Go - The Select Statement

Go select statement is useful to handle multiple channel operations and selects the ready channel against multiple channels.

The Select Statement

When you are working with multiple channels in Go language, the select statement waits on multiple channel operations and executes the code block for the channel that is ready.

Syntax

The syntax for a select statement in Go programming language is as follows −

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

Rules for Using a Select Statement in Go

The following rules apply to a select statement −

• You can have any number of case statements within a select. Each case is followed by the value to be compared to and a colon.
• The type for a case must be the a communication channel operation.
• When the channel operation occured the statements following that case will execute. No break is needed in the case statement.
• A select statement can have an optional default case, which must appear at the end of the select. The default case can be used for performing a task when none of the cases is true. No break is needed in the default case.

Example of Select Statement

The following example demonstrates how you can use the select statement to handle multiple channels:

package main

import "fmt"

func main() {
   var c1, c2, c3 chan int
   var i1, i2 int
   select {
      case i1 = <-c1:
         fmt.Printf("received ", i1, " from c1\n")
      case c2 <- i2:
         fmt.Printf("sent ", i2, " to c2\n")
      case i3, ok := (<-c3):  // same as: i3, ok := <-c3
         if ok {
            fmt.Printf("received ", i3, " from c3\n")
         } else {
            fmt.Printf("c3 is closed\n")
         }
      default:
         fmt.Printf("no communication\n")
   }    
}   

When the above code is compiled and executed, it produces the following result −

no communication

Nested select Statement

A nested select statement allows you to use one or more select statements inside a select statement.

Example

The following is an example of a nested select statement in Go language:

package main

import (
	"fmt"
	"time"
)

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

	// Goroutine to send data to channel1
	go func() {
		time.Sleep(1 * time.Second)
		ch1 <- "I am in channel 1"
	}()

	// Goroutine to send data to channel2
	go func() {
		time.Sleep(3 * time.Second)
		ch2 <- 10
	}()

	// Nested select statement
	select {
	case ch_data1 := <-ch1:
		fmt.Println("Received from channel1:", ch_data1)
		select {
		case ch_data2 := <-ch2:
			fmt.Println("Received from channel2:", ch_data2)
		default:
			fmt.Println("No data in channel2 yet")
		}
	case ch_data2 := <-ch2:
		fmt.Println("Received from channel2:", ch_data2)
	default:
		fmt.Println("No messages received yet from any channel")
	}
}

When the above code is compiled and executed, it produces the following result −

No messages received yet from any channel