Reverse a map in <value, key> format in Golang

I have a map for my program that looks like following:

fruit_map := map[string]string {
    "apple": "likey",
    "orange": "no likey",
}

I would like to reverse it so that it reads the following:

{
    "likey": "apple",
    "no likey": "orange",
}

There are no duplicates in values. Also, my map is small - about 200 keys. I did not find any built-in method to reverse a map like this. Is there any way to do this fast? I am not much bothered about space complexity, but the solution needs to be fast.

Thanks.

You may write a for loop to iterate over the key-value pair of original map, and put them in a new map (see function reverseMap)

Code@https://play.golang.org/p/5y1gABTpdb8

package main

import (
	"fmt"
)

func main() {
	fruit_map := map[string]string{
		"apple":  "likey",
		"orange": "no likey",
	}

	reversedMap := reverseMap(fruit_map)
	fmt.Println(reversedMap)
}

func reverseMap(m map[string]string) map[string]string {
	n := make(map[string]string, len(m))
	for k, v := range m {
		n[v] = k
	}
	return n
}

Output:

map[likey:apple no likey:orange]

BTW, it is not idiomatic to name go variable like fruit_map, you really should use camel-case, like fruitMap.

The other answers offer the simple solution based on handling the map directly.

An alternative solution is to encapsulate a bidirectional map as a self-contained utility, which has the advantage that you could write thorough unit tests for it and then be able to rely on it to operate correctly via a simple API.

Here's my example implementation (which is incomplete and doesn't yet have the necessary unit tests):

package main

import (
"fmt"
)
func main() {
biMap := NewBiMap()
biMap.Put("apple", "likey")
biMap.Put("orange", "no likey")
v, _ := biMap.GetByValue("no likey")
fmt.Println(v)
}
type BiMap struct {
ab map[string]string
ba map[string]string
}
func NewBiMap() *BiMap {
return &BiMap{make(map[string]string), make(map[string]string)}
}
func (m *BiMap) Put(key, value string) *BiMap {
m.ab[key] = value
m.ba[value] = key
return m
}
func (m *BiMap) GetByKey(key string) (value string, exists bool) {
value, exists = m.ab[key]
return
}
func (m *BiMap) GetByValue(value string) (key string, exists bool) {
key, exists = m.ba[value]
return
}
func (m *BiMap) Len() int {
return len(m.ab)
}
func (m *BiMap) DeleteKey(key string) *BiMap {
value, exists := m.ab[key]
if exists {
delete(m.ab, key)
delete(m.ba, value)
}
return m
}
func (m *BiMap) DeleteValue(value string) *BiMap {
key, exists := m.ba[value]
if exists {
delete(m.ab, key)
delete(m.ba, value)
}
return m
}

You are right, there is nothing built-in to accomplish this but it is really straightforward:

package main
import "fmt"
func main() {
fruit_map := map[string]string{
"apple":  "likey",
"orange": "no likey",
}
//create your new empty map that will hold your reversed contents.
reversed_fruit_map := make(map[string]string)
for k, v := range fruit_map{
reversed_fruit_map[v] = k
}
fmt.Println(reversed_fruit_map)
}

This outputs the following:

map[likey:apple no likey:orange]

Check it out on the playground. If this is common you can always extract out into your own function.

There isn't a built in function to do that, but it's simple enough with a for loop.

fruit_map := map[string]string {
"apple": "likey",
"orange": "no likey",
}
reversed_map := make(map[string]string)
for key,value := range fruit_map {
reversed_map[value] = key
}

see: http://play.golang.org/p/BQjqUsf9aU

Using a custom type with a Reverse() method implemented is another option.

package main
import "fmt"
type (
MapA map[string]string
MapB map[int]string
)
// Reverse returns a reverse map of the MapA object.
func (m MapA) Reverse() map[string]string {
n := make(map[string]string, len(m))
for k, v := range m {
n[v] = k
}
return n
}
// Reverse returns a reverse map of the MapB object.
func (m MapB) Reverse() map[string]int {
n := make(map[string]int, len(m))
for k, v := range m {
n[v] = k
}
return n
}
func main() {
myMapA := &MapA{
"apple":  "likey",
"orange": "no likey",
}
myMapB := &MapB{
10: "likey",
5:  "not sure",
0:  "no likey",
}
fmt.Println(myMapA.Reverse())
fmt.Println(myMapB.Reverse())
}

Extending @Dave C's answer , with generic

package main
import (
"fmt"
)
func main() {
myMap := map[int]string{
1: "one",
2: "two",
3: "three",
}
result := reverseMap(myMap)
fmt.Println(result)
}
func reverseMap[M ~map[K]V, K comparable, V comparable](m M) map[V]K {
reversedMap := make(map[V]K)
for key, value := range m {
reversedMap[value] = key
}
return reversedMap
}