Go parse JSON array of array

Question

I have data like this

"descriptionMap": [[[1,2], "a"], [[3,4], "b"]]

and I was trying to decode it with

  DescriptionMap []struct {
    OpcodeTableIdPair []int
    OpcodeDescription string   
  } `json:"descriptionMap"`

but I keep on getting empty arrays,

[[{[] } {[] }]]

Your structure will not match that JSON. In JSON you have array of arrays with array and string and in Go you have array of structures with array and string. On the second level there is a mismatch between array and structure. — Grzegorz Żur
– Grzegorz Żur, Commented Dec 9, 2016 at 9:47
@GrzegorzŻur I also tried DescriptionMap [][]struct to cater for that case but that won't work either — Shulhi Sapli
– Shulhi Sapli, Commented Dec 9, 2016 at 9:56
I believe there is no easy way of directly unmarshalling an array of mixed types ([num,num], "string"). You'll have to write a custom unmarshaller -- eagain.net/articles/go-json-array-to-struct — Nadh
– Nadh, Commented Dec 9, 2016 at 10:07

Ainar-G · Accepted Answer · 2016-12-09 10:12:22Z

3

You have a very unfortunate JSON schema which treats arrays as objects. The best you can do in this situation is something like this:

type Body struct {
    DescriptionMap []Description `json:"descriptionMap"`
}

type Description struct {
    IDPair      []int
    Description string
}

func (d *Description) UnmarshalJSON(b []byte) error {
    arr := []interface{}{}
    err := json.Unmarshal(b, &arr)
    if err != nil {
        return err
    }

    idPair := arr[0].([]interface{})
    d.IDPair = make([]int, len(idPair))
    for i := range idPair {
        d.IDPair[i] = int(idPair[i].(float64))
    }

    d.Description = arr[1].(string)

    return nil
}

Playground: https://play.golang.org/p/MPho12GJfc.

Notice though that this will panic if any of the types in the JSON don't match. You can create a better version which returns errors in such cases.

answered Dec 9, 2016 at 10:12

Ainar-G

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1 Comment

Shulhi Sapli Over a year ago

Thanks. This works really well. Unfortunately, I can't do anything with the JSON schema, I'm just consuming it.

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