golang append to a slice inside struct

Question

I'm trying to understand how to manipulate data structures in Go, and its approach to pointers (with copies or references).

my code is on Go Playground, here: https://play.golang.org/p/j_06RS5Xcz

I made up a map of slices of a struct that also has a slice of other thing inside.

here:

type Item struct {
    Name        string
    Description string
}

type Entity struct {
    Base   Item
    Others []Item
}

var database map[int][]Entity

func main() {
    database = make(map[int][]Entity)
    database[1] = []Entity{}

    e1 := Entity{}
    e1.Base = Item{"A", "aaa"}
    e1.Others = []Item{}

    database[1] = append(database[1], e1)

    // later, I want to add other items to my entity
    e1.Others = append(e1.Others, Item{"B", "bbb"})

    // other items field is empty
    fmt.Println(database)
}

// prints: map[1:[{{A aaa} []}]]

I want to append the Others items later in my program. it seems that I must use pointers to solve this, but I don't know how.

should my Entity be like this?

type Entity struct {
    Base   Item
    Others *[]Item
}

and if so, how should I append items to it? like this?

*e1.Others = append(*e1.Others, Item{"B", "bbb"})

.

if there is room for another question... it is also not clear to me if I 'must' do: database[1] = []Entity{} before database[1] = append(database[1], e1) or I could just append in this case. I tried the same on e1.Others = []Item{} but it didn't produced the same effect to append (I know that this is my misunderstanding, not a Go's fault).

thanks in advance :)

Roland Illig · Accepted Answer · 2016-08-18 01:36:06Z

2

In your current code, you have two objects of type Entity. One is named e1, the other is named database[1]. These two objects are completely independent, since they are structs. Therefore, when you change one of them, it will not affect the other. (Small exception: some changes to the Items field will be shared, but not all.)

If you want to first add the entity to the map and later modify it, you should use a map of pointers, map[int][]*Entity. Then, instead of Entity{}, you should create a pointer to an entity, with e1 := &Entity{}, and then the program will work. The changes to e1.Others will also affect database[1].Others, since these two variables now point to the same object.

But the print statement will be different. Instead of printing the struct, it will only print a pointer value. To fix this, add a String method:

func (e *Entity) String() string { return fmt.Sprint(*e) }

See https://play.golang.org/p/edU7E5Gnjw, where I also removed the needless empty slices. It is perfectly ok to append to a nil slice.

For further reading, I suggest http://research.swtch.com/godata, which will answer the questions that you currently have.

edited Aug 18, 2016 at 1:36

answered Aug 18, 2016 at 1:16

Roland Illig

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1 Comment

weeanon Over a year ago

thank you, very much ! the link you added is of great help too :D

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