如何在Golang中将类型从字符串转换为float64解码JSON？

I need to decode a JSON string with the float number like:

{"name":"Galaxy Nexus", "price":"3460.00"}

I use the Golang code below:

package main

import (
    "encoding/json"
    "fmt"
)

type Product struct {
    Name  string
    Price float64
}

func main() {
    s := `{"name":"Galaxy Nexus", "price":"3460.00"}`
    var pro Product
    err := json.Unmarshal([]byte(s), &pro)
    if err == nil {
        fmt.Printf("%+v
", pro)
    } else {
        fmt.Println(err)
        fmt.Printf("%+v
", pro)
    }
}

When I run it, get the result:

json: cannot unmarshal string into Go value of type float64
{Name:Galaxy Nexus Price:0}

I want to know how to decode the JSON string with type convert.

The answer is considerably less complicated. Just add tell the JSON interpeter it's a string encoded float64 with ,string (note that I only changed the Price definition):

package main

import (
    "encoding/json"
    "fmt"
)

type Product struct {
    Name  string
    Price float64 `json:",string"`
}

func main() {
    s := `{"name":"Galaxy Nexus", "price":"3460.00"}`
    var pro Product
    err := json.Unmarshal([]byte(s), &pro)
    if err == nil {
        fmt.Printf("%+v
", pro)
    } else {
        fmt.Println(err)
        fmt.Printf("%+v
", pro)
    }
}

Passing a value in quotation marks make that look like string. Change "price":"3460.00" to "price":3460.00 and everything works fine.

If you can't drop the quotations marks you have to parse it by yourself, using strconv.ParseFloat:

package main

import (
    "encoding/json"
    "fmt"
    "strconv"
)

type Product struct {
    Name       string
    Price      string
    PriceFloat float64
}

func main() {
    s := `{"name":"Galaxy Nexus", "price":"3460.00"}`
    var pro Product
    err := json.Unmarshal([]byte(s), &pro)
    if err == nil {
        pro.PriceFloat, err = strconv.ParseFloat(pro.Price, 64)
        if err != nil { fmt.Println(err) }
        fmt.Printf("%+v
", pro)
    } else {
        fmt.Println(err)
        fmt.Printf("%+v
", pro)
    }
}

Just letting you know that you can do this without Unmarshal and use json.decode. Here is Go Playground

package main

import (
    "encoding/json"
    "fmt"
    "strings"
)

type Product struct {
    Name  string `json:"name"`
    Price float64 `json:"price,string"`
}

func main() {
    s := `{"name":"Galaxy Nexus","price":"3460.00"}`
    var pro Product
    err := json.NewDecoder(strings.NewReader(s)).Decode(&pro)
    if err != nil {
        fmt.Println(err)
        return
    }
    fmt.Println(pro)
}

Avoid converting a string to []byte: b := []byte(s). It allocates a new memory space and copy the whole the content into it.

strings.NewReader interface is better. Below is the code from godoc:

package main

import (
    "encoding/json"
    "fmt"
    "io"
    "log"
    "strings"
)

func main() {
    const jsonStream = `
    {"Name": "Ed", "Text": "Knock knock."}
    {"Name": "Sam", "Text": "Who's there?"}
    {"Name": "Ed", "Text": "Go fmt."}
    {"Name": "Sam", "Text": "Go fmt who?"}
    {"Name": "Ed", "Text": "Go fmt yourself!"}
`
    type Message struct {
        Name, Text string
    }
    dec := json.NewDecoder(strings.NewReader(jsonStream))
    for {
        var m Message
        if err := dec.Decode(&m); err == io.EOF {
            break
        } else if err != nil {
            log.Fatal(err)
        }
        fmt.Printf("%s: %s
", m.Name, m.Text)
    }
}