Creating map with empty values

Question

I need to use a map for keys only, I don't need to store values. So I declared a map like this:

modified_accounts:=make(map[int]struct{})

The idea is to use empty struct because it doesn't consume storage.

However, when I tried to add an entry to the map,

modified_accounts[2332]=struct{}

I got a compile error:

./blockchain.go:291:28: type struct {} is not an expression

How do I add an empty key and no value to the map ?

Answer 1

You can do it declaring a empty variable

var Empty struct{}

func foo() {
    modified_accounts := make(map[int]struct{})
    modified_accounts[2332] = Empty
    fmt.Println(modified_accounts)
}

Or creating a new struct every time

func bar() {
    modified_accounts := make(map[int]struct{})
    modified_accounts[2332] = struct{}{}
    fmt.Println(modified_accounts)
}

To create a empty struct you should use struct{}{}

Answer 2

The error is exactly what you see in the below line :

./blockchain.go:291:28: type struct {} is not an expression

An expression is something that evaluates to something (something that has a value), struct{} is a type, and your statement is trying to assign a type(right) to a map's key's value, a variable(left)

What you need is to create a variable of this type, and assign that variable to the map's key as a value.

Either by using :

var x struct{}
modified_accounts[2332] = x

or

modified_accounts[2332] = struct{}{}

In either of the above way, you are creating a value of type struct{}, and assigning that value to the map's key.