range

for ... range clause can be used to iterate 5 types of variables: array, slice, string, map and channel, and the following sheet gives a summary of the items of for ... range loops:

For array, slice, string and map, if you don't care about the second item, you can omit it. E.g.:

package main

import "fmt"

func main() {
    m := map[string]struct{} {
        "alpha": struct{}{},
        "beta": struct{}{},
    }
    for k := range m {
        fmt.Println(k)
    }
}

The running result is like this:

alpha
beta

Likewise, if the program doesn't need the first item, a blank identifier should occupy the position:

package main

import "fmt"

func main() {
    for _, v := range []int{1, 2, 3} {
        fmt.Println(v)
    }
}

The output is:

1
2
3

For channel type, the close operation can cause for ... range loop exit. See the following code:

package main

import "fmt"

func main() {
    ch := make(chan int)
    go func(chan int) {
        for _, v := range []int{1, 2} {
            ch <- v
        }
        close(ch)
    }(ch)

    for v := range ch {
        fmt.Println(v)
    }
    fmt.Println("The channel is closed.")
}

Check the outcome:

1
2
The channel is closed.

We can see close(ch) statement in another goroutine make the loop in main goroutine end.