Golang で指数を使用して JSON 数値をアンマーシャリングする方法

Golang Json Unmarshal Numeric with Exponent

When unmarshaling a JSON string into a struct in Golang, numeric values with exponents are often interpreted as 0. This can be a challenge, as exponents are a standard part of the JSON specification.

To address this issue, the type of the numeric field must be modified to either float32 or float64. These floating-point types support the representation of exponents. For example:

type Person struct {
    Id    float64          `json:"id"`
    _Id   int64             
    Name  string           `json:"name"`
}

After changing the type, unmarshaling the JSON string into the struct will correctly parse the numeric value with the exponent.

Alternative Approach with a Helper Function

If you require the numeric field to be an integer, you can use a helper function to cast the floating-point value to the integer type after unmarshaling:

import (
    "encoding/json"
    "fmt"
    "math"
    "os"
    "reflect"
)

type Person struct {
    Id    float64          `json:"id"`
    _Id   int64             
    Name  string           `json:"name"`
}

var f Person
var b = []byte(`{"id": 1.2e+8, "Name": "Fernando"}`)

func main() {
    _ = json.Unmarshal(b, &f)

    if reflect.TypeOf(f._Id) == reflect.TypeOf((int64)(0)) {
        fmt.Println(math.Trunc(f.Id))
        f._Id = int64(f.Id)
    }
}

In this example, the helper function math.Trunc truncates the floating-point value to an integer. The truncated value is then assigned to the _Id field.

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