# 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,
},
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)

``````The value: 42