Go: using JSON Unmarshal within a for loop

Question

I'm new to Go and I'm having a problem with my code concerning using json.Unmarshal multiple times within a for loop.

In this code, the first two functions fetch a response from a url, convert it to byte format, and then unmarshal it to data. The final function iterates over a list of url's, and should unmarshal them in turn, appending an array with a set of flight codes each time.

Using the set of structs as I've used here, I'm able to use getFlightData for a single url, which will print a set of flight codes. However when attempting the same within a for loop, the array flightsToSunnyCities will print a set of square brackets with empty space inside. Printing thisNumber within the for loop as it iterates will similarly print empty space.

var data ScheduledFlight

func UnmarshalBodyToPointerFlight(Body []byte, welcome *ScheduledFlight) {
    err2 := json.Unmarshal(Body, &welcome)
    if err2 != nil {
        fmt.Println(err2)
        os.Exit(1)
    }
}

func GetFlightData(url string) *ScheduledFlight {
    res := FetchResponse(url)
    body := ResponseBodyToByte(res)
    UnmarshalBodyToPointerFlight(body, &data)
    return &data
}

func UnmarshalFlightStatsURL() []string {
    urlList := listOfURL()
    var flightsToSunnyCities []string
    for _, item := range urlList {

        var flightStats *ScheduledFlight = GetFlightData(item)
        var thisNumber string = flightStats.FlightNumber
        flightsToSunnyCities = append(flightsToSunnyCities, thisNumber)

    }
    fmt.Println(flightsToSunnyCities)
    return flightsToSunnyCities
}

The structs that I'm using are shown here:

type Welcome struct {
    ScheduledFlights []ScheduledFlight `json:"scheduledFlights"`
}

type ScheduledFlight struct {
    CarrierFSCode          string `json:"carrierFsCode"`
    FlightNumber           string `json:"flightNumber"`
    DepartureAirportFSCode string `json:"departureAirportFsCode"`
    ArrivalAirportFSCode   string `json:"arrivalAirportFsCode"`
}

I suspect this issue is arising from ScheduledFlights being of type []ScheduledFlight which would need to be accounted for, however I'm at a loss at what a solution would be. If anyone has any advice it would be much appreciated, thanks.

Burak Serdar · Accepted Answer · 2020-01-24 17:46:46Z

2

The way you unmarshal the JSON input below is wrong:

func UnmarshalBodyToPointerFlight(Body []byte, welcome *ScheduledFlight){
err2 := json.Unmarshal(Body, &welcome)

The function UnmarshalBodyToPointerFlight gets a pointer ScheduledFlight, and you want to unmarshal your data to the location pointed to by that pointer. To do that, you have to call:

err2 := json.Unmarshal(Body, welcome)

The way you're doing it, it is overwriting the welcome pointer (not the place it is pointing to) and unmarshaling to a new location, because you passed the pointer to the pointer. Because of this, you end up appending empty strings to your data.

answered Jan 24, 2020 at 17:46

Burak Serdar

52.5k4 gold badges54 silver badges74 bronze badges

Sign up to request clarification or add additional context in comments.

Collectives™ on Stack Overflow

Go: using JSON Unmarshal within a for loop

1 Answer 1

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Related