Why Do Nested Maps in Go Cause Runtime Panics When Accessed Directly, But Not When Using Append or Initialization?

Nested Maps in Golang: Anomalies Explored

When exploring nested maps in Golang, a series of code examples reveal apparent discrepancies:

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

This code runs successfully.

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)
}

You can also do this.

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

This can also be done.

However, when I run the code below I get an error.

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

There's a problem.

Investigating the cause

The cause of this problem is that the zero value of the map type is nil. Zero value is uninitialized. You cannot store values ​​in a nil map. This is a runtime panic.

In the last example, we initialized the (external) data map, but there are no entries. If you index like data["a"], there is no entry for the "a" key yet, so the indexing returns a zero value with a value type of nil. Therefore, if you try to assign to data"a", a runtime panic will occur.

The map must be initialized before storing any elements. For example:

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)

Output (Try it in Go Playground):

map[a:map[w:x] b:map[w:x] c:map[w:x]]

If you declare and initialize a variable map type using a compound literal, that is also considered an initialization. Masu.

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)

The output is the same. Try it at Go Playground.

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