Example
You define a map using the keyword map, followed by the types of its keys and its values:
// Keys are ints, values are ints.
var m1 map[int]int // initialized to nil
// Keys are strings, values are ints.
var m2 map[string]int // initialized to nil
Maps are reference types, and once defined they have a zero value of nil. Writes to nil maps will panic and reads will always return the zero value.
To initialize a map, use the make function:
m := make(map[string]int)
With the two-parameter form of make, it's possible to specify an initial entry capacity for the map, overriding the default capacity:
m := make(map[string]int, 30)
Alternatively, you can declare a map, initializing it to its zero value, and then assign a literal value to it later, which helps if you marshal the struct into json thereby producing an empty map on return.
m := make(map[string]int, 0)
You can also make a map and set its initial value with curly brackets ({}).
var m map[string]int = map[string]int{"Foo": 20, "Bar": 30}
fmt.Println(m["Foo"]) // outputs 20
All the following statements result in the variable being bound to the same value.
// Declare, initializing to zero value, then assign a literal value.
var m map[string]int
m = map[string]int{}
// Declare and initialize via literal value.
var m = map[string]int{}
// Declare via short variable declaration and initialize with a literal value.
m := map[string]int{}
We can also use a map literal to create a new map with some initial key/value pairs.
The key type can be any comparable type; notably, this excludes functions, maps, and slices. The value type can be any type, including custom types or interface{}.
type Person struct {
FirstName string
LastName string
}
// Declare via short variable declaration and initialize with make.
m := make(map[string]Person)
// Declare, initializing to zero value, then assign a literal value.
var m map[string]Person
m = map[string]Person{}
// Declare and initialize via literal value.
var m = map[string]Person{}
// Declare via short variable declaration and initialize with a literal value.
m := map[string]Person{}
