Difference Ways to Read Channels

Question

I am curious of why different ways of reading values from a channel result in different behaviors. Provided the code:

mychan := make(chan int)

go func() {
    i := 0
    for {
        mychan <- i
        <-time.After(time.Second * 1)
        i++
    }
}()

The goroutine "streams" a sequence of integer infinitely to a mychan channel. After this code, if you read directly with <-mychan like so:

fmt.Println(<-mychan)

This prints "0" as expected. If you keep repeating, it keeps reading:

fmt.Println(<-mychan)    // 1
fmt.Println(<-mychan)    // 2
//...

However, using the looping mechanism, it blocks infinitely.

for i := range mychan {
    fmt.Println(i)
}

That means that this mechanism can only reads from a closed channel, right? However, with select method, thing gets wierder:

for i:=0; i<=10;i++ {
    select {
    case <-mychan:
        fmt.Println(<-mychan)
    }
}

Now it prints alternately, like 1, 3, 5, 9, ... every 2s as if select was switching between mychan and some other invisible channel. Adding another case makes the case (no pun intended):

for i:=0; i<=10;i++ {
    select {
    case <-time.After(1 * time.Second):
        fmt.Println("foo")
    case <-mychan:
        fmt.Println(<-mychan)
    }
}

// now prints 1, foo, 3, foo, 5, foo, ... every 1s

As trivial the question as it may seems to some of you, I'd appreciate if anyone can explain and enlighten me.

Answer 1

The behavior from

for i := range mychan {
    fmt.Println(i)
}

Appears to result from running examples on go playground When I ran the code locally the program prints out one value every second indefinitely. It would be cool if there is some code analysis tool they are using on the server to figure out if the code will run forever.

As Volker points out your second example is reading the channel twice per print statement. You can fix this with

for i:=0; i<=10;i++ {
    select {
    case <-time.After(1 * time.Second):
        fmt.Println("foo")
    case x, open := <-mychan:
        if !open { return }
        fmt.Println(x)
    }
}