Golang map used for concurrent read/write

main.go

package main

import (
	"fmt"
	"sync"
	"time"
)

var countryCities = map[string][]string{}
var mutex = &sync.RWMutex{}

func main() {
	countryCities = make(map[string][]string)
	countryCities["England"] = []string{"London", "Leeds", "Liverpool", "Bristol", "Cardiff"}
	go func() {
		for country := range countryCities {
			go func() {
				printCity(country)
			}()
		}
	}()
	go func() {
		for _, city := range []string{"Sheffield", "Glasgow", "Manchester", "Bradford", "Edinburgh"} {
			mutex.Lock()
			countryCities["England"] = append(countryCities["England"], city)
			mutex.Unlock()
			time.Sleep(time.Millisecond * 1000)
		}
	}()
	time.Sleep(time.Second * 6)
	fmt.Println(countryCities)
}

func printCity(country string) {
	var index int
	for {
		mutex.Lock()
		if len(countryCities[country])-1 < index {
			time.Sleep(2 * time.Second)
			delete(countryCities, country)
			mutex.Unlock()
			return
		}
		fmt.Println(countryCities[country][index])
		index++
		mutex.Unlock()
		time.Sleep(time.Millisecond * 300)
	}
}