Pointing map to a new map inside a method looses effect after leaving method - go

Question

This code, visible on the Go PlayGround:
package main
import "fmt"
type MyType map[int]int
func (x MyType) updateIt() {
newItem := make(MyType)
for i := 0; i < 3; i++ {
newItem[i] = i
}
x = newItem
fmt.Println(x)
}
func main() {
x := make(MyType)
x.updateIt()
fmt.Println(x)
}
Produces:
map[0:0 1:1 2:2]
map[]
While I expect:
map[0:0 1:1 2:2]
map[0:0 1:1 2:2]
Why?
My understanding is that: x.updateIt() takes x as an argument, then creates newItem and changes the pointer of x so that it points to newItem. So x points to the data of newItem. This seems to be indeed the case inside updateIt, as visible from the first print line. But when x is print after the method, the change is lost.
I'm new to Go and this is unexpected behaviour for me, could you please explain?

Answer 1

You should be using a pointer receiver:
func (x *MyType) updateIt() {
newItem := make(MyType)
for i := 0; i < 3; i++ {
newItem[i] = i
}
*x = newItem
fmt.Println(x)
}
Playground: https://play.golang.org/p/K82TTjHdDgg
Explanation
Arguments "in front" of a function (so-called receivers) behave more or less like ordinary arguments.
Imagine a function like this:
func updateIt(x MyType) {
newItem := make(MyType)
for i := 0; i < 3; i++ {
newItem[i] = i
}
x = newItem
fmt.Println(x)
}
Here, you pass x by value, not by reference (or as a pointer).
What happens is that x gets replaced inside the function, but keeps its original value outside of it. By passing a pointer you can replace the value to which x is pointing, thereby changing the value of x outside of the function.
See also "A Tour of Go, Pointers": https://tour.golang.org/moretypes/1

Answer 2

Inside the main function you create a new map and assign the pointer of that new map to x.
Then the value of that pointer is passed to the updateIt function.
Then inside updateIt you replace it with pointer to another map you create inside updateIt. Then it updates this new map instead of updating the original map created inside main
The most important thing to understand here is that your program creates two different maps.
So avoid creating two maps and update the same map passed to your updateIt function.
func (x MyType) updateIt() {
for i := 0; i < 3; i++ {
x[i] = i
}
fmt.Println(x)
}
https://play.golang.org/p/-MemfTv1uJV