CODEWITHSUNDEEP
for-loopgoconcurrency
Pobe
Pobe

Reputation: 2793

for range vs static channel length golang

I have a channel taking events parsed from a log file and another one which is used for synchronization. There were 8 events for the purpose of my test.

When using the for range syntax, I get 4 events. When using the known number (8), I can get all of them.

func TestParserManyOpinit(t *testing.T) {
    ch := make(chan event.Event, 1000)
    done := make(chan bool)
    go parser.Parse("./test_data/many_opinit", ch, done)
    count := 0
    exp := 8
    evtList := []event.Event{}

    <-done
    close(ch)
    //This gets all the events
    for i := 0; i < 8; i++ {
            evtList = append(evtList, <-ch)
            count++
    }

    //This only gives me four
    //for range ch {
    //        evtList = append(evtList, <-ch)
    //        count++
    //}

    if count != exp || count != len(evtList) {
            t.Errorf("Not proper lenght, got %d, exp %d, evtList %d", count, exp, len(evtList))
    }

func Parse(filePath string, evtChan chan event.Event, done chan bool) {
    log.Info(fmt.Sprintf("(thread) Parsing file %s", filePath))
    file, err := os.Open(filePath)
    defer file.Close()

    if err != nil {
            log.Error("Cannot read file " + filePath)
    }
    count := 0
    scan := bufio.NewScanner(file)
    scan.Split(splitFunc)
    scan.Scan() //Skip log file header

    for scan.Scan() {
            text := scan.Text()
            text = strings.Trim(text, "\n")
            splitEvt := strings.Split(text, "\n")
            // Some parsing ...
            count++
            evtChan <- evt
    }

    fmt.Println("Done ", count) // gives 8
    done <- true
}

I must be missing something related to for loops on a channel.

I've tried adding a time.Sleep just before the done <- true part. It didn't change the result.

Upvotes: 0

Views: 527

Answers (1)

Adrian
Adrian

Reputation: 46562

When you use for range, each loop iteration reads from the channel, and you're not using the read value. Hence, half the values are discarded. It should be:

for ev := range ch {
        evtList = append(evtList, ev)
        count++
}

In order to actually utilize the values read in the loop iterator.

Ranging over channels is demonstrated in the Tour of Go and detailed in the Go spec.

Upvotes: 2

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