Maps in Go are collections that store data as key-value pairs, where each unique key is linked to a value. These are unordered, dynamic, and do not allow duplicate keys. The len() function gives the total number of elements in a map, and their default value is nil. Internally, maps use hash tables for efficient data management.
Creating Maps
Using var and := Syntax
Maps can be initialized using the var keyword or the shorthand := syntax.
Syntax
var mapName = map[KeyType]ValueType{key1: value1, key2: value2, ...}mapName := map[KeyType]ValueType{key1: value1, key2: value2, ...}Example
package mainimport ("fmt")func main() { var motorcycles = map[string]string{"brand": "Honda", "model": "CB500F", "year": "2022"} cars := map[string]int{"SUV": 5, "Sedan": 3, "Truck": 7} fmt.Printf("motorcycles\t%v\n", motorcycles) fmt.Printf("cars\t%v\n", cars)}Result
motorcycles map[brand:Honda model:CB500F year:2022]cars map[Truck:7 Sedan:3 SUV:5]Using make() Function
The make() function is commonly used to create maps.
Syntax
var mapName = make(map[KeyType]ValueType)mapName := make(map[KeyType]ValueType)Example
package mainimport ("fmt")func main() { var motorcycles = make(map[string]string) motorcycles["brand"] = "Yamaha" motorcycles["model"] = "R3" motorcycles["year"] = "2021" cars := make(map[string]int) cars["SUV"] = 4 cars["Sedan"] = 3 cars["Truck"] = 6 fmt.Printf("motorcycles\t%v\n", motorcycles) fmt.Printf("cars\t%v\n", cars)}Result
motorcycles map[brand:Yamaha model:R3 year:2021]cars map[Truck:6 Sedan:3 SUV:4]Creating an Empty Map
An empty map can be created using make() (preferred) or by defining its type.
Syntax
var mapName = make(map[KeyType]ValueType) // Recommendedvar mapName map[KeyType]ValueType // May cause runtime panic if written toExample
package mainimport ("fmt")func main() { var motorcycles = make(map[string]string) var cars map[string]string fmt.Println(motorcycles == nil) // false fmt.Println(cars == nil) // true}Result
falsetrueAllowed Key Types
Map keys can be of types that support the equality operator (==), including:
- Booleans
- Numbers
- Strings
- Arrays
- Pointers
- Structs
- Interfaces (if the dynamic type supports equality)
Invalid key types include:
- Slices
- Maps
- Functions (since the equality operator is not defined for them)
For values, any type is allowed.
Accessing Map Elements
Elements of a map can be accessed using their keys.
Syntax
value = mapName[key]Example
package mainimport ("fmt")func main() { motorcycles := map[string]string{"brand": "Suzuki", "model": "GSX-R750", "year": "2023"} fmt.Println(motorcycles["brand"])}Result
SuzukiUpdating and Adding Elements
Maps allow you to update existing elements or add new ones by assigning values to keys.
Syntax
mapName[key] = valueExample
package mainimport ("fmt")func main() { cars := make(map[string]string) cars["brand"] = "BMW" cars["model"] = "M3" cars["year"] = "2019" fmt.Println(cars) cars["year"] = "2022" // Updating an element cars["color"] = "blue" // Adding an element fmt.Println(cars)}Result
map[brand:BMW model:M3 year:2019]map[brand:BMW color:blue model:M3 year:2022]Removing Elements
The delete() function is used to remove elements from a map.
Syntax
delete(mapName, key)Example
package mainimport ("fmt")func main() { motorcycles := map[string]string{"brand": "Ducati", "model": "Monster", "year": "2020"} fmt.Println(motorcycles) delete(motorcycles, "year") fmt.Println(motorcycles)}Result
map[brand:Ducati model:Monster year:2020]map[brand:Ducati model:Monster]Checking for Specific Elements
To check if a certain key exists, use the val, ok pattern.
Syntax
val, ok := mapName[key]Example
package mainimport ("fmt")func main() { motorcycles := map[string]string{"brand": "Triumph", "model": "Bonneville", "year": "2017"} val1, ok1 := motorcycles["brand"] val2, ok2 := motorcycles["color"] _, ok3 := motorcycles["model"] fmt.Println(val1, ok1) fmt.Println(val2, ok2) fmt.Println(ok3)}Result
Triumph true falsetrueMaps Are References
Maps are reference types, meaning changes in one variable reflect in others pointing to the same map.
Example
package mainimport ("fmt")func main() { cars := map[string]string{"brand": "Tesla", "model": "Model 3", "year": "2021"} electricCars := cars fmt.Println(cars) fmt.Println(electricCars) electricCars["year"] = "2023" fmt.Println("After change to electricCars:") fmt.Println(cars) fmt.Println(electricCars)}Result
map[brand:Tesla model:Model 3 year:2021]map[brand:Tesla model:Model 3 year:2021]After change to electricCars:map[brand:Tesla model:Model 3 year:2023]map[brand:Tesla model:Model 3 year:2023]Iterating Over Maps
Maps can be iterated using the range keyword. Keep in mind the order is not guaranteed.
Example
package mainimport ("fmt")func main() { motorcycles := map[string]int{"Sport": 1, "Adventure": 2, "Cruiser": 3, "Touring": 4} for k, v := range motorcycles { fmt.Printf("%v : %v, ", k, v) }}Result
Touring : 4, Cruiser : 3, Sport : 1, Adventure : 2,Iterating in a Specific Order
To maintain a specific order, use an additional structure.
Example
package mainimport ("fmt")func main() { motorcycles := map[string]int{"Sport": 1, "Adventure": 2, "Cruiser": 3, "Touring": 4} order := []string{"Sport", "Adventure", "Cruiser", "Touring"} for _, key := range order { fmt.Printf("%v : %v, ", key, motorcycles[key]) }}Result
Sport : 1, Adventure : 2, Cruiser : 3, Touring : 4,