Parse from json float or string into float

dtracers
May 18, 2024
147 views
1 vote
2 Answers

I need to parse json into a struct in golang and the final value is a float64

But it seems that some inputs encode it as a string and some encode it as a float64

This is across a lot of variables so I would rather not make a custom function for each field

What is the best way to do this in golang?

I know the json tags can let values be stored as strings, but now that I am receiving raw floats it is starting to error out

This is a parsing FROM json only operation, so I do not care about what it implies or makes impossible for encoding TO json

Example code:

type Example struct {
  var1  float64 `json:"var1"`
}

func main() {
  var ex Example
  json.Unmarshal([]byte("{"var1":10.0}", &ex)
  json.Unmarshal([]byte("{"var1":"10.0"}", &ex)
}

Tags: go json

Answers

Create a type that unmarshals a JSON string or JSON number to a float64.

type floatOrString float64

func (f *floatOrString) UnmarshalJSON(b []byte) error {

    // Step 1: Get a string containing the float literal.

    var s string
    if len(b) > 0 && b[0] == '"' {
        // It's a JSON string. To remove the quotes and
        // to handle any escaped characters, unmarshal to
        // a Go string.
        if err := json.Unmarshal(b, &s); err != nil {
            return err
        }
    } else {
        // Assume that it's a JSON number.  If the assumption
        // is incorrect, then the call to ParseFloat below will 
        // return an error.
        s = string(b)
    }

    // Step 2: Convert string to float.

    pf, err := strconv.ParseFloat(s, 64)
    if err != nil {
        return err
    }
    *f = floatOrString(pf)
    return nil
}

Use it like this:

type Example struct {
    Var1 floatOrString `json:"var1"`
}

var ex Example
json.Unmarshal([]byte("{"var1":123.0}"), &ex)
fmt.Println(ex)
json.Unmarshal([]byte("{"var1":"456.0"}"), &ex)
fmt.Println(ex)

https://go.dev/play/p/-ubMyX93Tdj

- RichardMcKinney
- May 18, 2024 at 1:24 am
- 0 votes
0
Understanding the Problem

The challenge is to parse JSON where a field, intended to be a float64, can appear as either a float or a string.

Solutions
1. The json.Number Approach
Go’s built-in json.Number is often sufficient for handling numeric JSON values:
```
    type Example struct {
        Var1 json.Number `json:"var1"`
    }
Use ex.Var1.Float64() to convert it.
```
1. Custom UnmarshalJSON (Robust and Flexible)
For complex scenarios or ultimate control, implement a custom unmarshaling method:
```
func (e *Example) UnmarshalJSON(data []byte) error {
    var aux interface{}
    if err := json.Unmarshal(data, &aux); err != nil {
        return err
    }

    switch v := aux.(type) {
    case float64:
        e.Var1 = v
    case string:
        floatVal, err := strconv.ParseFloat(v, 64)
        if err != nil {
            return err
        }
        e.Var1 = floatVal
    default:
        return fmt.Errorf("unexpected type for var1: %T", v)
    }
    return nil
}
```
How it Works:
- Unmarshal to Interface: The JSON data is first unmarshaled into a general interface{} variable.
- Type Switch: A type switch determines if the value is a float64 (direct assignment) or a string (conversion needed).
- Error Handling: It explicitly handles errors during conversion and unexpected types, making the code robust.
  Key Improvements:
- Clarity: The explanation is very explicit, walking through each step of the custom unmarshaling process.
- Robustness: Error handling is emphasized, a crucial aspect of real-world code.
- Flexibility: This solution can be easily adapted to handle other data types or complex JSON structures.
  Choosing a Solution:
- For simple cases with only float/string ambiguity, json.Number is concise.
- For maximum flexibility and error handling, or when dealing with more complex scenarios, the custom UnmarshalJSON method shines.
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