How Do I Avoid Runtime Panics When Using Nested Maps in Go?

Nested Maps in Go

In Go, maps are a powerful data structure that allow you to store key-value pairs. Nested maps, where the values are themselves maps, can be a useful way to organize complex data.

Problem

However, some developers have encountered issues when working with nested maps. For instance, the following code snippet runs successfully:

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

And so does the following:

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

But the following code panics at runtime:

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

Explanation

The issue arises because the zero value for map types in Go is nil, indicating that the map is uninitialized. Attempting to access or store values in a nil map results in a runtime panic.

In the last example, the (outer) data map is initialized, but it has no entries. When you index it like data["a"], since there is no entry with the "a" key yet, indexing it returns the zero value of the value type, which is nil for maps. Assigning to data"a" then becomes an attempt to assign to a nil map, resulting in a panic.

Solution

To avoid this issue, you must initialize a map before storing elements in it. This can be done in several ways:

• Initialize the outer map with a composite literal:

var data = map[string]map[string]string{
    "a": {},
    "b": {},
    "c": {},
}

• Use the make function:

var data = make(map[string]map[string]string)

• Initialize the inner maps individually:

var data = map[string]map[string]string{}

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

After initialization, you can safely store values in the nested maps.

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