Go Maps - howtogo.dev

var m map[string]int declares a nil map. Reading from a nil map returns the zero value, but writing to one panics.

var m map[string]int

make allocates a usable, empty map. This is the standard way to get a map you intend to write into.

m := make(map[string]int)

A map literal declares and initializes in one step, with each key: value pair separated by commas.

m := map[string]int{
    "a": 1,
    "b": 2,
}

Indexing a map returns two values: the value, and whether the key was actually present. This comma-ok idiom is the only way to distinguish a stored zero value from a missing key.

v, ok := m["a"]
// v = 1, ok = true

v, ok = m["z"]
// v = 0, ok = false

delete removes a key from the map. Deleting a key that doesn't exist is a no-op, not an error.

delete(m, "b")

Printing a map shows its remaining keys in sorted order courtesy of fmt, though iteration with for range visits keys in randomized order at runtime.

Terminal

$ go run maps.go
map[a:1]