Maps

Make Go Specification

• In Go, a map is a reference to a hash table, and a map type is written map[K]V
  • K and V are the types of its keys and values.
• All of the keys in a given map are of the same type, and all of the values are of the same type,
  • The keys need not be of the same type as the values.

• The key type K must be comparable using ==, so that the map can test whether a given key is equal to one already within it.

• A map is an unordered group of elements of one type, called the element type

• indexed by a set of unique keys of another type, called the key type.

In other words a map maps keys to values.

• The zero value of a map is nil. A nil map has no keys, nor can keys be added.
• The make function returns a map of the given type, initialized and ready for use.

type Students struct {
    ages map[string]int
}

type Coordinates struct {
    X,Y float64
}

ages := make(map[string]int, 5)
students := Students{ages}
students.ages["jane"] = 18
students.ages["jake"] = 21

Notice the Students struct has a field of ages which is a map type We can use the make builtin function to create a new empty map value which takes the map type and a capacity

type Coordinates struct {
    X,Y float64
}

var points = map[string]Coordinates{
    "Points 1": Coordinates{
        X: 2.5, Y: 3.75,
    },
}

var points2 = make(map[string]Coordinates)
points2["Points 2"] = Coordinates{
    X: 3.75, Y: 5.5,
}

• Notice the difference between points2 and points. For points we are using a map literal
• Map literals are like struct literals, but the keys are required.
• For points2 we used the make builtin and assigned a key value of Points 2

Maps Playground