How to convert interface{} to map - dictionary

Question

I am trying to create a function that could accept following
*struct
[]*struct
map[string]*struct
Here struct could be any struct not just a specific one.
Converting interface to *struct or []*struct is working fine.
But giving error for map.
After reflect it shows it is map[] but giving error when try to iterate over range.
Here is code
package main
import (
"fmt"
"reflect"
)
type Book struct {
ID int
Title string
Year int
}
func process(in interface{}, isSlice bool, isMap bool) {
v := reflect.ValueOf(in)
if isSlice {
for i := 0; i < v.Len(); i++ {
strct := v.Index(i).Interface()
//... proccess struct
}
return
}
if isMap {
fmt.Printf("Type: %v\n", v) // map[]
for _, s := range v { // Error: cannot range over v (type reflect.Value)
fmt.Printf("Value: %v\n", s.Interface())
}
}
}
func main() {
b := Book{}
b.Title = "Learn Go Language"
b.Year = 2014
m := make(map[string]*Book)
m["1"] = &b
process(m, false, true)
}
Is there any way to convert interface{} to map and iterate or get it's elements.

Answer 1

If the map value can be any type, then use reflect to iterate through the map:
if v.Kind() == reflect.Map {
for _, key := range v.MapKeys() {
strct := v.MapIndex(key)
fmt.Println(key.Interface(), strct.Interface())
}
}
playground example
If there's a small and known set of struct types, then a type switch can be used:
func process(in interface{}) {
switch v := in.(type) {
case map[string]*Book:
for s, b := range v {
// b has type *Book
fmt.Printf("%s: book=%v\n" s, b)
}
case map[string]*Author:
for s, a := range v {
// a has type *Author
fmt.Printf("%s: author=%v\n" s, a)
}
case []*Book:
for i, b := range v {
fmt.Printf("%d: book=%v\n" i, b)
}
case []*Author:
for i, a := range v {
fmt.Printf("%d: author=%v\n" i, a)
}
case *Book:
fmt.Ptintf("book=%v\n", v)
case *Author:
fmt.Printf("author=%v\n", v)
default:
// handle unknown type
}
}

Answer 2

You don't need reflect here. Try:
v, ok := in.(map[string]*Book)
if !ok {
// Can't assert, handle error.
}
for _, s := range v {
fmt.Printf("Value: %v\n", s)
}
Same goes for the rest of your function. It looks like you're using reflection when you would be better served by a type switch.
Alternatively, if you insist on using reflection here (which doesn't make a lot of sense) you can also use Value.MapKeys with the result from your ValueOf (see the answer https://stackoverflow.com/a/38186057/714501)

Answer 3

This may help:
b := []byte(`{"keyw":"value"}`)
var f interface{}
json.Unmarshal(b, &f)
myMap := f.(map[string]interface{})
fmt.Println(myMap)

Answer 4

Another way to convert an interface{} into a map with the package reflect is with MapRange.
I quote:
MapRange returns a range iterator for a map. It panics if v's Kind is
not Map.
Call Next to advance the iterator, and Key/Value to access each entry.
Next returns false when the iterator is exhausted. MapRange follows
the same iteration semantics as a range statement.
Example:
iter := reflect.ValueOf(m).MapRange()
for iter.Next() {
key := iter.Key().Interface()
value := iter.Value().Interface()
...
}