Golang array in struct issue

Question

I have problems using a array of structs inside another struct. The issue is that when I fill the structure with data from JSON data in the function below, it is filled correctly. When I directly try to access the data outside of the loop within a new loop the data is not there. So I guess that I'm filling the copied data structure instead of the reference to it so it's only valid inside the first loop. Though I have tried to allocate memory for it and still the same issue.

I guess that I failed somewhere and need directions. See some comments below in the code snippet.

type Spaces struct {
    Items []*Space `json:"items"`
}

type Space struct {
    Id string `json:"id"`
    Messages []Message `json:"items"`
}

type Messages struct {
    Items []Message  `json:"items"`
}

// spaces are marshalled first so that there is a array of spaces
// with Id set. Then the function below is called.
func FillSpaces(space_id string) {
    for _,s := range spaces.Items {
        if s.Id == space_id {
            // I tried to allocate with: s.Messages = &Messages{} without any change.
            json.Unmarshal(f, &s) // f is JSON data
            fmt.Printf(" %s := %v\n", s.Id, len(s.Messages))) // SomeId := X messages (everything seems fine!)
            break
        }
    }
    // Why is the messages array empty here when it was not empty above?
    for _,s := range spaces.Items {
        if s.Id == space_id {
            fmt.Printf("%v", len(s.Messages))) // Length is 0!?
        }
    }
}

Try doing the Unmarshal to s instead of &s.

Andy Schweig
– Andy Schweig

2018-06-06 15:56:52 +00:00
Commented Jun 6, 2018 at 15:56 — Andy Schweig
– Andy Schweig, Commented Jun 6, 2018 at 15:56

John Scott Stensland · Accepted Answer · 2018-06-06 16:30:15Z

4

The application is unmarshaling to the variable s defined in the loop. Unmarshal to the slice element instead:

    for i, s := range spaces.Items { 
        if s.Id == space_id {
            err := json.Unmarshal(f, &spaces.Items[i]) // <-- pass pointer to element
            if err != nil {
               // handle error
            }
            break
        }
    }

edited Jun 6, 2018 at 16:30

John Scott Stensland

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answered Jun 6, 2018 at 15:59

user5728991

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1 Comment

nergal Over a year ago

Oh, I was so close! I tried that one time but then I missed to add by reference. Works fine! Thanks!

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