range slice and map

range_slice_map.go

package main

import (
	"fmt"
	"sync"
)

func main() {
	testSlice()
	testMap()
}

func testSlice() {
	s := []int{1, 2, 3}
	once := sync.Once{}
	for i, v := range s {
		once.Do(func() {
			s = []int{}
		})
		fmt.Println(i, v)
	}

	s = []int{1, 2, 3}
	once = sync.Once{}
	for i, v := range s {
		once.Do(func() {
			s[0] = 10
			s[1] = 20
			s[2] = 30
		})
		fmt.Println(i, v)
	}

	s = []int{1, 2, 3}
	once = sync.Once{}
	for i := range s {
		once.Do(func() {
			s[0] = 10
			s[1] = 20
			s[2] = 30
		})
		fmt.Println(i, s[i])
	}
}

func testMap() {
	m := map[int]int{
		0: 0,
		1: 1,
		2: 2,
	}
	once := sync.Once{}
	for k, v := range m {
		once.Do(func() {
			m[3] = 3
		})
		// 不一定能打印出 3
		fmt.Println(k, v)
	}
}