How Can I Preserve Trailing Zeros When Marshaling Floats to JSON in Go?

JSON Marshal and Float Trailing Zeros

Problem:

When marshaling float numbers to JSON using json.Marshal(), the trailing zeros are stripped, potentially causing issues when parsing the JSON with external programs.

Solution:

To preserve the trailing zeros in the JSON output, one approach is to define a custom float type and provide a custom MarshalJSON() method for it.

type KeepZero float64

func (f KeepZero) MarshalJSON() ([]byte, error) {
    if float64(f) == float64(int(f)) {
        return []byte(strconv.FormatFloat(float64(f), 'f', 1, 32)), nil
    }
    return []byte(strconv.FormatFloat(float64(f), 'f', -1, 32)), nil
}

In this implementation:

• The KeepZero type wraps the float64 type.
• The MarshalJSON() method checks if the float is an integer (with no decimal part). If so, it uses the strconv.FormatFloat() function with a precision of 1 decimal place to ensure the trailing zero is retained.
• Otherwise, it uses a precision of -1 to avoid specifying a fixed number of decimal places.

Example:

type Pt struct {
    Value KeepZero
    Unit  string
}

func main() {
    data, err := json.Marshal(Pt{40.0, "some_string"})
    fmt.Println(string(data), err)
}

This example will produce the desired JSON output:

{"Value":40.0,"Unit":"some_string"}

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