How to Preserve Trailing Zeros in JSON Output for Go\'s Floating-Point Numbers?

Preserving Trailing Zeros in JSON Output for Floating-Point Numbers

In Go, the json.Marshal() function is commonly used for serializing data structures into JSON format. However, it has a tendency to strip trailing zeros from floating-point numbers during the conversion process. This can be a problem if the consuming application expects the numbers to have trailing zeros.

To address this issue, one approach is to create a custom floating-point type that implements the json.Marshaler interface. This allows you to define how the type is serialized into JSON. Here's an example implementation:

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 code:

• The KeepZero type is a new floating-point type that wraps the native float64 type.
• The MarshalJSON method implements the json.Marshaler interface for the KeepZero type.
• The strconv.FormatFloat function is used to format the floating-point number as a string. If the number is an integer (i.e., it has no fractional part), the string is formatted with a single decimal place to preserve the trailing zero. Otherwise, the string is formatted without a fixed decimal point.

To use the KeepZero type, you can replace the original float64 field in your data structure with a KeepZero field. For example:

type Pt struct {
    Value KeepZero
    Unit  string
}

When you call json.Marshal on a Pt object, the Value field will be serialized using the custom MarshalJSON method, preserving the trailing zero if necessary.

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

This will produce the following JSON output:

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

This solution allows you to retain the trailing zeros in your floating-point numbers when serializing them to JSON, as required by the consuming application.

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