10. Maps

Maps are mapping keys to values. The key is then used to “unlock” the data, e. g. to give us access or the ability to overwrite the value.

When creating a map, the data type of the key is first defined. In the following code example, we set the key to be a string. The type of the actual value is the following.

type Person struct {
	Name string
	Age  int
}
 
// map[<Key-Type>]<Value-Type> 
var personMap map[string]Person
 
func main() {
	personMap = make(map[string]Person)
	personMap["CEO"] = Person{"Max", 23}
	personMap["CTO"] = Person{"John", 26}
 
 
	fmt.Println(personMap["CEO"])
}
 

“CEO” is the key, while the Person-instance is the value.

This can be rewritten to literals:

type Person struct {
	Name string
	Age  int
}
 
var personMap = map[string]Person{
	"CEO": Person{"Max", 23},
	"CTO": Person{"Kate", 26},
}
 
func main() {
	fmt.Println(personMap["CTO"])
}

Here, we turned from declaration to declaration and initialization. Actually, providing the Person-type here in the initialization is optional: “If the top-level type is just a type name, you can omit it from the elements of the literal.”

 
var personMap = map[string]Person{
	"CEO": {"Max", 23},
	"CTO": {"Kate", 26},
}

Mutating maps

Insert or update an element in map m:

m[key] = elem

Retrieve an element:

elem = m[key]

Delete an element:

delete(m, key)

Test that a key is present with a two-value assignment:

elem, ok = m[key]

If key is in m, ok is true. If not, ok is false.

If key is not in the map, then elem is the zero value for the map’s element type.

Note: If elem or ok have not yet been declared you could use a short declaration form:

elem, ok := m[key]

Creating maps with make

To understand what makedoes, check this make and new

When creating a map with make one can’t initialize it with values directly. Instead, an empty map is created and then filled with values.

	people := make(map[string]Person)
	people["CTO"] = Person{"Max", 22}

Next Chapter: Methods