Iterate through the fields of a struct in 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_numbe         


        

Answer 1

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 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

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 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)