Golang - using/iterating through JSON parsed map - json

Question

With PHP and JavaScript (and Node) parsing JSON is a very trivial operation. From the looks of it Go is rather more complicated. Consider the example below:
package main
import ("encoding/json";"fmt")
type fileData struct{
tn string
size int
}
type jMapA map[string] string
type jMapB map[string] fileData
func parseMapA(){
var dat jMapA
s := `{"lang":"Node","compiled":"N","fast":"maybe"}`
if err := json.Unmarshal([]byte(s), &dat); err != nil {
panic(err)
}
fmt.Println(dat);
for k,v := range dat{
fmt.Println(k,v)
}
}
func parseMapB(){
var dat jMapB
s := `{"f1":{"tn":"F1","size":1024},"f2":{"tn":"F2","size":2048}}`
if err := json.Unmarshal([]byte(s), &dat); err != nil {
panic(err)
}
fmt.Println(dat);
for k,v := range dat{
fmt.Println(k,v)
}
}
func main() {
parseMapA()
parseMapB()
}
The parseMapA() call obligingly returns:
map[lang:Node Compiled:N fast:maybe]
lang Node
compiled N
fast maybe
However, parseMapB() returns:
map[f1:{ 0}, f2:{ 0}]
f2 { 0}
f1 { 0}
I am into my first few hours with Go so I imagine I am doing something wrong here. However, I have no idea what that might be. More generally, what would the Go equivalent of the Node code
for(p in obj){
doSomethingWith(obj[p]);
}
be in Go?

Answer 1

In Go, unmarshaling works only if a struct has exported fields, ie. fields that begin with a capital letter.
Change your first structure to:
type fileData struct{
Tn string
Size int
}
See http://play.golang.org/p/qO3U7ZNrNs for a fixed example.
Moreover, if you intend to marshal this struct back to JSON, you'll notice that the resulting JSON use capitalized fields, which is not what you want.
You need to add field tags:
type fileData struct{
Tn string `json:"tn"`
Size int `json:"size"`
}
so the Marshal function will use the correct names.