Golang : Count JSON objects and convert to slice/array

Problem :

You have an array of JSON objects and you want to count the number of JSON objects. You also want to convert the JSON objects into slice or array and access the data. How to do that?

NOTE : Useful in situation where you want to know in advance the total number of objects to create a list. Such as the JSON result return by Elastic Search and you want to paginate the result. The list also need some summary information to become something.... like a book index.

Solution :

First convert the JSON objects to type interface{} and then count the number with len() function. To access the data. you will have to convert the type interface{} to map[string]interface{}

Here is an example :

 package main

 import (
  "encoding/json"
  "fmt"
  "os"
 )

 var jsonStr = `[
 {
 "ID1": "Y",
 "ID2": 1888,
 "ID3": "F",
 "ID4": [
 {
 "Contact Zip": "12345",
 "Date of Contact": "08/14/2015"
 }
 ]
 },
 {
 "ID1": "Y",
 "ID2": 1889,
 "ID3": "M",
 "ID4": [
 {
 "Contact Zip": "12346",
 "Date of Contact": "08/13/2015"
 }
 ]
 }
 ]`

 func main() {
  var jsonObjs interface{}

  json.Unmarshal([]byte(jsonStr), &jsonObjs)

  // convert the json objects to slice/array of interface{}

  objSlice, ok := jsonObjs.([]interface{})

  if !ok {
 fmt.Println("cannot convert the JSON objects")
 os.Exit(1)
  }

  // count the number of JSON object
  fmt.Println("Number of JSON objects : ", len(objSlice))

  // iterate the JSON slice
  for _, obj := range objSlice {

 // convert each obj to map[string]interface{} from interface{}
 // because, interface{} alone does not support indexing

 objMap, ok := obj.(map[string]interface{})

 if !ok {
 fmt.Println("cannot convert interface{} to type map[string]interface{}")
 }
 fmt.Println(objMap)

 // now we can access the data with key index
 fmt.Println("ID4 : ", objMap["ID4"])
  }
 }

Output :

Number of JSON objects : 2

map[ID1:Y ID2:1888 ID3:F ID4:[map[Contact Zip:12345 Date of Contact:08/14/2015]]]

ID4 : [map[Date of Contact:08/14/2015 Contact Zip:12345]]

map[ID1:Y ID2:1889 ID3:M ID4:[map[Contact Zip:12346 Date of Contact:08/13/2015]]]

ID4 : [map[Contact Zip:12346 Date of Contact:08/13/2015]]

Play at http://play.golang.org/p/rmNCyyXz5C

Reference :

https://www.socketloop.com/tutorials/golang-decode-unmarshal-unknown-json-data-type-with-map-string-interface