structure with nested maps golang - struct

Question

Hi I'm new to go and was trying to figure out how maps work.
I have made up a little test program and can't seem to get it to work.
What I'm doing wrong?
package main
import (
"fmt"
)
type Stats struct {
cnt int
category map[string]Events
}
type Events struct {
cnt int
event map[string]Event
}
type Event struct {
value int64
}
func main() {
stats := new(Stats)
stats.cnt = 33
stats.category["aa"].cnt = 66
stats.category["aa"].event["bb"].value = 99
fmt.Println(stats.cnt, stats.category["aa"].event["bb"].value)
}

Answer 1

There are couple of issues with the code:
Map needs to be initialized using make function. Currently they are nil
Return value from map is non-addressable, this because if map is growing it needs to relocated which will cause memory address to change. Hence we need to extract value explicitly from map to a variable, update it and assigning it back.
Use pointer
I have updated the solution to show both updated it value returned and assigning it back and pointer.
http://play.golang.org/p/lv50AONXyU
package main
import (
"fmt"
)
type Stats struct {
cnt int
category map[string]Events
}
type Events struct {
cnt int
event map[string]*Event
}
type Event struct {
value int64
}
func main() {
stats := new(Stats)
stats.cnt = 33
stats.category = make(map[string]Events)
e, f := stats.category["aa"]
if !f {
e = Events{}
}
e.cnt = 66
e.event = make(map[string]*Event)
stats.category["aa"] = e
stats.category["aa"].event["bb"] = &Event{}
stats.category["aa"].event["bb"].value = 99
fmt.Println(stats)
fmt.Println(stats.cnt, stats.category["aa"].event["bb"].value)
}

Answer 2

Adding this as a different approach to the problem:
type Stats struct {
cnt int
categories map[string]*Events
}
func (s *Stats) Category(n string) (e *Events) {
if s.categories == nil {
s.categories = map[string]*Events{}
}
if e = s.categories[n]; e == nil {
e = &Events{}
s.categories[n] = e
}
return
}
type Events struct {
cnt int
events map[string]*Event
}
func (e *Events) Event(n string) (ev *Event) {
if e.events == nil {
e.events = map[string]*Event{}
}
if ev = e.events[n]; ev == nil {
ev = &Event{}
e.events[n] = ev
}
return
}
type Event struct {
value int64
}
func main() {
var stats Stats
stats.cnt = 33
stats.Category("aa").cnt = 66
stats.Category("aa").Event("bb").value = 99
fmt.Println(stats)
fmt.Println(stats.cnt, stats.Category("aa").Event("bb").value)
}
playground

Answer 3

There are a few issues with your approach.
You aren't initializing you maps. You need to create them first.
Maps return copies of their values. So when you pull out "aa" and modify it, you are getting a copy of "aa", changing it, then throwing it away. You need to put it back in the map, or use pointers.
Here's a working example (non-pointer version) on Play.
Notice the construction of the maps, and the re-assignment back to the map when modifying a value.
package main
import (
"fmt"
)
type Stats struct {
cnt int
category map[string]Events
}
type Events struct {
cnt int
event map[string]Event
}
type Event struct {
value int64
}
func main() {
stats := &Stats{category: map[string]Events{}}
stats.cnt = 33
tmpCat, ok := stats.category["aa"]
if !ok {
tmpCat = Events{event: map[string]Event{}}
}
tmpCat.cnt = 66
tmpEv := tmpCat.event["bb"]
tmpEv.value = 99
tmpCat.event["bb"] = tmpEv
stats.category["aa"] = tmpCat
fmt.Println(stats.cnt, stats.category["aa"].event["bb"].value)
}