30-select.md

This Go code demonstrates the use of the select statement to handle multiple channel operations concurrently. Let's go through it with inline comments:

package main

import (
    "fmt"
    "time"
)

func main() {
    // Creating two unbuffered channels, 'c1' and 'c2'
    c1 := make(chan string)
    c2 := make(chan string)

    // Launching two goroutines to send messages to 'c1' and 'c2' after a specific time delay
    go func() {
        time.Sleep(1 * time.Second)
        c1 <- "one"
    }()
    go func() {
        time.Sleep(2 * time.Second)
        c2 <- "two"
    }()

    // Looping twice to handle messages from 'c1' and 'c2'
    for i := 0; i < 2; i++ {
        // The 'select' statement allows the program to wait on multiple communication operations
        // It will execute the first case that is ready, blocking the others
        select {
        case msg1 := <-c1:
            fmt.Println("received", msg1)
        case msg2 := <-c2:
            fmt.Println("received", msg2)
        }
    }
}

Output

received one
received two
Note that the total execution time is only ~2 seconds since both the 1 and 2 second Sleeps execute concurrently.

real    0m2.245s

Explanation:

1. package main: Indicates that this Go file belongs to the main executable package.

2. import (...): Imports necessary packages, including "fmt" for formatting and printing, and "time" for handling time-related operations.

3. func main() { ... }: The main function, where the execution of the program begins.

4. c1 := make(chan string): Creates an unbuffered channel named 'c1'.

5. c2 := make(chan string): Creates another unbuffered channel named 'c2'.

6. Two goroutines are launched using anonymous functions and the go keyword. These goroutines will send messages to 'c1' and 'c2' after specific time delays using time.Sleep.

7. The for loop runs twice to handle messages from both 'c1' and 'c2'.

8. select { ... }: The select statement allows the program to wait on multiple communication operations. It blocks until one of its cases can execute, at which point it will execute that case.

9. case msg1 := <-c1:: If a message is received from 'c1', it prints "received" along with the received message.

10. case msg2 := <-c2:: If a message is received from 'c2', it prints "received" along with the received message.

The use of select here allows the program to handle multiple channels concurrently, effectively waiting for the first one to send a message. The output will depend on which goroutine completes its work first.