Golang: update slice in loop for empty interface - go

Question

For example, we have 3 CSV files and common for all is Email column. In first file are Name and Email, in another are Email (plus different info) and no Name field. So, if I need to fill in 2 and 3 files field Name based on the correspondence of the Name and Еmail from the first file than... I wrote code like this:
package main
import (
"fmt"
"io/ioutil"
"log"
"path/filepath"
"strings"
"github.com/jszwec/csvutil"
)
type User struct {
Name string `csv:"name"`
Email string `csv:"email"`
}
type Good struct {
User
Dt string `csv:"details"`
}
type Strange struct {
User
St string `csv:"status"`
Dt string `csv:"details"`
}
var lst map[string]string
func readCSV(fn string, dat interface{}) error {
raw, err := ioutil.ReadFile(fn)
if err != nil {
return fmt.Errorf("Cannot read CSV: %w", err)
}
if err := csvutil.Unmarshal(raw, dat); err != nil {
return fmt.Errorf("Cannot unmarshal CSV: %w", err)
}
return nil
}
func fixNames(fl string, in interface{}) error {
if err := readCSV(fl, in); err != nil {
return fmt.Errorf("CSV: %w", err)
}
switch in.(type) {
case *[]Good:
var vals []Good
for _, v := range *in.(*[]Good) {
v.Name = lst[strings.TrimSpace(strings.ToLower(v.Email))]
vals = append(vals, v)
}
in = vals
case *[]Strange:
var vals []Strange
for _, v := range *in.(*[]Strange) {
v.Name = lst[strings.TrimSpace(strings.ToLower(v.Email))]
vals = append(vals, v)
}
in = vals
}
b, err := csvutil.Marshal(in)
if err != nil {
return fmt.Errorf("Cannot marshal CSV: %w", err)
}
ext := filepath.Ext(fl)
bas := filepath.Base(fl)
err = ioutil.WriteFile(bas[:len(bas)-len(ext)]+"-XIAOSE"+ext, b, 0644)
if err != nil {
return fmt.Errorf("Cannot save CSV: %w", err)
}
return nil
}
func main() {
var users []User
if err := readCSV("./Guitar_Contacts.csv", &users); err != nil {
log.Fatalf("CSV: %s", err)
}
lst = make(map[string]string)
for _, v := range users {
lst[strings.TrimSpace(strings.ToLower(v.Email))] = v.Name
}
var usersGood []Good
if err := fixNames("./Guitar-Good.csv", &usersGood); err != nil {
log.Fatalf("fix: %s", err)
}
var usersStrange []Strange
if err := fixNames("./Guitar-Uknown.csv", &usersStrange); err != nil {
log.Fatalf("fix: %s", err)
}
fmt.Println("OK")
}
in this code I don't like part in func fixNames where is switch:
switch in.(type) {
case *[]Good:
var vals []Good
for _, v := range *in.(*[]Good) {
v.Name = lst[strings.TrimSpace(strings.ToLower(v.Email))]
vals = append(vals, v)
}
in = vals
case *[]Strange:
var vals []Strange
for _, v := range *in.(*[]Strange) {
v.Name = lst[strings.TrimSpace(strings.ToLower(v.Email))]
vals = append(vals, v)
}
in = vals
}
because I just repeat code in part where *in.(SOME_TYPE). I want one loop and one action for different types, structs where are Name and Email fields...
Also was idea to do it with reflection smth. like this:
v := reflect.ValueOf(in)
v = v.Elem()
for i := 0; i < v.Len(); i++ {
fmt.Println(v.Index(i))
}
but I do not know what to do next, how to add in that v value for Name

Answer 1

You don't need reflection for this particular case. You can clean the code up by realizing that you are only working on the User part of the structs, and that you can simplify the type switch:
fix:=func(in *User) {
in.Name = lst[strings.TrimSpace(strings.ToLower(in.Email))]
}
switch k:=in.(type) {
case *[]Good:
for i := range *k {
fix( &(*k)[i].User )
}
case *[]Strange:
for i := range *k {
fix( &(*k)[i].User )
}
}
You have to repeat the for loop, but above code does the correction in place.
You can clean up a bit more by not passing a reference to the slice.

Answer 2

With reflect package, you can do that like this.
func fixNames(fl string, in interface{}) error {
//other code
v := reflect.ValueOf(in)
if v.Kind() == reflect.Ptr {
arr := v.Elem()
fmt.Println(arr.Len())
if arr.Kind() == reflect.Slice || arr.Kind() == reflect.Array {
for i := 0; i < arr.Len(); i++ {
elem := arr.Index(i)
f := elem.FieldByName("Name")
f.SetString("NameOfUser")
}
}
}
// other code
}
Also playground example: https://play.golang.org/p/KrGvLVprslH