json error, cannot unmarshal object into Go value

Question

I have this JSON data:

{
"InfoA" : [256,256,20000],
"InfoB" : [256,512,15000],
"InfoC" : [208,512,20000],
"DEFAULT" : [256,256,20000]
}

With JSON-to-Go, I get this Go type definition:

type AutoGenerated struct {
    InfoA   []int `json:"InfoA"`
    InfoB   []int `json:"InfoB"`
    InfoC   []int `json:"InfoC"`
    DEFAULT []int `json:"DEFAULT"`
}

With this code (play.golang.org)

package main

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

func main() {
    type paramsInfo struct {
        InfoA   []int `json:"InfoA"`
        InfoB   []int `json:"InfoB"`
        InfoC   []int `json:"InfoC"`
        DEFAULT []int `json:"DEFAULT"`
    }
    rawJSON := []byte(`{
"InfoA" : [256,256,20000],
"InfoB" : [256,512,15000],
"InfoC" : [208,512,20000],
"DEFAULT" : [256,256,20000]
}`)
    var params []paramsInfo
    err := json.Unmarshal(rawJSON, &params)
    if err != nil {
        fmt.Println(err.Error())
        os.Exit(1)
    }
}

I get error json: cannot unmarshal object into Go value of type []main.paramsInfo

I don't understand why. Can you help me?

What part of the error is confusing you? It tells you exactly the problem. — Jonathan Hall
– Jonathan Hall, Commented Nov 22, 2017 at 11:18

icza · Accepted Answer · 2017-11-22 11:17:24Z

9

The JSON source is a single object, yet you try to unmarshal it into a slice. Change the type of params to paramsInfo (non-slice):

var params paramsInfo
err := json.Unmarshal(rawJSON, &params)
if err != nil {
    fmt.Println(err.Error())
    os.Exit(1)
}
fmt.Printf("%+v", params)

And with that the output (try it on the Go Playground):

{InfoA:[256 256 20000] InfoB:[256 512 15000] InfoC:[208 512 20000]
    DEFAULT:[256 256 20000]}

answered Nov 22, 2017 at 11:17

icza

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3 Comments

LeMoussel Over a year ago

How can I do without explictly declaring "InfoA ", "InfoAB", etc in structs ? I implemented a solution (play.golang.org/p/3NoMXoyOCU), but I got empty result. I don't see how it might be implemented.

icza Over a year ago

@LeMoussel See this answer and this answer.

LeMoussel Over a year ago

Solution: params2 := make(map[string][]int) _= json.Unmarshal(rawJSON, &params2) fmt.Printf("\nparams2: %+v", params2)

Wojciech Kaczmarek · Accepted Answer · 2017-12-07 10:11:01Z

2

You are decoding a single JSON object but you are attempting to put it into the []paramsInfo slice.

It works fine when you decode JSON array of objects:

rawJSON := []byte(`[{
    "InfoA" : [256,256,20000],
    "InfoB" : [256,512,15000],
    "InfoC" : [208,512,20000],
    "DEFAULT" : [256,256,20000]
}]`)

(note the square brackets around your object)

By the way, in the if branch handling an error you don't need to call err.Error() to get the error string; fmt.Println(err) is sufficient and it's actually a Go idiom to use it like this. The implementation of fmt.Print* is taking care of handling the error type (contrary to print for example).

edited Dec 7, 2017 at 10:11

answered Nov 22, 2017 at 11:32

Wojciech Kaczmarek

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