Iterate Struct (map key) value in golang

Question

I've looked up Structs as keys in Golang maps

I understand iteration over the maps in golang has no guaranteed order. I've followed the example in golang blog, and tried using a struct as a map key.

Here's my code

package main

func main() {
    req := make(map[mapKey]string)

    req[mapKey{1, "r"}] = "robpike"
    req[mapKey{2, "gri"}] = "robert griesemer"
    req[mapKey{3, "adg"}] = "andrew gerrand"
    req[mapKey{4, "rsc"}] = "russ cox"

    var keys []int
    for k := range req {
        keys = append(keys, k.Key)
    }

    for _, k := range keys {
        fmt.Printf("short name : %s , long name : %s\n",req[k], req[k]) // How do I iterate here
    }

    sort.Ints(keys)
}

type mapKey struct {
    Key    int
    Option string
}

What I want the results to be is

short name : r , long name : rob pike
short name : gri , long name : robert griesemer
short name : adg , long name : andrew gerrand
short name : rsc , long name : russ cox

And I don't know how I can get the struct value and key iterated by separated data structure.

Oh I think I have found something similar [golang map prints out of order](http://stackoverflow.com/questions/12108215/golang-map-prints-out-of-order) — Ye Lin Aung, Feb 13 '15 at 08:54

score 3 · Accepted Answer · answered Feb 13 '15 at 09:19

Short version? You can't do it that way.

Long version you, you can use a custom sorter:

func main() {
    req := make(map[mapKey]string)

    req[mapKey{1, "r"}] = "robpike"
    req[mapKey{2, "gri"}] = "robert griesemer"
    req[mapKey{3, "adg"}] = "andrew gerrand"
    req[mapKey{4, "rsc"}] = "russ cox"

    var keys mapKeys
    for k := range req {
        keys = append(keys, k)
    }
    sort.Sort(keys)
    for _, k := range keys {
        fmt.Printf("short name : %s , long name : %s\n", k.Option, req[k])
    }
}

type mapKey struct {
    Key    int
    Option string
}

type mapKeys []mapKey

func (mk mapKeys) Len() int           { return len(mk) }
func (mk mapKeys) Swap(i, j int)      { mk[i], mk[j] = mk[j], mk[i] }
func (mk mapKeys) Less(i, j int) bool { return mk[i].Key < mk[j].Key }

play

Keep in mind that if your mapKey struct have a field that doesn't support equality (aka a struct or a slice) using req[k] won't work.

In that case you can switch to type mapKeys []*mapKey and map[*mapKey]string.

Iterate Struct (map key) value in golang

1 Answers1