Table of Contents
Maps
A map is an unordered collection of key-value pairs, where each key is unique. Maps in Go are called hashes or dictionaries in other programming languages.
Declaring a map
You can define a map using the following syntax.
① make(map[keyType]valueType)
② map[keyType]valueType { key1: value1, key2: value2, ......., keyX: valueX}① make(map[keyType]valueType)
You can initialize a map using the built-in make() function. The make() function returns a map of the given type, initialized and ready for use. Keys and corresponding values can be added to a map like the code below.
package main
import "fmt"
func main() {
var map1 = make(map[int]string)
fmt.Println(map1) //=> map[]
//Add keys and values to 'map1'
map1[1] = "Go"
map1[2] = "Ruby"
fmt.Println(map1) //=> map[1:Go 2:Ruby]
}② map[keyType]valueType { key1: value1, key2: value2, …, keyX: valueX}
By using the map literal, you can initialize a map with some initial data.
package main
import "fmt"
func main() {
var map2 = map[int]string { 1: "Go", 2:"Ruby"}
fmt.Println(map2) //=> map[1:Go 2:Ruby]
}Nil maps
If you declare a map with the map literal without initial data, it would generate a nil-map. Needless to say, a nil-map does not contain any data. Moreover, any attempt to add any data to a nil-map causes a runtime error.
package main
import "fmt"
func main() {
var nilMap map[int]string
if nilMap == nil {
fmt.Println("nil") //=> nil
}
nilMap[1] = "GO" //=> assignment to entry in nil map
}Modyfing a map
You can add data to a map and modify the data of a map like the folloing code.
package main
import "fmt"
func main(){
var mapEx = make(map[int]string)
//add keys and values to a map
mapEx[1] = "GO"
mapEx[2] = "Ruby"
fmt.Println(mapEx) //=> map[1:GO 2:Ruby]
//modify the data of a map
mapEx[1] = "Python"
mapEx[2] = "Java"
fmt.Println(mapEx) //=> map[1:Python 2:Java]
}Retrieving values from a map
You can retrieve the value assigned to a key in a map using the syntax map[key].
package main
import "fmt"
func main(){
var mapEx = make(map[int]string)
mapEx[1] = "GO"
mapEx[2] = "Ruby"
fmt.Println(mapEx[1]) //=> GOChecking the existence of a key in a map
When you retrieve the value assigned to a given key using the syntax map[key], it returns an additional boolean value as well. It returns true if the key exists in the map and returns false if it does not exist in the map.
package main
import "fmt"
func main(){
var mapEx = make(map[int]string)
mapEx[1] = "GO"
mapEx[2] = "Ruby"
//Trying to retrieve a key that exists in mapEx.
lang, ok := mapEx[1]
//Trying to retrieve a key that does not exist in mapEx.
lang2, ok2 := mapEx[4]
fmt.Println(lang, ok) //=> GO true
fmt.Println(lang2, ok2) //=> false
}If you just want to check for the existence of a key without retrieving the value associated with that key, then you can use an _ (underscore) in place of the first value.
package main
import "fmt"
func main(){
var mapEx = make(map[int]string)
mapEx[1] = "GO"
mapEx[2] = "Ruby"
_, ok := mapEx[1]
_, ok2 := mapEx[4]
fmt.Println(ok) //=> true
fmt.Println(ok2) //=> false
}Deleting a key from a map
You can delete a key from a map using the built-in delete(map, key) function. The delete() function does not do anything if the key does not exist in the map.
package main
import "fmt"
func main(){
var mapEx = make(map[int]string)
mapEx[1] = "GO"
mapEx[2] = "Ruby"
delete(mapEx, 1)
fmt.Println(mapEx) //=> map[2:Ruby]
}Range
The range keyword is used in for loop to iterate over items of an array, slice, channel or map. When you are ranging over an array or a slice, it returns the index and the element at that index. When you are ranging over a map, it returns the key and the key-value.
Array
package main
import "fmt"
var arryEx = [3]string { "Go", "Ruby", "Python" }
func main(){
for index, value := range arryEx {
fmt.Println(index, value)
//=> 0 Go
//=> 1 Ruby
//=> 2 Python
}
}Slice
package main
import "fmt"
var sliceEx = []string { "Go", "Ruby", "Python" }
for index, ele := range sliceEx {
fmt.Println(index, ele)
//=> 0 Go
//=> 1 Ruby
//=> 2 Python
}
}Map
package main
import "fmt"
var mapEx = map[string]string{ "Name":"Sam", "Gender":"Male" }
for key, value := range mapEx {
fmt.Println(key, value)
//=> Name Sam
//=> Gender Male
}
}Skipping the index, key or value
You can skip the index or value by assigning to _. If you only want the index, drop the value entirely.
Index Only
package main
import "fmt"
var arryEx = [3]string { "Go", "Ruby", "Python" }
func main(){
for index, _ := range arryEx {
fmt.Println(index)
//=> 0
//=> 1
//=> 2
}
}Value Only
package main
import "fmt"
var arryEx = [3]string { "Go", "Ruby", "Python" }
func main(){
for _, value := range arryEx {
fmt.Println(value)
//=> Go
//=> Ruby
//=> Python
}
}