Why the map doesn't return its value?

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]
}

Remember to create a minimal test-case. Almost none of that code is relevant. — user2864740
– user2864740, Commented Aug 13, 2014 at 7:22

Denys Séguret · Accepted Answer · 2014-08-13 13:25:05Z

9

You're not using the right function, the one using the map whose key is a name. You probably want

a := CodeByCountry("Australia")

This works.

But it makes no sense to create the map each time you need it. Take the map creation out of the functions, and put it for example in the init :

package main

import "fmt"

var byCode = make(map[string]string)
var byName = make(map[string]string)

func init() {
    m := byCode
    m["AF"] = "Afghanistan"
    m["AL"] = "Albania"
    m["DZ"] = "Algeria"
    m["AS"] = "American Samoa"

    m = byName
    m["Austria"] = "AT"
    m["Mozambique"] = "MZ"
    m["Solomon Islands"] = "SB"
    m["United States"] = "US"
    m["Anguilla"] = "AI"
    m["Australia"] = "AU"
}

func CountryCode(s string) string {
    return byCode[s]
}

func CodeByCountry(s string) string {
    return byName[s]
}

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

Another solution for the initialization, as it seems bijective, would be to have one function adding a pair and filling both maps :

func addInMaps(code,name string) {
     byCode[code] = name
     byName[name] = code
}

edited Aug 13, 2014 at 13:25

answered Aug 13, 2014 at 7:22

Denys Séguret

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4 Comments

hey Over a year ago

Indeed.. I figured it out just after I posted the question. I pasted all the code because obviously I couldn't reproduce the issue with a small map.

hey Over a year ago

I'm not sure how to move the map creation into an init function. If I do that I get an "undefined" error when I assign values to the map

OneOfOne Over a year ago

@hey if your data is static (aka you don't have to load it from a database or something, you can skip init all together and use something like this: play.golang.org/p/ooli1FRDcQ

OneOfOne Over a year ago

Another approach to only generate one of the maps and use it to populate the other : play.golang.org/p/F3vICTHVRN

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