How to decode JSON with type convert from string to float64

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\n", pro)
    } else {
	    fmt.Println(err)
	    fmt.Printf("%+v\n", 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\n", pro)
	} else {
		fmt.Println(err)
		fmt.Printf("%+v\n", 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\n", m.Name, m.Text)
	}
}

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\n", pro)
    } else {
        fmt.Println(err)
        fmt.Printf("%+v\n", pro)
    }
}