How to get empty array when parsing XML?

php editor Baicao brings you a method to obtain an empty array when parsing XML. When processing XML data, sometimes we encounter a situation where the node is empty. At this time, we need to obtain an empty array to represent the node. In PHP, you can get an empty array by using an xpath expression or by using the children() method of the SimpleXMLElement object. You can use xpath expressions to get an empty array by adding "[]" after the node name, and use the children() method to get an empty array by passing an empty string as a parameter. These methods can help us accurately obtain the value of an empty array when parsing XML.

Question content

I have been tasked with writing a go utility that accepts an xml file, parses it, and returns it as json.

The following is an example of xml:

<?xml version="1.0" encoding="utf-8"?>
<tracks clid="020">
  <track uuid="551" category="s" route="8" vehicle_type="trolleybus" >
    <point
      latitude="53.61491"
      longitude="55.90922"
      avg_speed="24"
      direction="270"
      time="13122022:072116"
    />
  </track>
  <track uuid="552" category="s" route="6" vehicle_type="trolleybus">
    <point
      latitude="53.68321"
      longitude="57.90922"
      avg_speed="42"
      direction="181"
      time="13122022:072216"
    />
  </track>
</tracks>

I wrote the following code:

package main

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

type tracks struct {
    xmlname xml.name `xml:"tracks" json:"-"`
    clid    string   `xml:"clid,attr" json:"clid"`
    tracks  []track  `xml:"track" json:"track_list"`
}

type track struct {
    xmlname     xml.name `xml:"tracks"`
    uuid        string   `xml:"uuid,attr" json:"uuid"`
    category    string   `xml:"category,attr" json:"category"`
    route       string   `xml:"route,attr" json:"route"`
    vehicletype string   `xml:"vehicle_type,attr" json:"vehicle_type"`
    point       point    `xml:"point" json:"point"`
}

type point struct {
    latitude  string `xml:"latitude,attr" json:"latitude"`
    longitude string `xml:"longitude,attr" json:"longitude"`
    avgspeed  string `xml:"avg_speed,attr" json:"avg_speed"`
    direction string `xml:"direction,attr" json:"direction"`
    time      string `xml:"time,attr" json:"time"`
}

func main() {
    rawxmldata := `
        <?xml version="1.0" encoding="utf-8"?>
        <tracks clid="020">
            <track uuid="551" category="s" route="8" vehicle_type="trolleybus">
                <point latitude="53.61491" longitude="55.90922" avg_speed="24" direction="270" time="13122022:072116"/>
            </track>
            <track uuid="552" category="s" route="6" vehicle_type="trolleybus">
                <point latitude="53.68321" longitude="57.90922" avg_speed="42" direction="181" time="13122022:072216"/>
            </track>
        </tracks>
    `

    var tracks tracks

    err := xml.unmarshal([]byte(rawxmldata), &tracks)
    if err != nil {
        log.fatal(err)
    }

    jsondata, err := json.marshal(tracks)
    if err != nil {
        log.fatal(err)
    }

    fmt.printf(string(jsondata))
}

go.dev

But, unfortunately, it doesn't work. I get the following message in the console:

2009/11/10 23:00:00 expected element type <tracks> but have <track>

What did i do wrong? How can I solve this problem?

SOLUTION

I guess I should move the discussion to the answer, since I think you're pretty close. As I mentioned, you need to check the errors returned by xml.unmarshal. It might look like this:

if err := xml.unmarshal([]byte(rawxmldata), &tracks); err != nil {
        panic(err)
    }

Now that you have valid xml data in your code, we can generate meaningful errors; after completing the above error checking, running the code will produce:

panic: expected element type <tracks> but have <track>

goroutine 1 [running]:
main.main()
        /home/lars/tmp/go/main.go:48 +0x12f

This happens because there is a small typo in your data structure; in the definition of the track structure, you have:

type track struct {
    xmlname     xml.name `xml:"tracks"`
    uuid        string   `xml:"uuid,attr" json:"uuid"`
    category    string   `xml:"category,attr" json:"category"`
    route       string   `xml:"route,attr" json:"route"`
    vehicletype string   `xml:"vehicle_type,attr" json:"vehicle_type"`
    point       point    `xml:"point" json:"point"`
}

You mistagged the xmlname attribute as tracks when it should actually be track:

type track struct {
    xmlname     xml.name `xml:"track"`
    uuid        string   `xml:"uuid,attr" json:"uuid"`
    category    string   `xml:"category,attr" json:"category"`
    route       string   `xml:"route,attr" json:"route"`
    vehicletype string   `xml:"vehicle_type,attr" json:"vehicle_type"`
    point       point    `xml:"point" json:"point"`
}

Finally - and this is not directly related to the question - you should avoid naming variables error, as this is the name of an incorrect built-in data type. I would modify your call to json.marshal to look like this:

jsondata, err := json.marshal(tracks)
    if err != nil {
        panic(err)
    }

You don't need

panic() on errors; it's just a convenient way to get rid of your code.

After making these changes, if we compile and run the code, we will get the output (in the format jq):

{
  "clid": "020",
  "track_list": [
    {
      "xmlname": {
        "space": "",
        "local": "track"
      },
      "uuid": "551",
      "category": "s",
      "route": "8",
      "vehicle_type": "trolleybus",
      "point": {
        "latitude": "53.61491",
        "longitude": "55.90922",
        "avg_speed": "24",
        "direction": "270",
        "time": "13122022:072116"
      }
    },
    {
      "xmlname": {
        "space": "",
        "local": "track"
      },
      "uuid": "552",
      "category": "s",
      "route": "6",
      "vehicle_type": "trolleybus",
      "point": {
        "latitude": "53.68321",
        "longitude": "57.90922",
        "avg_speed": "42",
        "direction": "181",
        "time": "13122022:072216"
      }
    }
  ]
}

Note that you don't even need the xmlname element in your structure; if we remove it entirely, then we have:

type track struct {
    uuid        string `xml:"uuid,attr" json:"uuid"`
    category    string `xml:"category,attr" json:"category"`
    route       string `xml:"route,attr" json:"route"`
    vehicletype string `xml:"vehicle_type,attr" json:"vehicle_type"`
    point       point  `xml:"point" json:"point"`
}

Then we get the output (in the format jq):

{
  "clid": "020",
  "track_list": [
    {
      "uuid": "551",
      "category": "s",
      "route": "8",
      "vehicle_type": "trolleybus",
      "point": {
        "latitude": "53.61491",
        "longitude": "55.90922",
        "avg_speed": "24",
        "direction": "270",
        "time": "13122022:072116"
      }
    },
    {
      "uuid": "552",
      "category": "s",
      "route": "6",
      "vehicle_type": "trolleybus",
      "point": {
        "latitude": "53.68321",
        "longitude": "57.90922",
        "avg_speed": "42",
        "direction": "181",
        "time": "13122022:072216"
      }
    }
  ]
}

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