# 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

```go
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"])
}
```

```bash
{40.68433 -74.39967}
```

## Map Literals

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

### Example

```go
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)
}
```

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

#### Shorter version

```go
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)
}

```

```bash
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

```go
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)
}

```

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