Golang maps

• Basics
• Map literals
• Mutating maps
• Types

Basics

A map maps keys to values, ie: associative arrays. Zero value of a map is nil. make function returns a new map.

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"])
}

Map literals

These are essentially the same as [Golang Structs](Golang Structs), keys are required.

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)
}

If top-level type is just a type name, it can be omitted from literal elements. Saves some typing.

package main
 
import "fmt"
 
type Vertex struct {
	Lat, Long float64
}
 
var m = map[string]Vertex{
	"Bell Labs": {40.68433, -74.39967},
	"Google":    {37.42202, -122.08408},
}
 
func main() {
	fmt.Println(m)
}

Mutating maps

Nothing shocking here either. Map elements can be added/deleted/modified/etc.

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)
}

Types

A type can be declared as a thin wrapper around a map:

import "fmt"
 
type Dictionary map[string]string
 
func (d Dictionary) Search(word string) string {
	return d[word]
}
 
func main() {
	d := Dictionary{"key": "This is the value"}
	fmt.Println(d.Search("key"))
}