Adding elements to JSON dynamically from a Go struct

March 17, 2023

The output that I want to get is in the format:

{"data": [ [ 0, [ "Brendan", "Fraser" ] ] , [ 1, [ "Keanu", "Reeves" ] ] ] }

To do this, I have defined the following struct:

type actors struct {
    Data [][]interface{} `json:"data"`
    }

The individual values Brendan , Fraser and Keanu, Reeves are coming from another data struct that is accessed in the format rec.records[0].value .

That is:

rec.records[0].value would correspond to Brendan

rec.records[1].value would correspond to Fraser and so on.

To assign values to this struct I am doing:

numberOfActCnt := 0
numberOfColParsed := 0
numberOfColumns := 3

var act actors
var res []actors

    for i := 0; i < 2; i++ {
        numberOfActCnt = numberOfActCnt + numberOfColParsed
        for j := 0; j < 1; j++ {
            act = actors{[][]interface{}{{i, []string{rec.records[numberOfActCnt].value, rec.records[numberOfActCnt+1].value}}}}
            re, _ := json.Marshal(act)
            fmt.Println(string(re))
            numberOfColParsed = numberOfColumns - 1
        }
    }

This gives me the output:

{"data":[[0,["Brendan","Fraser"]]]}
{"data":[[1,["Keanu","Reeves"]]]}

How do I get the output in the format? :

{"data": [ [ 0, [ "Brendan", "Fraser" ] ] , [ 1, [ "Keanu", "Reeves" ] ] ] }

I think my struct is defined correctly, however how do I append values to the same json?

>Solution :

First, loop over all the records and store them inside the Data [][]any slice using append and then, after the loop, marshal the result.

numberOfActCnt := 0
numberOfColParsed := 0
numberOfColumns := 3

var res struct { Data [][]any `json:"data"` }
for i := 0; i < 2; i++ {
    numberOfActCnt = numberOfActCnt + numberOfColParsed
    for j := 0; j < 1; j++ {
        res.Data = append(res.Data, []any{i, []string{rec.records[numberOfActCnt].value, rec.records[numberOfActCnt+1].value}})
        numberOfColParsed = numberOfColumns - 1
    }
}
re, err := json.Marshal(res)
if err != nil {
    panic(err)
}
fmt.Println(string(re))

Note: any is an alias for interface{} since Go1.18, if you are on an older version of Go then you’ll need to keep using interface{}.