dqwr32867 2016-02-29 18:10
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I'm reading data structures from JSON. There's a little bit of conversions going on and at the end I have a struct where one of the fields is of type interface{}. It's actually a map, so JSON puts it inside a map[string]inteface{}.

I actually know that the underlying structure is map[string]float64 and I would like to use it like that, so I try to do an assertion. The following code reproduces the behaviour:

type T interface{}

func jsonMap() T {
    result := map[string]interface{}{
        "test": 1.2,
    }
    return T(result)
}

func main() {
    res := jsonMap()

    myMap := res.(map[string]float64)

    fmt.Println(myMap)
}

I get the error:

panic: interface conversion: main.T is map[string]interface {}, not map[string]float64

I can do the following:

func main() {
    // A first assertion
    res := jsonMap().(map[string]interface{})

    myMap := map[string]float64{
        "test": res["test"].(float64), // A second assertion
    }

    fmt.Println(myMap)
}

This works fine, but I find it very ugly since I need to reconstruct the whole map and use two assertions. Is there a correct way to force the first assertion to drop the interface{} and use float64? In other words, what is the correct way to do the original assertion .(map[string]float64)?

Edit:

The actual data I'm parsing looks like this:

[
 {"Type":"pos",
 "Content":{"x":0.5 , y: 0.3}} ,

{"Type":"vel",
"Content":{"vx": 0.1, "vy": -0.2}}
]

In Go I use a struct and encoding/json in the following way.

type data struct {
    Type string
    Content interface{}
}

// I read the JSON from a WebSocket connection
_, event, _ := c.ws.ReadMessage()

j := make([]data,0)
json.Unmarshal(event, &j)
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1条回答 默认 最新

  • doubo3384 2016-03-01 09:51
    关注

    You cannot type assert map[string]interface{} to map[string]float64. You need to manually create new map.

    package main
    
    import (
        "encoding/json"
        "fmt"
    )
    
    var exampleResponseData = `{
            "Data":[
                {
                    "Type":"pos",
                    "Content":{
                        "x":0.5,
                        "y":0.3
                    }
                },
                {
                    "Type":"vel",
                    "Content":{
                        "vx":0.1,
                        "vy":-0.2
                    }
                }
            ]
        }`
    
    type response struct {
        Data []struct {
            Type    string
            Content interface{}
        }
    }
    
    func main() {
        var response response
        err := json.Unmarshal([]byte(exampleResponseData), &response)
        if err != nil {
            fmt.Println("Cannot process not valid json")
        }
    
        for i := 0; i < len(response.Data); i++ {
            response.Data[i].Content = convertMap(response.Data[i].Content)
        }
    }
    
    func convertMap(originalMap interface{}) map[string]float64 {
        convertedMap := map[string]float64{}
        for key, value := range originalMap.(map[string]interface{}) {
            convertedMap[key] = value.(float64)
        }
    
        return convertedMap
    }
    

    Are you sure you cannot define Content as map[string]float64? See example below. If not, how can you know that you can cast it in the first place?

    type response struct {
        Data []struct {
            Type    string
            Content map[string]float64
        }
    }
    
    var response response
    err := json.Unmarshal([]byte(exampleResponseData), &response)
    
    本回答被题主选为最佳回答 , 对您是否有帮助呢?
    评论

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