  • repeat while

• condition branch

  • if

  • switch

    where

  • guard

• control transfer statements

  • break
  • continue
  • fallthrough
  • return
  • throw

Loop

For-In Loops

var pets = ["cat", "dog"]
for pet in pets {print(pet)
}
for index in 0..<2 {print(pets[index])
}

// if we don't need the value from the 0..<2, use the underscore to ignore the value.
var n = 1
for _ in 0..<2 {print(n)
        n += 1
}

var words = [11: "fine", 2: "happiness"]
for (key, word) in words {print("\(key): \(word)")
}

Stride Function

let minutes = 60
let interval = 5
for tickMark in stride(from: 0, to: minutes, by: interval) {
        // render the mark every 5 minutes. 
        // 0, 5, 10, ... , 50, 55
}

// closed range
for tickMark in stride(from: 0, through: minutes, by: inerval {
        // render the mark every 5 minutes.
        // 0, 5, 10, ... , 50, 55, 60
}

The for-in can be only use to the object which conform to the Sequence protocol.

While

var a = 0
while a < 10 {print(a)  // 0 to 9
        a += 1
}

Repeat While

The content that repeat while enclose with {} will run at least once.

var a = 0
while a < 0 {print(a)
        a += 1
}
// will not print

repeat {print(a)
        a += 1
}while a < 0
// will print 0

Condition

If

var a = 4
if a < 5 {// smaller than 5} else if a > 5 {// bigger than 5} else {// equal to 5}

Switch

The body of each case must contain at least one executable statement.

No Implicit Fallthrough: no need to write‘break’in each case, switch do only one case.

var s: String = "as"
switch s {
        case "aa", "AA":
                print("not the target")
        case "as":
                print("yes")
        default:
                print("I don't know")
}

Interval Matching

var a = 40
var amount: String?
switch a {
        case ..<0:
                amount = nil 
        case 0: 
                amount = "no"
        case 1...10:
                amount = "few"
        case 11..<50:
                amount = "some"
        case 50..<100:
                amount = "many"
        default:
                amount = "much"
}

Tuples

• Use underscore as a wildcard pattern.
• The point (0, 0) would match case (0, 0) first, and so all other matching cases would be ignored.

let point = (1, 1)
switch point {case (0, 0):
    print("It's origin")
case (_, 0):
    print("It's on x axis")
case (0, _):
    print("It's on y axis")
case (-2...2, -2...2):
    print("It's in the box")
default:
    print("Out of box")
}

Value Bindings

• The case can name value or values from the matched test, for use in the body of the case.
• It can only use in tuple test.

let point = (2, 0)
switch point {case (let x, 0):
    print("(\(x),0) is on x axis")
case (0, let y):
    print("(0, \(y) is on y axis")
case let(x, y):
    print("(\(x), \(y)) not on axis")
}

Where

• Add a condition for the value or values from the test.

let point = (2, -2)
switch point {case let(x, y) where x == y:
    print("(\(x), \(y)) is on the line y = x")
case let(x, y) where x == -y:
    print("(\(x), \(y) is on the line y = -x")
case let(x, y):
    print("(\(x), \(y)) others")
}

Compound Cases

• Multiple cases can compounded into one.
• The pattern can be written over multiple lines.

let c = "a"
switch c {
case "a", "e", "i", "o", "u":
    print("It's a vowel.")
case "b", "c", "d", "f", "g",
"h", "j", "k", "l", "m", "n",
"p", "q", "r", "s", "t", "v",
"w", "x", "y", "z":
    print("It's a consonant.")
default:
    print("It's not an alphabet.")
}

• Compound with tuple binding

  Every pattern in a case must include the temporary object(distance), so the code in the body of the case can always access a value for distance.

  let point = (1, 0)
  switch point {case (0, let distance), (let distance, 0):
      print("point is on the axis, distance \(distance)")
  default:
      print("point isn't on the axis")
  }

  —

  ## Control Transfer Statements

  ### Continue

  Tells a loop to stop what it’s doing and start again at the beginning of the next iteration through the loop.

  var nums: [Int] = []
  for n in  1...10 {
      if n % 2 == 0 {continue}
      nums.append(n)
  }
  print(nums)

  • resule

    [1, 3, 5, 7, 9]

### Break

```swift
var nums: [Int] = []
for n in  1...10 {
    if n == 5 {break}
    nums.append(n)
}
print(nums)
```

- result
    
    [1, 2, 3, 4]
    

Break in a switch

- comments aren’t statement, use a `break` when the case body is empty.

```swift
let n = 5
switch n {
case 1:
    print("one")
case 2:
    print("I want to break")
    break
        print("two")   // it won't do this
case 3:
    print("three")
default:
    break
}
```

Fallthrough

In Swift , oneswitch only execute one case . If we want to execute multiple cases, use the fallthrough .

let n = 5
switch n {
case 5:
    print("five")
    fallthrough    // go on the code in the switch
default:
    print("believe it or not")
    break
}

Labeled Statements

The break and continue can only affect the current for, while, switch, repeat while. So when we want to break or continue the outside loop or switch, we can use the labeled statements.

• break

  var a = 1
  var sum = 0
  aLoop: while a < 10 {
      while a < 5 {
          if a == 3 {
                          a += 5
              break aLoop
          }
          a += 1
          print("add a: \(a)")
      }
      print(a)
          a += 1
  }

  • result

    add a: 2
    add a: 3

• continue

  var a = 1
  var sum = 0
  aLoop: while a < 10 {
      while a < 5 {
          if a == 3 {
              a+=5   // now the a is 8
              continue aLoop
          }
          a += 1
          print("add a: \(a)")
      }
      print(a)
      a += 1
  }

  • result

    add a: 2
    add a: 3
    8
    9

    It continue the outside while loop.

---

## Early Exit

A `guard` must have else clause after it.
It’s to exit early.

```swift
func play(ball: String?) {
    guard let str = ball else {print("no ball")
        return   // the guard's else can't fall through, here we use the return to end the excution.
    }
        print(str)   // it's available after the guard's statement.
}

play(ball: nil)
```

## Checking API Availability

```swift
if #available(iOS 10, macOS 10.12, *) { } else {// Fall back to earlier iOS and macOS APIs}
```