Go language -select statement

Preface

        select yes go A choice statement provided in the language .select The syntax of is similar to switch sentence , It also belongs to the control statement .

What then? select The statement we didn't put in and if、switch Statement together ？

         because select It's cooperation channel The channel uses . Every case It has to be a communication operation , Either send or receive .

Feature summary

• select It can only be used for channel operation , Every case Must be a channel;
• If there are more than one case It is permissible to （channel There's no obstruction ）, Then select one at random case Statement execution ;
• without case Statement can execute （channel happening ）, Cut no default sentence , be select Statements block ;
• without case Statement can execute （channel happening ）, Yes default sentence , execute default sentence ;
• Generally, timeout statements are used instead of default sentence ;
• If case Statement channel by nil, Then ignore the branch , It's equivalent to from select The branch was deleted in ;
• If select Statements in for In circulation , Generally not used default sentence , Because it will cause CPU The problem of high occupancy .

select sentence

select Grammar format

select {
    case communication clause  :
       statement(s);      
    case communication clause  :
       statement(s);
    /*  You can define any number of  case */
    default : /*  Optional  */
       statement(s);
}

Example

select It can only be used for channel operation , Every case Must be a channel. And channel Is read / Writing is OK .

func main() {
	ch1 := make(chan int)
	ch2 := make(chan string)

	go func() {
		ch1 <- 100
	}()

	go func() {
		num2 := <-ch2
		fmt.Println(num2)
	}()

	select {
	case num1 := <-ch1: // Read channel data 
		fmt.Println("ch1 The data in is :", num1)
	case ch2 <- "201": //channel Write data 
		fmt.Println("ch2 There's data written in ")
	}
}

 

Randomness

If more than one case Statements can be executed , Then execute one randomly .

Example , Two article case Statements can be executed . Multiple executions can reveal ,case Statements are executed at random .

func main() {
	ch1 := make(chan int)
	ch2 := make(chan int)

	go func() {
		ch1 <- 100
	}()
	go func() {
		ch2 <- 200
	}()

	select {
	case num1 := <-ch1:
		fmt.Println("ch1 The data in is :", num1)
	case num2 := <-ch2:
		fmt.Println("ch2 The data in is :", num2)
	}
}

 

Deadlock

If all case Statements are blocked , Cut no default Statement and timeout statement , The program will report an error deadlock .

func main() {
	ch1 := make(chan int)
	ch2 := make(chan int)

	select {
	case <-ch1:
		fmt.Println("ch1")
	case <-ch2:
		fmt.Println("ch2")
	}
}

 

default sentence

If all case Statements are blocked , Yes default In words , Will execute default sentence .

Pay attention to if select Statements in for In circulation ,default Statements may cause CPU Occupy too much .

func main() {
	ch1 := make(chan int)
	ch2 := make(chan int)

	select {
	case num1 := <-ch1:
		fmt.Println("ch1 The data in is :", num1)
	case num2 := <-ch2:
		fmt.Println("ch2 The data in is :", num2)
	default:
		fmt.Println(" The channel is blocked ...default")
	}
}

 

Timeout statement

Generally, timeout statements are used instead of default sentence .

func main() {
	ch1 := make(chan int)
	ch2 := make(chan int)

	select {
	case num1 := <-ch1:
		fmt.Println("ch1 The data in is :", num1)
	case num2 := <-ch2:
		fmt.Println("ch2 The data in is :", num2)
	case <-time.After(3 * time.Second):
		fmt.Println("timeout...")
	}
}