go, trouble iterating over array

Question

I can't access the values in an array somehow. Here is how the array is declared:

messages := strings.Split(request_string, end_of_message_terminator)

But when I make a loop over this array, I get empty strings as values. That's the loop:

for i, v := range messages {
    fmt.Printf("messages are (3) %q\n", messages)
    go func(){

        fmt.Printf("message is %s\n", messages[i])
        fmt.Printf("i is %s\n", i)
        fmt.Printf("v is %s\n", v)
        respond_to_message(messages[i], response_writer())
    }()


}

And that's the output:

messages are (3) ["asti" ""]
messages are (3) ["asti" ""]
message is                  
i is %!s(int=1)             
v is                        
message is                  
i is %!s(int=1)             
v is

Does anyone know what the problem is? Why can I access the individual values in this slice?

Just in case you want to run this code, here is the entire program:

package main


import (
    "fmt"
    "net"
    "os"
    "strings"
//  "io/ioutil"


)


func main() {

    end_of_message_terminator := "||"
    beginning_of_next_message := ""
    request := make([]byte, 512)

    service_port := ":7777"
    tcpAddr, err := net.ResolveTCPAddr("tcp4", service_port)
    checkError(err)
    listener, err := net.ListenTCP("tcp", tcpAddr)
    checkError(err)



    for {
        conn, err := listener.Accept()

        if err != nil {

            continue

        }

        read_len, err := conn.Read(request)

        if read_len == 0 {
            continue
        }

        request_string := string(request[:read_len])
        fmt.Printf("Request String '%s'\n", request_string)

        messages := strings.Split(request_string, end_of_message_terminator)
        fmt.Printf("messages are (1) %q\n", messages)

        messages[0] = beginning_of_next_message + messages[0]

        if messages[len(messages) - 1] != "" {
            beginning_of_next_message = messages[len(messages) - 1]
            messages[len(messages) - 1] = ""

        }

        if len(messages) == 1 {
            continue
        }

        fmt.Printf("messages are (2) %q\n", messages)
        for i, v := range messages {
            fmt.Printf("messages are (3) %q\n", messages)
            go func(){

                fmt.Printf("message is %s\n", messages[i])
                fmt.Printf("i is %s\n", i)
                fmt.Printf("v is %s\n", v)

            }()


        }
        conn.Close()

    }

}






func checkError(err error) {

    if err != nil {

        fmt.Fprintf(os.Stderr, "Fatal error: %s", err.Error())
        os.Exit(1)

    }

}

You need to run this program first:

netcat -l -p 8000

And then, finally (after starting both this program and running netcat -l -p 8000) you need to run:

printf "asti||"  | netcat localhost 7777

github.com/golang/go/wiki/CommonMistakes

squiguy
– squiguy

2016-07-14 17:25:25 +00:00
Commented Jul 14, 2016 at 17:25 — squiguy
– squiguy, Commented Jul 14, 2016 at 17:25
golang.org/doc/faq#closures_and_goroutines

Mr_Pink
– Mr_Pink

2016-07-14 19:21:58 +00:00
Commented Jul 14, 2016 at 19:21 — Mr_Pink
– Mr_Pink, Commented Jul 14, 2016 at 19:21

abhink · Accepted Answer · 2016-07-14 17:27:11Z

2

In your goroutine you are not binding i properly:

for i, v := range messages {
    fmt.Printf("messages are (3) %q\n", messages)
    go func(i int){

        fmt.Printf("message is %s\n", messages[i])
        fmt.Printf("i is %s\n", i)
        fmt.Printf("v is %s\n", v)
        respond_to_message(messages[i], response_writer())
    }(i) // you'll have to pass i here
}

You don't see any output because for each the value of i gets set to len(messages) - 1 which in this case is 2 - 1 = 1 and the value messages[1] == ""

So each goroutine is printing messages[1] which is an empty string.

answered Jul 14, 2016 at 17:27

abhink

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1 Comment

Adrian Over a year ago

Right answer but wrong reason - it's not that i is set to len(messages) - 1. It's because when you queue up all the goroutines closed over i, if they don't run until the loop finishes, then i still the value from the last iteration of the loop. If you removed the go (so that the closures ran immediately) you'd find the expected behavior, but no concurrency.

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