为什么地图不返回其值？

Question

I have a function which simply returns the value of a map. However for some reasons it doesn't do that. Any idea why ? I've pasted here the code. You may also play it using

Try it!

package main

import "fmt"

func main() {    
    a := CountryCode("Australia")    
    fmt.Println(a)    
}

func CountryCode(s string) string {    
    m := make(map[string]string)
    // [snip]
    m["AU"] = "Australia"
    // [snip]
    return m[s]
}

func CodeByCountry(s string) string {
    m := make(map[string]string)
    // [snip]
    m["Australia"] = "AU"
    // [snip]
    return m[s]
}