structure with nested maps golang

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:

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)
}

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