【Swift 筆記 Draft】第 4 講: Flow Control 流程控制

循環 loop 語句：

1. for-in
2. for-condition-increment
3. while
4. do ... while

分支語句：

5. if condition
6. switch

跳轉語句：

7. fallthrough
8. where clause
9. break, continue
10. 可跳轉的標籤


for-in:

for index in 1 ... 10 { ...}

for _ in 1 ... 10 { ... } //當 index 在 { ... } 不被使用時


let nums = [1, 2, 3, 4, 5]
for num in nums { ... }

let dict1 = ["key1":1,"key2":2 ]
for (key,value) in dict1 { ... }

for character in "hello, world" { ... } 

for-condition-increment:

for initialization; condition; increment { ... }


while

while condition { ... }


do ... while

do { ... } while condition


無限循環：

for ; ; { ... }
while true { ... }
do { ... } while true


if condition

if condition { ...}

if condition { ...} else { ... }

if condition1 { ... }
else if condition2 { ... }
else { ... }


switch
可用於　Integer / Floating Point / Character / String / Tuple 等類型的數據做比較。

switch condition {
   case value1:
      statement1
   case value2, value3:
      statement2
   ...
   default:
      statementN+1
}

例如：範圍 range 比較：

let weight = 80
switch weight {
  case 0 ..< 50:
    println("lower than standard")
  case 50:
    println("perfect")
  case 51 ... 200:
    println("above the standard)
  default:
    println("Not applicable")
}



where clause

where: 在 case 中增加篩選條件

let weight = 80
switch weight
{
  case let (x) where x < 50:
    println("lower than standard")
  case let (x) where x == 50:
    println("perfect")
  case let (x) where x > 50:
    println("above the standard")
  default:
    println("Not applicable")
}

將 weight 這值 binding 到一個臨時的 constant / variable。(例如：x)

fallthrough:

fallthrough 為貫穿 case 而設計。例如：

var n = 1
var x = 2

switch x {
  case 1:
       n++
  case 2:
       n++
       fallthrough
  case 3:
       n++
  default:
       n++
}
println("n = \(n)") //此時　n = 3


break, continue

continue: 結束本次loop，進入下一個 loop
break: 結束當前 statements 執行

可跳轉的標籤Label:

forLabel:for index1 in 1 ... 5
{
   for index2 in 1 ... 5
   {
      ...
      break forLabel
   }
}