go channel get data isn't as expected

Question

I was the new learning of go version 1.22, in the following code, for i := range ch1; if will throughout all the data in the ch1, if there is already have data, it will printed. but why my output is

0 1
2 3
4 5
6 7
8 9

there are some data missing, index 1, 3, 5, 7, 9 are not output, what is the problem, anyone can help me here? thx!

package main

import "fmt"

func main() {
    ch1 := make(chan int, 10)
    go func() {
        for i := 0; i < 10; i++ {
            ch1 <- i
        }
        close(ch1)
    }()
    for i := range ch1 {
        x, ok := <-ch1
        if !ok {
            break
        }
        fmt.Println(i, x)
    }
}

the output should be

0 0
1 1
2 2
3 3
4 4
5 5
6 6
7 7
8 8
9 9

Answer 1

You are reading from the channel twice per iteration:

1. for i := range ch1 reads a value from the channel.
2. x, ok := <-ch1 reads the next value from the channel.

You should decide which one you want to use. Don't mix them.

---

on 1) With iteration over a channel there is no index. A range over a channel will stop the iteration after all items are read from the channel and the channel is closed.

    for x := range ch1 {
        fmt.Println(x)
    }

on 2) In this case the ok variable indicates if the channel is empty and closed:

    for {
        x, ok := <-ch1
        if !ok {
            break
        }
        fmt.Println(x)
    }