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

384k90 gold badges813 silver badges779 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

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

3

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

edited Sep 2, 2016 at 15:30

answered Sep 2, 2016 at 14:45

Uvelichitel

8,5481 gold badge21 silver badges37 bronze badges

1 Comment

Denys Séguret Over a year ago

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"}} }.

Collectives™ on Stack Overflow

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

2 Answers 2

Comments

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related