Maps

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.

Example

package main

import "fmt"

type Vertex struct {
    Lat, Long, float64
}

var m map[string]Vertex

func main() {
    m = make(map[string]Vertex)
    m["Bell Labs"] = Vertex{
        40.68433, -74.39967,
    }
    fmt.Println(m["Bell Labs"])
}

{40.68433 -74.39967}

Map Literals

Map literals are like struct literals, but the keys are required.

Example

package main

import "fmt"

type Vertex struct {
	Lat, Long float64
}

var m = map[string]Vertex{
	"Bell Labs": Vertex{
		40.68433, -74.39967,
	},
	"Google": Vertex{
		37.42202, -122.08408,
	},
}

func main() {
	fmt.Println(m)
}

map[Bell Labs:{40.68433 -74.39967} Google:{37.42202 -122.08408}]

Shorter version

package main

import "fmt"

type Vertex struct {
	Lat, Long float64
}

var m = map[string]Vertex{
	"Bell Labs": {40.68433, -74.39967},		// type Vertex ignored here
	"Google":    {37.42202, -122.08408},	// type Vertex ignored here
}

func main() {
	fmt.Println(m)
}

map[Bell Labs:{40.68433 -74.39967} Google:{37.42202 -122.08408}]

Map Mutations

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.

Example

package main

import "fmt"

func main() {
	m := make(map[string]int)

	m["Answer"] = 42
	fmt.Println("The value:", m["Answer"])

	m["Answer"] = 48
	fmt.Println("The value:", m["Answer"])

	delete(m, "Answer")
	fmt.Println("The value:", m["Answer"])

	v, ok := m["Answer"]
	fmt.Println("The value:", v, "Present?", ok)
}

The value: 42
The value: 48
The value: 0
The value: 0 Present? false