Iterate through the fields of a struct in Go - go

Question

Basically, the only way (that I know of) to iterate through the values of the fields of a struct is like this:
type Example struct {
a_number uint32
a_string string
}
//...
r := &Example{(2 << 31) - 1, "...."}:
for _, d:= range []interface{}{ r.a_number, r.a_string, } {
//do something with the d
}
I was wondering, if there's a better and more versatile way of achieving []interface{}{ r.a_number, r.a_string, }, so I don't need to list each parameter individually, or alternatively, is there a better way to loop through a struct?
I tried to look through the reflect package, but I hit a wall, because I'm not sure what to do once I retrieve reflect.ValueOf(*r).Field(0).
Thanks!

Answer 1

After you've retrieved the reflect.Value of the field by using Field(i) you can get a
interface value from it by calling Interface(). Said interface value then represents the
value of the field.
There is no function to convert the value of the field to a concrete type as there are,
as you may know, no generics in go. Thus, there is no function with the signature GetValue() T
with T being the type of that field (which changes of course, depending on the field).
The closest you can achieve in go is GetValue() interface{} and this is exactly what reflect.Value.Interface()
offers.
The following code illustrates how to get the values of each exported field in a struct
using reflection (play):
import (
"fmt"
"reflect"
)
func main() {
x := struct{Foo string; Bar int }{"foo", 2}
v := reflect.ValueOf(x)
values := make([]interface{}, v.NumField())
for i := 0; i < v.NumField(); i++ {
values[i] = v.Field(i).Interface()
}
fmt.Println(values)
}

Answer 2

If you want to Iterate through the Fields and Values of a struct then you can use the below Go code as a reference.
package main
import (
"fmt"
"reflect"
)
type Student struct {
Fname string
Lname string
City string
Mobile int64
}
func main() {
s := Student{"Chetan", "Kumar", "Bangalore", 7777777777}
v := reflect.ValueOf(s)
typeOfS := v.Type()
for i := 0; i< v.NumField(); i++ {
fmt.Printf("Field: %s\tValue: %v\n", typeOfS.Field(i).Name, v.Field(i).Interface())
}
}
Run in playground
Note: If the Fields in your struct are not exported then the v.Field(i).Interface() will give panic panic: reflect.Value.Interface: cannot return value obtained from unexported field or method.

Answer 3

Go 1.17 (Q3 2021) should add a new option, through commit 009bfea and CL 281233, fixing issue 42782.
reflect: add VisibleFields function
When writing code that reflects over a struct type, it's a common requirement to know the full set of struct fields, including fields available due to embedding of anonymous members while excluding fields that are erased because they're at the same level as another field with the same name.
The logic to do this is not that complex, but it's a little subtle and easy to get wrong.
This CL adds a new reflect.VisibleFields() function to the reflect package that returns the full set of effective fields that apply in a given struct type.
fields := reflect.VisibleFields(typ)
for j, field := range fields {
...
}
Example,
type employeeDetails struct {
id int16
name string
designation string
}
func structIterator() {
fields := reflect.VisibleFields(reflect.TypeOf(struct{ employeeDetails }{}))
for _, field := range fields {
fmt.Printf("Key: %s\tType: %s\n", field.Name, field.Type)
}
}

Answer 4

Maybe too late :))) but there is another solution that you can find the key and value of structs and iterate over that
package main
import (
"fmt"
"reflect"
)
type person struct {
firsName string
lastName string
iceCream []string
}
func main() {
u := struct {
myMap map[int]int
mySlice []string
myPerson person
}{
myMap: map[int]int{1: 10, 2: 20},
mySlice: []string{"red", "green"},
myPerson: person{
firsName: "Esmaeil",
lastName: "Abedi",
iceCream: []string{"Vanilla", "chocolate"},
},
}
v := reflect.ValueOf(u)
for i := 0; i < v.NumField(); i++ {
fmt.Println(v.Type().Field(i).Name)
fmt.Println("\t", v.Field(i))
}
}
and there is no *panic* for v.Field(i)

Answer 5

use this:
type x struct {
Id int
jsj int
}
func main() {
x2 := x{jsj: 10, Id: 5}
v := reflect.ValueOf(x2)
for i := 0; i < v.NumField(); i++ {
fmt.Println(v.Field(i))
}
}
====>10
====>5

Answer 6

Taking Chetan Kumar solution and in case you need to apply to a map[string]int
package main
import (
"fmt"
"reflect"
)
type BaseStats struct {
Hp int
HpMax int
Mp int
MpMax int
Strength int
Speed int
Intelligence int
}
type Stats struct {
Base map[string]int
Modifiers []string
}
func StatsCreate(stats BaseStats) Stats {
s := Stats{
Base: make(map[string]int),
}
//Iterate through the fields of a struct
v := reflect.ValueOf(stats)
typeOfS := v.Type()
for i := 0; i< v.NumField(); i++ {
val := v.Field(i).Interface().(int)
s.Base[typeOfS.Field(i).Name] = val
}
return s
}
func (s Stats) GetBaseStat(id string) int {
return s.Base[id]
}
func main() {
m := StatsCreate(BaseStats{300, 300, 300, 300, 10, 10, 10})
fmt.Println(m.GetBaseStat("Hp"))
}

Answer 7

Use reflect package. First, get the type of variable with reflect.TypeOf and get numbers of elements with reflect.NumField.To obtain the values of the fields iteratively of a structure must reflect the variable and use the function rg.Elem().Field(i)
package main
import (
"fmt"
"reflect"
)
type Gopher struct {
Name string
Color string
Year int
}
func main() {
g := Gopher{Name: "AAA", Color: "BBBB", Year: 2021}
gtype := reflect.TypeOf(g)
numFields := gtype.NumField()
rg := reflect.ValueOf(&g)
for i := 0; i < numFields; i++ {
fmt.Println(rg.Elem().Field(i))
}
}

Answer 8

In Go, you can use the reflect package to iterate through the fields of a struct. The reflect package allows you to inspect the properties of values at runtime, including their type and value. Here's an example of how to iterate through the fields of a struct:
Go Playground
package main
import (
"fmt"
"reflect"
)
type Movie struct {
Name string
Year int
}
func main() {
p := Movie{"The Dark Knight", 2008}
val := reflect.ValueOf(p)
typ := val.Type()
for i := 0; i < val.NumField(); i++ {
field := val.Field(i)
fieldType := typ.Field(i)
fmt.Printf("Field Name: %s, Field Value: %v\n", fieldType.Name, field.Interface())
}
}
Output:
Field Name: Name, Field Value: The Dark Knight
Field Name: Age, Field Value: 2008