Golang maps

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 struct literals, 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"))
}