How to decode JSON with type convert from string to float64

Question

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.

score 221 · Accepted Answer · answered Mar 05 '12 at 21:01

221

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)
    }
}

answered Mar 05 '12 at 21:01

Dustin

89,080
21
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133

Thank you! I think this is the best solution for my problem. Could you tell me where is the official document about the usage of ",string"? – yanunon Mar 06 '12 at 02:35
4

Note that the leading comma in front of `json:",string"` is necessary - it won't work without it. – Darrrrrren Nov 25 '14 at 15:18
2

@dustin any idea how to do that other way around? Convert json number to go string? e.g. `"N": 1234` to `N: "1234"`? – Kamil Dziedzic Feb 12 '16 at 16:32
That's really awesome. Except it doesn't work on slices. Does anyone know a way how to do this for []int ? Simply adding the ",string" flag produces: "cannot unmarshal string into go value of int" (http://play.golang.org/p/aFWSH4lUxv) – Dalibor Filus Mar 03 '16 at 10:58
@KamilDziedzic did you get a solution of your query? – maverick May 13 '20 at 16:34
1

Official document: https://golang.org/pkg/encoding/json/#Marshal `The "string" option signals that a field is stored as JSON inside a JSON-encoded string. It applies only to fields of string, floating point, integer, or boolean types. This extra level of encoding is sometimes used when communicating with JavaScript programs` – dao leno May 21 '20 at 03:05
3

Does anybody know how to accept both number and string? – aolivera Aug 08 '21 at 17:59
When serializing a float with 0 decimals, eg 123.00, it is serialized as `"field": "123"` without any decimals. Is there a way for me to always include 2 decimals? – Gustav Sep 20 '22 at 15:31
sounds interesting, do you think it works from int to string ? I tried this https://go.dev/play/p/wLNA1Zu0CAU, but it returns **json: cannot unmarshal number into Go struct field Product.Price of type string** – Yuseferi Jun 05 '23 at 16:20

score 21 · Answer 2 · edited Aug 02 '16 at 20:26

21

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)
}

edited Aug 02 '16 at 20:26

answered Nov 19 '15 at 07:38

Salvador Dali

214,103
147
703
753

Is there a way to do the reverser? i.e decode a number into a string variable? – theNoble247 Jul 21 '21 at 04:31
@theNoble247 did you find a solution for it? ( reverser) – Yuseferi Jun 05 '23 at 16:57

score 6 · Answer 3 · answered Oct 23 '18 at 07:11

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)
    }
}

Your answer makes a great point that I quoted. Did I add to it? See https://stackoverflow.com/a/62740786/12817546 — , Jul 07 '20 at 21:50

score 4 · Answer 4 · answered Feb 26 '12 at 12:45

4

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)
    }
}

answered Feb 26 '12 at 12:45

Mostafa

26,886
10
57
52

Is there a way that not changing the 'Product' struct and implement a decode function or interface to parse the string to float? – yanunon Feb 26 '12 at 13:26
1

@yanunon Yes, you can use a map of type `map[string]interface{}` for `Unmarshal`, and parse that into your struct. – Mostafa Feb 26 '12 at 14:11
@yanunon Or if you want **real** flexibility, you can write your own `Unmarshal`, which calls default `Unmarshal` with a `map[string]interface{}`, but uses `reflect` and `strconv` packages to do the parsing. – Mostafa Feb 26 '12 at 14:18

How to decode JSON with type convert from string to float64

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