Euphoria - switch 语句

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switch 语句用于根据表达式的值运行特定的一组语句。它通常替换一组if…elsif 语句，使您的程序更易于控制和阅读。

语法

简单 switch 语句的语法如下：

switch expression do
   case <val> [, <val-1>....] then
      -- Executes when the expression matches one of the values  
   
   case <val> [, <val-1>....] then
      -- Executes when the expression matches one of the values  
      .....................
   
   case else
      -- Executes when the expression does not matches any case.  
end if

case 中的<val>必须是原子、字面量字符串、常量或枚举。可以通过逗号分隔值来为单个 case 指定多个值。默认情况下，当遇到下一个 case 时，控制流将流向 switch 块的末尾。

示例

#!/home/euphoria-4.0b2/bin/eui

atom marks = 'C'

switch marks do
   case 'A' then
      puts(1, "Excellent!\n" )
   
   case 'B', 'C' then
      puts(1, "Well done!\n" )
   
   case 'D' then
      puts(1, "You passed!\n" )
   
   case 'F' then
      puts(1, "Better try again!\n" )
   
   case else
      puts(1, "Invalid grade!\n" )
end switch

这将产生以下结果：

Well done!

switch...with fallthru 语句

当case 语句与给定表达式值匹配时，switch 的case 语句将被执行，并且默认情况下它会退出。默认情况下，当遇到下一个 case 时，控制流将流向 switch 块的末尾。

可以通过在 switch 语句中使用with fallthru 来更改特定 switch 块的默认设置，以便在每次遇到新 case 时，控制流都传递到下一个可执行语句：

语法

简单switch...with fallthru 语句的语法如下：

switch expression with fallthru do
   case <val> [, <val-1>....] then
      -- Executes when the expression matches one of the values  
      break -- optional to come out of the switch from this point.
   
   case <val> [, <val-1>....] then
      -- Executes when the expression matches one of the values  
      break -- Optional to come out of the switch from this point.
      .....................
   
   case else
      -- Executes when the expression does not matches any case.  
      break -- Optional to come out of the switch from this point.
end if

示例

#!/home/euphoria-4.0b2/bin/eui

atom marks = 'C'

switch marks with fallthru do
   case 'A' then
      puts(1, "Excellent!\n" )
   
   case 'B', 'C' then
      puts(1, "Well done!\n" )
   
   case 'D' then
      puts(1, "You passed!\n" )
   
   case 'F' then
      puts(1, "Better try again!\n" )
   
   case else
      puts(1, "Invalid grade!\n" )
end switch

这将产生以下结果：

Well done!
You passed!
Better try again!
Invalid grade!

您可以使用可选的break 语句从 switch 语句内部的某个点退出，如下所示：

#!/home/euphoria-4.0b2/bin/eui

atom marks = 'C'

switch marks with fallthru do
   case 'A' then
      puts(1, "Excellent!\n" )
      break
   
   case 'B', 'C' then
      puts(1, "Well done!\n" )
      break
   
   case 'D' then
      puts(1, "You passed!\n" )
      break
   
   case 'F' then
      puts(1, "Better try again!\n" )
      break
   
   case else
      puts(1, "Invalid grade!\n" )
      break
end switch

这将产生以下结果：

Well done!

switch....label 语句

switch 语句可以有一个可选的label 来命名 switch 块。此名称可用于嵌套的 switch break 语句中，以跳出封闭的 switch 而不是仅跳出拥有它的 switch。

switch 标签用于命名块，标签名称必须是包含一个或多个单词的双引号常量字符串。label 关键字区分大小写，应写为label。

语法

简单switch...label 语句的语法如下：

switch expression label "Label Name" do
   case <val> [, <val-1>....] then
      -- Executes when the expression matches one of the values
      break "LEBEL NAME" 
   
   case <val> [, <val-1>....] then
      -- Executes when the expression matches one of the values 
      break "LEBEL NAME"  
      .....................
   
   case else
      -- Executes when the expression does not matches any case.
      break "LEBEL NAME"   
end if

示例

#!/home/euphoria-4.0b2/bin/eui

atom marks = 'C'
atom scale = 'L'

switch marks label "MARKS" do
   case 'A' then
      puts(1, "Excellent!\n" )
   
   case 'B', 'C' then
      puts(1, "Well done!\n" )
      
      switch scale label "SCALE" do
         case 'U' then
             puts(1, "Upper scale!\n" )
             break "MARKS"
         
         case 'L' then
             puts(1, "Lower scale!\n" )
             break "MARKS"
         
         case else
             puts(1, "Invalid scale!\n" )
             break "MARKS"
      end switch
   
   case 'D' then
      puts(1, "You passed!\n" )
   
   case 'F' then
      puts(1, "Better try again!\n" )
   
   case else
      puts(1, "Invalid grade!\n" )
end switch

这将产生以下结果：

Well done!
Lower scale!

注意 - 如果您没有使用with fallthru 语句，则无需使用标签，因为 switch 语句会自动退出。