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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)
}
831
Golang Create Nested Map We can define a nested map the same way we declare a normal map. We start by setting the data type of the key (top-level map) and the type of the value. Since this is a nested map, the value of the top-level map is a map. The previous code creates a simple restaurant menu using nested maps.
You can iterate through a map in Golang using the for... range statement where it fetches the index and its corresponding value. In the code above, we defined a map storing the details of a bookstore with type string as its key and type int as its value. We then looped through its keys and values using the for..
Nested Maps are those in which values in key:value pairs of the outer map are also maps. In other words maps inside a map, hence nested maps.
In Go programming, a structure or struct is a user-defined type to store a collection of different fields into a single field. For example, suppose you have a player and want to store his name and age. You can create two variables, name, and age, to store the values.
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)
}
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
26
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