Appending to a slice in a struct in a map

Question

I'm trying to append to a slice within a map:

type MyOuterStruct struct{
    name string
    inners map[string]MyInnerStruct
}

type MyInnerStruct struct{
    name string
    list []int
}

func main() {
    
    ms := MyOuterStruct{
        name: "outer",
        inners: map[string]MyInnerStruct{
            "a": {name: "inner"},
        },
    }
    
    ms.inners["a"].list = append(ms.inners["a"].list, 4, 5, 6)
    
    fmt.Println(ms.inners["a"].list)

    //cannot assign to struct field ms.inners["a"].list in map
}

I know the issue is I'm assigning to an "unaddressable" field, but I'm not sure how to structure this properly.

I've tried the following

myCurrentList := ms.inners["a"].list
myCurrentList = append(myCurrentList, 4, 5, 6)
ms.inners["a"].list = myCurrentList

But this wasn't correct either.

How should I structure the MyInnerStruct so that I can have access to a dynamic slice list than I can append to?

Why the downvote? I can edit the question if it requires refinement. Appending to arrays and slices within maps and structs seems to be a common issue on StackOverflow. — ZAR, May 05 '21 at 15:01
Votes are also a reflection on usefulness to the community, so even a valid question may not be deemed useful. There are thousands of examples of users misunderstanding the language fundamentals, so questions like this are unlikely to be upvoted. — JimB, May 05 '21 at 21:07

score 3 · Accepted Answer · answered May 05 '21 at 14:58

The problem is that ms.inners["a"] returns a copy of the value stored for that key. So when you modify a member of that copy, it is not reflected on the copy stored in the map.

The easiest solution is to define the map as:

map[string]*MyInnerStruct

This way, ms.inners["a"] returns a pointer to the value stored in the map. Another solution is:

x:=ms.inners["a"]
x.list = myCurrentList
ms.inners["a"]=x

This copies the struct stored in the map to x, modifies it, and copies it back.

Yes! Thank you for your help and clear explanation. This now makes sense to me. Will accept answer once SO allows. — ZAR, May 05 '21 at 15:04

Appending to a slice in a struct in a map

1 Answers1