Golang 运行时错误：数组解组索引超出范围

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我是 golang 新手，仍在学习东西，但我偶然发现了这个问题。我有从这个 api 获取的 json 数据。 json的相关部分如下所示：

{
   "features": [
     {
       "properties": {
         "display_name": "name", 
         "address": {"county": "county", "country":"country", "country_code":"cc"}
       },
       "bbox": [13.9171885,44.2603464,15.2326512,45.6729436],
       "geometry": {
          "coordinates": [
               [
                 [
                   [14.4899021,41.4867039],
                   [14.5899021,41.5867039]
                 ]
               ],
               [
                 [
                   [15.4899021,41.4867039],
                   [15.5899021,41.5867039]
                 ]
               ],
            ]
        }
     }
   ]
}

我正在尝试用这种结构来解组它：

// feature struct
type feature struct {
    properties struct {
        name    string `json:"display_name"`
        address struct {
            country string `json:"country"`
            code    string `json:"country_code"`
        } `json:"address"`
    } `json:"properties"`
    bbox []float64 `json:"bbox"`
    geo  struct {
        coordinates [][][][]float64 `json:"coordinates"`
    } `json:"geometry"`
}

// geojsonentry struct
type geojsonentry struct {
    features []feature `json:"features"`
}

我正在使用此函数调用 api：

func (server *server) importregionsmultipolygon(w http.responsewriter, r *http.request) {
    var locations []locationmulti
    var errors []error

    for _, county := range multipolygons {
        res, err := http.get(baseurl + "?osm_ids=" + county.type + county.osmid + "&format=json&polygon_geojson=1")
        if err != nil {
            errors = append(errors, err)
        }

        bytes, err := ioutil.readall(res.body)

        if err != nil {
            errors = append(errors, err)
        }

        location, err := parsegeojson(bytes)

        if err != nil {
            errors = append(errors, err)
        } else {
            locations = append(locations, location)
        }
    }

    if errors != nil {
        http.error(w, errors[0].error(), http.statusbadrequest)
        return
    } else {
        file, _ := json.marshalindent(locations, "", " ")
        if err := ioutil.writefile("./static/regions/geodata-multi.json", file, 0644); err != nil {
            http.error(w, "error writing file", http.statusbadrequest)
            return
        } else {
            responses.json(w, http.statuscreated, locations)
        }
    }
}

locationmulti 看起来像这样：

// locationmulti struct
type locationmulti struct {
    name        string
    lat         string
    lng         string
    country     string
    countrycode string
    coordinates [][][][]float64
}

函数 parsegeojson 如下所示：

func parsegeojson(bytes []byte) (locationmulti, error) {
    var entry geojsonentry
    var err error

    if err := json.unmarshal(bytes, &entry); err != nil {
        fmt.println("error parsing json", err)
    }

    fmt.printf("%v\n", &entry)

    location := locationmulti{
        name:        entry.features[0].properties.name,
        lat:         fmt.sprintf("%f", (entry.features[0].bbox[1]+entry.features[0].bbox[3])/2),
        lng:         fmt.sprintf("%f", (entry.features[0].bbox[0]+entry.features[0].bbox[2])/2),
        country:     entry.features[0].properties.address.country,
        countrycode: entry.features[0].properties.address.code,
        coordinates: entry.features[0].geo.coordinates,
    }

    return location, err
}

我收到错误：

Error parsing json json: cannot unmarshal array into Go value of type controllers.Entry
2020/04/28 17:36:39 http: panic serving [::1]:61457: runtime error: index out of range

我做错了什么，我应该如何解组这种json？

解决方案

unmarshal方法本身被正确调用，问题是您请求的是JSON格式，它是一个数组，而不是预期的GeoJSON。json返回错误的解决方案。unmarshal正在替换

res, err := http.get(baseurl + "?osm_ids=" + county.type + county.osmid + "&format=json&polygon_geojson=1")

与

res, err := http.get(baseurl + "?osm_ids=" + county.type + county.osmid + "&format=geojson&polygon_geojson=1")

编辑：虽然另外我会重写 parsegeojson 以避免恐慌：

func parsegeojson(bytes []byte) (locationmulti, error) {
    var entry entry
    var err error

    if err := json.unmarshal(bytes, &entry); err != nil {
        fmt.println("error parsing json", err)
        return locationmulti{}, err
    }

    fmt.printf("%v\n", &entry)

    if len(entry.features) == 0 {
        return locationmulti{}, fmt.errorf("parsed entry has no features")
    }

    feature := entry.features[0]
    if len(feature.bbox) < 4 {
        return locationmulti{}, fmt.errorf("bbox of parsed entry has too few points")
    }

    location := locationmulti{
        name:        feature.properties.name,
        lat:         fmt.sprintf("%f", (feature.bbox[1]+feature.bbox[3])/2),
        lng:         fmt.sprintf("%f", (feature.bbox[0]+feature.bbox[2])/2),
        country:     feature.properties.address.country,
        countrycode: feature.properties.address.code,
        coordinates: feature.geo.coordinates,
    }

    return location, err
}

正如 @mkopriva 所指出的，问题可能是在检索 bytes 时出现。以下产生预期输出：

package main

import (
    "encoding/json"
    "fmt"
)

type feature struct {
    properties struct {
        name    string `json:"display_name"`
        address struct {
            country string `json:"country"`
            code    string `json:"country_code"`
        } `json:"address"`
    } `json:"properties"`
    bbox []float64 `json:"bbox"`
    geo  struct {
        coordinates [][][]float64 `json:"coordinates"`
    } `json:"geometry"`
}

type entry struct {
    features []feature `json:"features"`
}

var data = `{
   "features": [
     {
       "properties": {
         "display_name": "name",
         "address": {"county": "county", "country":"country", "country_code":"cc"}
       },
       "bbox": [13.9171885,44.2603464,15.2326512,45.6729436],
       "geometry": {
          "coordinates": [
             [
               [14.4899021,41.4867039],
               [14.5899021,41.5867039]
             ]
          ]
        }
     }
   ]
}`

func main() {
    var entry entry

    bytes := []byte(data)
    if err := json.unmarshal(bytes, &entry); err != nil {
        fmt.println("error parsing json", err)
    }

    fmt.printf("%v\n", entry)
}

结果：

{[{{name {country cc}} [13.9171885 44.2603464 15.2326512 45.6729436] {[[[14.4899021 41.4867039] [14.5899021 41.5867039]]]}}]}

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