标题：[实践OK]golang随机数生成踩过的坑记录一下，在学习go的if和switch的变量作用域时遇到的。
出处：向东博客 专注WEB应用 构架之美 --- 构架之美，在于尽态极妍 | 应用之美，在于药到病除
时间：Sun, 06 Mar 2022 22:36:51 +0000
作者：jackxiang
地址：https://jackxiang.com/post/11243/

内容：
cat short-if.go 

package main
import(
        "fmt"
        "math/rand"
        "time"
)
func main(){
        rand.Seed(time.Now().Unix())
        if num := rand.Intn(3); num == 0 {
                fmt.Println(num)
                fmt.Println("Space Adventures")
        } else if num == 1 {
                fmt.Println(num)
                fmt.Println("SpaceX")
        }else{
                fmt.Println(num)
                fmt.Println("Virgin Galactic")
        }

}


go build short-if.go   
./short-if 
1
SpaceX

./short-if 
2
Virgin Galactic

./short-if 
0
Space Adventures


如果不加：
        rand.Seed(time.Now().Unix())

其值一直是：2

原因是：https://blog.csdn.net/weixin_33816821/article/details/94581126


Go语言的switch语法学习之num的简短声明也可用于switch语句的一部分：

package main
import(
        "fmt"
        "math/rand"
        "time"
)
func main(){
        rand.Seed(time.Now().Unix())
        switch num := rand.Intn(10); num {
        case 0:
                fmt.Println("Space Adventure")
        case 1:
                fmt.Println("SpaceX")
        case 2:
                fmt.Println("Virgin Galactic")
        default:
                fmt.Println("Random spaceline #",num)
        }
}



./short-switch 
Virgin Galactic

./short-switch  
Random spaceline # 4


cat scope-rules.go   #几月有多少天的一个判断：

package main
import(
        "fmt"
        "math/rand"
        "time"
)
var era = "AD"
func main(){
        year := 2018
        rand.Seed(time.Now().Unix())
        switch month := rand.Intn(12) + 1;month {
        case 2:
            day  := rand.Intn(28) + 1
            fmt.Println(era, year, month, day)
        case 4, 6, 9, 11:
            day := rand.Intn(30) + 1
            fmt.Println(era, year, month, day)
        default:
            day := rand.Intn(31) + 1
            fmt.Println(era, year, month, day)
        }
}




./scope-rules 
AD 2018 10 24


        default:
            day := rand.Intn(31) + 1
            fmt.Println(era, year, month, day)
        }
        fmt.Println(month,day)  //作用域不再有了。
go build scope-rules.go      
# command-line-arguments
./scope-rules.go:22:14: undefined: month
./scope-rules.go:22:20: undefined: day


包括switch的每个case也是都拥有自己独立的作用域，每个分支里面的day变量在分支结束后，day变量将不再处于作用域之内，switch分支的作用域是唯一一种无需使用大括号标识的作用域。


cat random-date.go

package main
import(
        "fmt"
        "math/rand"
        "time"
)
var era = "AD"
func main(){
        rand.Seed(time.Now().Unix())
        year := 2018
        month := rand.Intn(12) + 1
        daysInMonth := 31
        switch month { 
        case 2:
            daysInMonth = 28
        case 4, 6, 9, 11:
            daysInMonth = 30
        }
        day := rand.Intn(daysInMonth) + 1
        fmt.Println(era, year, month, day)
}




#./random-date 
AD 2018 11 30

#./random-date 
AD 2018 11 30


研究下闰年：
cat random-date-study.go

package main
import(
        "fmt"
        "math/rand"
        "time"
)
var era = "AD"
func main(){
        rand.Seed(time.Now().Unix())
        year := 2018 + rand.Intn(5)
        //year := 2000
        leep := year%400 == 0 || (year%4 == 0 && year%100 != 0)  //是否是闰年Boole
        month := rand.Intn(12) + 1
        daysInMonth := 31
        switch month { 
        case 2:
            if leep{
               daysInMonth = 29
            } else {
               daysInMonth = 28
            }
        case 4, 6, 9, 11:
            daysInMonth = 30
        }
        for i := 0;i < 10 ;i++ {
          day := rand.Intn(daysInMonth) + 1
          fmt.Println(era, year, month, day)
        }
}


./random-date-study
AD 2000 2 18
AD 2000 2 4
AD 2000 2 6
AD 2000 2 8
AD 2000 2 7
AD 2000 2 21
AD 2000 2 7
AD 2000 2 29
AD 2000 2 12
AD 2000 2 12


Generated by Jackxiang's Bo-blog 2.1.1 Release