# 04.6 switch语句

在本节介绍`switch`语句主要是因为`switch`可以用于正则表达式！首先看一段`switch`代码块：

> ```go
> switch asString {
>     case "1":
>         fmt.Println("One")
>     case "0":
>         fmt.Println("Zero")
>     default:
>         fmt.Println("Do not care!")
> }
> ```

这段代码能够区分不同的`asString`值所对应的不同操作。

> *`switch`代码块设置default子句是非常棒的实践。由于`switch`的`case`语句是依赖顺序的，所以`default`子句总是在最后声明。*

`switch`的使用还可以更加灵活：

> switch {
>
> case number < 0:
>
> ​ fmt.Println("Less than zero")
>
> case number > 0:
>
> ​ fmt.Println("Bigger than zero")
>
> default:
>
> ​ fmt.Println("zero")
>
> }

上面的代码块能够在某个数字正数、负数、以及0的情况下执行不同的任务。如你所见，`switch`的分支语句可以是条件语句，那么其分支语句同样也可以是正则表达式！

关于`switch`的用法将在`switch.go`中分5部分展示。

第一部分：

> ```go
> package main
>
> import (
>     "fmt"
>     "os"
>     "regexp"
>     "strconv"
> )
>
> func main() {
>     arguments := os.Args
>     if len(arguments) <2 {
>         fmt.Println("Usage: switch number")
>         os.Exit(1)
>     }
> ```

`regex`包用于生成正则表达。

第二部分代码：

> ```go
> number, err := strconv.Atoi(arguments[1])
>     if err != nil {
>         fmt.Println("The value is not an integer",number)
>     }else {
>         switch {
>         case number<0:
>             fmt.Println("Less than zero")
>         case number >0:
>             fmt.Println("Bigger than zero")
>         default:
>             fmt.Println("Zero")
>         }
>     }
> ```

第三部分代码：

> ```go
> asString := arguments[1]
>     switch  asString{
>     case "5":
>         fmt.Println("Five")
>     case "0":
>         fmt.Println("Zero")
>     default:
>         fmt.Println("Do not care")
>     }
> ```

这部分代码说明case`子句可以包含硬编码的变量，这种情况通常是`switch\`后跟有一个变量。

第四部分代码：

> ```go
> var negative = regexp.MustCompile(`-`)
>     var floatingPoint = regexp.MustCompile(`\d?\.\d`)
>     var mail = regexp.MustCompile(`^[^@]+@[^@.]+\.[^@.]+`)
>     switch {
>     case negative.MatchString(asString):
>         fmt.Println("Negative number")
>     case floatingPoint.MatchString(asString):
>         fmt.Println("Floating Point")
>     case mail.MatchString(asString):
>         fmt.Println("It is an email")
>         fallthrough
>     default:
>         fmt.Println("Something else")
>     }
> ```

这部分代码十分有趣。首先，我们定义了3个正则表达`negative`,`floatingPoint`,以及`mail`。然后使用`regexp.MatchString()`在`switch`中匹配不同的情况。

最后，`fallthrough`关键字告诉Go执行接下来的分支，即`default`分支。这意味着无论`mail.MatchString(asString)`是否成功匹配，`default`子句都会执行。

最后一部分：

> ```go
> var aType error = nil
>     switch aType.(type) {
>     case nil:
>         fmt.Println("It is a nil interface")
>     default:
>         fmt.Println("It it not a nil interface")
>
>     }
>
> }
> ```

这段代码说明`switch`能够区分不同类型，你将在第7章中了解到接口的知识。执行`switch.go`将会产生如下输出：

> ```
> $ go run switch.go
> Usage: switch number
> exit status 1
> hanshanjiedeMacBook-Pro:chapter4 hanshanjie$ go run switch.go mike@g.com
> The value is not an integer 0
> Do not care
> It is an email
> Something else
> It is a nil interface
> hanshanjiedeMacBook-Pro:chapter4 hanshanjie$ go run switch.go 5
> Bigger than zero
> Five
> Something else
> It is a nil interface
> hanshanjiedeMacBook-Pro:chapter4 hanshanjie$ go run switch.go 0
> Zero
> Zero
> Something else
> It is a nil interface
> hanshanjiedeMacBook-Pro:chapter4 hanshanjie$ go run switch.go 1.2
> The value is not an integer 0
> Do not care
> Floating Point
> It is a nil interface
> hanshanjiedeMacBook-Pro:chapter4 hanshanjie$ go run switch.go -1.5
> The value is not an integer 0
> Do not care
> Negative number
> It is a nil interface
> ```


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