structure with nested maps golang

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)
}

There are couple of issues with the code:

1. Map needs to be initialized using make function. Currently they are nil

2. 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.

3. 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)
}

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)
}

<kbd>playground</kbd>

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)
}