“ Tour of Go”中的并发

I'm going through 'A Tour of Go' and have been editing most of the lessons to make sure I fully understand them. I have a question regarding: https://tour.golang.org/concurrency/1

package main

import (
    "fmt"
    "time"
)

func say(s string) {
    for i := 0; i < 5; i++ {
        time.Sleep(100 * time.Millisecond)
        fmt.Println(s)
    }
}

func main() {
    go say("world")
    say("hello")
}

Leaving main the way it is produces a random ordering of hellos and worlds because the threads are executing in different orders each time the program runs. I have two questions:

1. If I remove go from the line with world and add it to the line with hello, world is printed 5 times and hello is not printed at all. Why is that?
2. If I add go in front of both lines, nothing is printed at all. Why is that?

I have some experience with concurrency with C++ (although it was a while ago) and some more recent experience with Python, but would describe my overall experience with concurrency fairly novice-level.

Thanks!

The program terminates before you get a chance to see the results.

You can fix this by adding a statement that ensures main doesn't exit before the other routines are finished.

From Golang - Concurrency:

With a goroutine we return immediately to the next line and don't wait for the function to complete.

They give a code example:

package main

import "fmt"

func f(n int) {
  for i := 0; i < 10; i++ {
    fmt.Println(n, ":", i)
  }
}

func main() {
  go f(0)
  var input string
  fmt.Scanln(&input)
}

In regards to the code above:

This is why the call to the Scanln function has been included; without it the program would exit before being given the opportunity to print all the numbers.