Length of channel is zero after adding element

Question

I learn buffered channels in Go and some magic is hidden from me. I have this code

package main

import (
    "fmt"
)

func write(ch chan int) {
    for i := 1; i <= 5; i++ {
        ch <- i
        fmt.Printf("Channel's length is %d\n", len(ch))
    }
    close(ch)
}
func main() {
    ch := make(chan int, 2)
    go write(ch)

    for v := range ch {
        fmt.Println(v)
    }
}

The output is

Channel's length is 0
Channel's length is 1
Channel's length is 2
1
2
3
4
Channel's length is 2
Channel's length is 0
5

Why is the channel's length on the first iteration in write goroutine is zero? What do I don't know?

Answer 1

As per the concept of GO buffer, you can push elements in channels as per defined buffer size (in your case it is 2). But at the time an element is pushed into channel, same is being read by the main GO routine which leads to a decrease in the size of channel to zero. So, if you put some time.Sleep before your read statement you will get the expected result.

`

package main
import (
    "fmt"
    "time"
)

func write(ch chan int) {
    for i := 1; i <= 5; i++ {
        ch <- i
        fmt.Printf("Channel's length is %d\n", len(ch))
    }
    close(ch)
}
func main() {
    ch := make(chan int, 2)
    go write(ch)
    time.Sleep(2*time.Second)
    for v := range ch {
        fmt.Println(v)
        time.Sleep(2*time.Second)
    }
}`

Output for the above code is:

Channel's length is 1

Channel's length is 2

1

Channel's length is 2

2

Channel's length is 2

3

Channel's length is 2

4

5