How to append items to a map[string][]Struct

Question

I am trying to append items to this struct I have:

type AuditSource struct {
    Source      map[string][]Pgm  `json:"Source"`
}

type Pgm struct {
    ID            uint   `json:"id,omitempty"`
    SourceIP      Inet   `json:"sourceip,omitempty"`
    MulticastPort int `json:"multicastport,omitempty"`
}


func NewAuditSource(lid string) (a *AuditSource) {
    a = &AuditSource{
        Id:              make(map[string]uint64),
        Source:          make(map[string][]Pgm),
    }
    return
}

func (as *AuditSource) AuditSourceDifferences(a, b int) bool{

        if a != b {
            as.Source["Primary"][0].MulticastPort =a //ERRORS out every time
            as.Source["Primary"][1].MulticastPort =b 

        }
       return true
}

Any idea why my struct map[string][]Pgm errors out every time I try to add something to it? Do I need to initialize []Pgm perhaps?

Could you post the error you are seeing? Unless you're initializing `a.Source["Primary"]` elsewhere, `a.Source["Primary"]` will be nil. — kingkupps, Apr 13 '20 at 21:27
The line you've highlighted as an error is trying to *read* the item at index `0`, which probably doesn't exist because you haven't appended anything to the slice (which is how you'd actually *add* something to it). — Adrian, Apr 13 '20 at 21:53

score 0 · Answer 1 · answered Apr 13 '20 at 21:26

0

You seem to have initialized the map, so it is likely that the error is because you're accessing nonexistent elements of the slice. When you initialize the slice, it is empty, and there is no 0th or 1st element:

src:=a.Source["Primary"]
src=append(src,Pgm{MulticastPort:a})
src=append(src,Pgm{MulticastPort:b})
a.Source["Primary"]=src

answered Apr 13 '20 at 21:26

Burak Serdar

27,223
3
14
34

1

This seems needlessly verbose since you can append to a `nil` slice just fine. `a.Source["Primary"] = append(a.Source["Primary"], Pgm{MulticastPort:a})` would work just fine. – Adrian Apr 13 '20 at 21:54

score 0 · Answer 2 · answered Apr 14 '20 at 09:49

There are some error in the code:

Inet type is not defined
AuditSource type does not contains Id used in NewAuditSource function
AuditSourceDifferences function does not contains the if statement
You are appending on a (probable) not existent value of the array using the index 0 and 1 in AuditSourceDifferences function
You have used the same name (a) to the input value (a, b int) and the struct receiver (a *AuditSource)

Try with the following code

package yourPackage

type AuditSource struct {
    Source map[string][]Pgm `json:"Source"`
    Id     map[string]uint64
}

type Pgm struct {
    ID            uint `json:"id,omitempty"`
    SourceIP      Inet `json:"sourceip,omitempty"`
    MulticastPort int  `json:"multicastport,omitempty"`
}

func NewAuditSource(lid string) (a *AuditSource) {
    a = &AuditSource{
        Id:     make(map[string]uint64),
        Source: make(map[string][]Pgm),
    }
    return
}

func (a *AuditSource) AuditSourceDifferences(i, j int) bool {
    if i != j {
        a.Source["Primary"] = append(a.Source["Primary"], Pgm{MulticastPort: i},Pgm{MulticastPort: j})
    }
    return true
}

How to append items to a map[string][]Struct

2 Answers2