Golang - using/iterating through JSON parsed map

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?

possible duplicate of Unmarshal json error even a matching data structure is provided — Ainar-G
– Ainar-G, Commented Jun 17, 2015 at 15:39

SirDarius · Accepted Answer · 2015-06-17 15:36:49Z

2

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.

answered Jun 17, 2015 at 15:36

SirDarius

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2 Comments

DroidOS Over a year ago

I am not quite sure why JSON parsing is so much more complicated in Go when compared with PHP and Node but I imagine there is a reason somewhere.

SirDarius Over a year ago

For starters, Go is a compiled language with static typing, as opposed to PHP and Javascript. On the other hand, try to do the same in C, and then you'll find that Go isn't that hard after all :)

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