How to Safely Handle Nested Maps in Go and Avoid Runtime Errors?

Nested Maps in Golang

In Golang, nested maps can be used to create hierarchical data structures. However, some unexpected behavior can occur when working with nested maps.

Multiple Bracket Support

Go supports multiple brackets for accessing nested maps. For example, the following code successfully prints the value associated with the key "w":

func main() {
    var data = map[string]map[string]string{}
    data["a"]["w"] = "x"
    data["b"]["w"] = "x"
    data["c"]["w"] = "x"
    fmt.Println(data)
}

Nil Map

The issue arises when the nested map is not initialized. The zero value for map types is nil. Indexing a nil map will result in a runtime error. For instance, if the above code is modified as follows:

func main() {
    var data = map[string]map[string]string{}
    data["a"]["w"] = "x"
    data["b"]["w"] = "x"
    data["c"]["w"] = "x"
    fmt.Println(data)
}

The program will fail with the error:

panic: runtime error: assignment to entry in nil map

To resolve this issue, the nested map must be initialized before assigning values to its entries:

func main() {
    var data = map[string]map[string]string{}

    data["a"] = map[string]string{}
    data["b"] = make(map[string]string)
    data["c"] = make(map[string]string)

    data["a"]["w"] = "x"
    data["b"]["w"] = "x"
    data["c"]["w"] = "x"
    fmt.Println(data)
}

Alternatively, composite literals can be used to initialize the nested maps:

func main() {
    var data = map[string]map[string]string{
        "a": map[string]string{},
        "b": map[string]string{},
        "c": map[string]string{},
    }

    data["a"]["w"] = "x"
    data["b"]["w"] = "x"
    data["c"]["w"] = "x"
    fmt.Println(data)
}

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