Goroutine导致for循环中的可互换动作

Question

I'm very new to go and I'm not sure why this code has this output. I understand that sleep will cause the new goroutine to start the other thread for the specified amount of time. I'm trying to sequentially map out the logic and it looks like "world" should always print before "hello".

package main

import (
    "fmt"
    "time"
)

func say(s string) {
    for i := 0; i < 5; i++ {
        time.Sleep(1 * time.Millisecond)
        fmt.Println(s, i)
    }
}

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

Actual Output:

world 0
hello 0
hello 1
world 1
world 2
hello 2
hello 3
world 3
world 4
hello 4

Expected Output:

world 0
hello 0
world 1
hello 1
world 2
hello 2

...etc

Answer 1

I understand that sleep will cause the new goroutine to start the other thread for the specified amount of time

This is partly incorrect!

This would be correct on a machine with only one core where only one thread can be executed at a time.

On a machine with multiple cores go can execute as many goroutines parallel as there are cores. With parallel executed goroutines there is no guarantee at all, what will be executed before or after.