How to build a map of struct and append values to it using GO?

Question

I tried many ways to build a map of struct and append values to it and I did not find any way to do it.

The keys of the map are strings. The struct is made of two parts: "x" integer and "y" a slice of strings.

The errors I face to build the map are (for m) :

main.go:11: syntax error: unexpected comma, expecting semicolon, newline, or }

When I try to add new keys and values to the map, the errors are:

go:33: syntax error: missing operand

package main

import "fmt"

type TEST struct {
    x  int
    y []string  
}

// none of these var gives the expected result

var m = map[string]*struct{x int, y []string}{"foo": {2, {"a", "b"}}, }

var m2 = map[string]struct{x int, y []string}{"foo": {2, {"a", "b"}}, }


var n = map[string]*struct{x int
            y []string
            }{"foo": {2, {"a", "b"}}, }
            
var o = map[string]*struct{
            x int
            y []string
            }{"foo": {2, {"a", "b"}}, }         



func main() {

    m["foo"].x = 4
    fmt.Println(m["foo"].x)
    
// how can I had a new key ?

    m["toto"].x = 0
    m["toto"] = {0, {"c", "d"}}

// and append the string "e" to {"c", "d"} ?
   
    m["toto"].y = append(m["toto"].y, "e")

    
    a := new(TEST)
    a.x = 0
    a.y = {"c", "d"}
    
    m["toto"] = a
    
    fmt.Println(m) 
    
}

Denys Séguret · Accepted Answer · 2016-09-02 13:10:19Z

13

Here's a way to write it:

package main

import "fmt"


type TEST struct {
    x   int
    y   []string  
}

var m = map[string]*TEST { "a": {2, []string{"a", "b"}} }


func main() {

    a := new(TEST)
    a.x = 0
    a.y = []string{"c", "d"}

    m["toto"] = a

    fmt.Println(m) 

}

Note: two types aren't the same just because their struct have identical fields.

edited Sep 2, 2016 at 13:10

answered Sep 2, 2016 at 13:02

Denys Séguret

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Comments

Uvelichitel · Accepted Answer · 2016-09-02 15:30:43Z

Long story. If you for some reason prefer unnamed types you must be quite verbose in composite literal describing both types and values

var m = map[string]*struct {
    x int
    y []string
}{"foo": {2, []string{"a", "b"}}}

or with semicolons

var m = map[string]*struct {x int; y []string}{"foo": {2, []string{"a", "b"}}}

and without indirection

var m1 = map[string]struct {
    x int
    y []string
}{2, []string{"a", "b"}}}

To add new key

m["todo"] = &struct {
        x int
        y []string
    }{0, []string{"c", "d"}}

You can also assign TEST struct but only without indirection because pointers *TEST and *youunnamedstruct are not assignable nevertheless structs having identical fields assignable themself

m1["todo"] = TEST{0, []string{"c", "d"}}

You can append only to indirect map struct field

m["todo"].y = append(m["todo"].y, "e")

because direct map struct fields are not addressable

You don't have to be this verbose, and you don't have to repeat the struct definition in the struct literal. This is correct: var m = map[string]*struct {x int; y []string} { "a": {2, []string{"a", "b"}} }.

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