A map is a data structure that allows to store values with arbitrary indexes called keys.

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.

A map literal in go is defined like this:

1catalog := map[int]Book{
2  1: Book{ID: 1, Title: "For the Love of Go"}
3}

so you specify the type of the key inside square brackets, and the type of the values afterwards.

If the top-level type is just a type name, you can omit it from the elements of the literal.

1catalog := map[int]Book{
2  1: {ID: 1, Title: "For the Love of Go"}
3}

Operations

To access elements, you just need to specify their key in square brackets:

1catalog[1]

To insert or update an element:

1catalog[key] = elem

To delete an element:

1delete(catalog, key)

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

1val, ok := catalog[key]

ok will return true if key is in the map, otherwise false. In the latter, val will be zero value.

 1package main
 2
 3import "fmt"
 4
 5type Vertex struct {
 6	Lat, Long float64
 7}
 8
 9var m map[string]Vertex // nil
10
11func main() {
12	m = make(map[string]Vertex) // not nil
13	m["Bell Labs"] = Vertex{
14		40.68433, -74.39967,
15	}
16	fmt.Println(m["Bell Labs"])
17}

References

• John Arundel, For the Love of Go

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