convert interface{} directly to int in Golang, where interface stores a number as string

Question

I got a map[string]interface{} because decoding to JSON; with normal data, interface most be only a number but in type string, like this :

var a interface{}
a="3"

And then all data will be stored into a struct.

type someStruct struct {
   ID string
   Number1 int
   Number2 int
   Number3 int
   Number4 int
}

So I need to convert interface to int, but can´t do it easily and efficiently because only the code would be https://play.golang.org/p/oktbvTUbk93, pretty annoying and code does not seem to be readable if you take in mind the fact that I should handle all of possible errors

I would want to convert it directly to int, I´ve been searching for solutions but any of this convertions works as I want https://play.golang.org/p/Dw67U6kZwHC

In case you wondering why I don´t just decode it into struct directly, it´s because it´s dynamic data, the actual decoding occurs like this :

type dataIn struct {
   Code int         `json:"code"`
   ID   string      `json:"id"`
   Data interface{} `json:"data"`
}

And then I handle Data according to code and id, and they all are different data structures, so I can´t directly handle it with JSON

Use the [blank identifier](https://golang.org/ref/spec#Blank_identifier) to use values. For example, `_, _ = ok, err`. — peterSO, Mar 30 '18 at 07:15
If the problem space is wide your code will be wide too. No magic here. Do some programming. — Volker, Mar 30 '18 at 07:46
The json package can encode and decode numbers as strings: https://play.golang.org/p/MdrfqQjXLkw. See [this question](https://stackoverflow.com/questions/48907574/go-unmarshal-nested-unknown-fields/48909049) for how to deal with inconsistent fields. — Peter, Mar 30 '18 at 11:11
@peterSO If I do that it will continue to be inefficient, because there will be one variable assigment and then I can store it into destine — John Balvin Arias, Mar 30 '18 at 14:28
@Peter "In case you wondering why I don´t just decode it into struct directly, it´s because it´s dynamic data" — John Balvin Arias, Mar 30 '18 at 14:29
@Volker maybe, but I was looking opinions for a optimal solution — John Balvin Arias, Mar 30 '18 at 14:30

icza · Answer 1 · 2018-03-30T09:07:16.720

Create a helper function which does parsing and validating for you, in one place:

func parseInt(i interface{}) (int, error) {
    s, ok := i.(string)
    if !ok {
        return 0, errors.New("not string")
    }
    return strconv.Atoi(s)
}

And you can use this where needed. Here's a complete example, in which I also used another helper function, which takes care of the error handling:

m := map[string]interface{}{
    "number1": "1",
    "number2": "2",
    "number3": "3",
    "number4": "4",
    "ID":      "asdsa",
    "Title":   "asdas",
}

getInt := func(key string) int {
    n, err := parseInt(m[key])
    if err != nil {
        panic(err) // Decide what you wanna do with error
    }
    return n
}

// converting to struct
data := element{
    ID:      m["ID"].(string),
    Title:   m["Title"].(string),
    Number1: getInt("number1"),
    Number2: getInt("number2"),
    Number3: getInt("number3"),
    Number4: getInt("number4"),
}

fmt.Printf("%+v\n", data)

Output of the above (try it on the Go Playground):

{ID:asdsa Title:asdas Number1:1 Number2:2 Number3:3 Number4:4}

Also note that the open source package github.com/icza/dyno should help you handle dynamic objects at ease. (Disclosure: I'm the author.) It has a dyno.GetInteger() function for example, which is capable of extracting an int64 value out of multiple types (such as integers, floats, strings etc.).

your solution was pretty simple compared as I tried to do it by the way. Although this solve the actual question, there is a solution in order not to need to convert anything in first place — John Balvin Arias, Mar 30 '18 at 16:27

score 0 · Accepted Answer · answered Mar 30 '18 at 06:56

0

Sounds like what you need is a custom unmarshal json method on your main struct. First, unmarshal into your main struct to get your code and id, then use those in a switch statement to determine which struct to use for the rest of the data and unmarshal into that, stirring that struct in your Data field.

answered Mar 30 '18 at 06:56

darkliquid

4,003
1
25
17

For that scenario I would have to unmarshal twice, I think this is not possible, could you add an example please – John Balvin Arias Mar 30 '18 at 14:47
https://golang.org/pkg/encoding/json/#RawMessage is a good example of exactly this kind of behaviour (look at the decode example) – darkliquid Mar 30 '18 at 15:23
Also, this is the kind of thing I was talking about using a custom unmarshal method: https://play.golang.org/p/ZyKhnXQ-nFB – darkliquid Mar 30 '18 at 15:41
I didn´t know I could do that, also that custom marshal seems to be what I need because using rawMessage it would make my code bigger, Thanks! – John Balvin Arias Mar 30 '18 at 16:19

score 0 · Answer 3 · answered Mar 30 '18 at 08:14

I still didn't get the part where you stated the struct is generated dynamically. Anyways, you can have a struct method attached, that does the int conversion. If the Data field, which is of type interface{} is always gonna hold integers, you an try this:

type DataIn struct {
  Code int         `json:"code"`
  ID   string      `json:"id"`
  Data interface{} `json:"data"`
}

func (s DataIn) toInt() int {
   switch t := s.Data.(type)
   case int:
     i, _ := strings.Atoi(fmt.Sprintf("%v",s.Data))
     return i
}

// using it
sampleData := someStruct{
  Number1: datain.toInt(),
}

DataInt does not hold numbers, it holds map[string]interfarce{}, where inside interface the are numbers as strings — John Balvin Arias, Mar 30 '18 at 14:21

convert interface{} directly to int in Golang, where interface stores a number as string

3 Answers3