Golang slices append

I am having a problem when appending to my slice using Golang.

Here is my code:

func MatchBeaconWithXY(w http.ResponseWriter, r *http.Request) ([]types.BeaconDataXY, error) {
context := appengine.NewContext(r)
returnBeaconData := []types.BeaconDataXY{}

beacondata, err := GetBeaconData(w, r)
if err != nil {
	log.Errorf(context, "error getting beacondata %v", err)
	w.WriteHeader(http.StatusInternalServerError)
	return nil, err
}

for index, element := range beacondata {
	q := datastore.NewQuery("physicalbeacondata").Filter("NamespaceID =", element.NamespaceID).Filter("InstanceID =", element.InstanceID)

	beacondatastatic := []types.BeaconDataStatic{}
	_, err := q.GetAll(context, &beacondatastatic)
	if err != nil {
		log.Errorf(context, "cant get query %v", err)
		w.WriteHeader(http.StatusInternalServerError)
		return nil, err
	}

	var beacondataXY = new(types.BeaconDataXY)
	beacondataXY.NamespaceID = element.NamespaceID
	beacondataXY.InstanceID = element.InstanceID
	beacondataXY.XCoord = beacondatastatic[0].XCoord
	beacondataXY.YCoord = beacondatastatic[0].YCoord
	beacondataXY.Distance = element.Distance

	returnBeaconData = append(returnBeaconData, beacondataXY...)

	log.Infof(context, "beaondataXY tot %v", beacondataXY)
}

The beacondataxy.go contains this:

package types

type BeaconDataXY struct {
	InstanceID  string
	NamespaceID string
	XCoord      float64
	YCoord      float64
	Distance    float64
}

The error message is this:

> utils.go:139: cannot use beacondataXY (type *types.BeaconDataXY) as
> type []types.BeaconDataXY in append

I don't really know how to handle slices in Golang, even after reading some tutorials that makes perfect sense. I'm not sure what I'm doing wrong.

I want to have an array/slice with types inside, return BeaconData is of []types. BeaconDataXY and it should contain single types of BeaconDataXY.

Thanks for all help.

EDIT:

The code now looks like this:

func MatchBeaconWithXY(w http.ResponseWriter, r *http.Request) ([]types.BeaconDataXY, error) {
	context := appengine.NewContext(r)
	//returnBeaconData := []types.BeaconDataXY{}
	returnBeaconData := make([]types.BeaconDataXY, 1)

	beacondata, err := GetBeaconData(w, r)
	if err != nil {
		log.Errorf(context, "error getting beacondata %v", err)
		w.WriteHeader(http.StatusInternalServerError)
		return nil, err
	}

	for _, element := range beacondata {
		q := datastore.NewQuery("physicalbeacondata").Filter("NamespaceID =", element.NamespaceID).Filter("InstanceID =", element.InstanceID)

		beacondatastatic := []types.BeaconDataStatic{}
		_, err := q.GetAll(context, &beacondatastatic)
		if err != nil {
			log.Errorf(context, "cant get query %v", err)
			w.WriteHeader(http.StatusInternalServerError)
			return nil, err
		}

		var beacondataXY = types.BeaconDataXY{}
		beacondataXY.NamespaceID = element.NamespaceID
		beacondataXY.InstanceID = element.InstanceID
		beacondataXY.XCoord = beacondatastatic[0].XCoord
		beacondataXY.YCoord = beacondatastatic[0].YCoord
		beacondataXY.Distance = element.Distance

		returnBeaconData = append(returnBeaconData, beacondataXY)

		//log.Infof(context, "beaondataXY tot %v", beacondataXY)
	}

With this assignment:

var beacondataXY = new(types.BeaconDataXY)

you are creating a variable of type *types.BeaconDataXY. Just create a new BeaconDataXY like this:

var beacondataXY = types.BeaconDataXY{}

When appending to your array do it like this:

returnBeaconData = append(returnBeaconData, beacondataXY)

The "..." would assume that beacondataXY is an array but it isn't, you just want to append beacondataXY to returnBeaconData. See https://golang.org/ref/spec#Appending_and_copying_slices for an explanation of what "..." means in this context.

Try returnBeaconData = append(returnBeaconData, *beacondataXY)

new() built-in function returns a pointer, you can alternatively write:

var beacondataXY = types.BeaconDataXY{}