Golang: Map

By Xah Lee. Date: . Last updated: .

Map is a unordered collection of key value pairs. (similar to Python Dictionary or Ruby Hash Table or Java Map, JavaScript Map ).

Each key value pair is called a element.

Syntax of Map Type

map[key_type]value_type
Syntax of the map type. Example: var x map[string]int declare a map, with key of string and value of int.
package main

import "fmt"

func main() {

	// declare xx to be map type
	var xx map[string]int

	// print the type of xx
	fmt.Println(fmt.Sprintf("%T", xx) == "map[string]int")
}

Create Map, Literal Expression

map[key_type]value_type{ key1: val1, key2: val2 }
Create a map, with keys and values. Example: map[string]int{"a": 1, "b": 2}
package main

import "fmt"

func main() {
	var xx = map[string]int{"a": 1, "b": 2}
	fmt.Println(xx) // map[a:1 b:2]
}

Create Map with make

make(map[key_type]value_type)
Create a empty map. Example: make(map[string]int)
package main

import "fmt"

func main() {

	var xx = make(map[string]int)

	xx["a"] = 1
	xx["b"] = 2

	fmt.Println(xx) // map[a:1 b:2]
}

Get Value

m[k]
Return 2 values. First value is the value of key k in map m. Second value is true if the key exist, else false. You can get both values by var a, b = m[k]

When a key does not exist, the first value is map's value's type's Golang: Zero Value. (example: if the map's value's type is integer, it return 0 if the key doesn't exist.)

package main

import "fmt"

func main() {

	var mm = map[string]int{"a": 1, "b": 2}

	// get
	fmt.Println(mm["a"] == 1)

	// return 2 values. second is true if exist, else false
	var v1, v2 = mm["b"]
	fmt.Println(v1 == 2, v2 == true)
}

Set Value, Add Key

m[k] = v
Sets value v to key k in map m. Create it if the key does not exist.
package main

import "fmt"

func main() {

	var xx = map[string]int{"a": 1, "b": 2}

	// set value to existing key
	xx["a"] = 9
	fmt.Println(xx["a"] == 9)

	// if key don't exist, add it
	xx["c"] = 8
	fmt.Println(xx["c"] == 8)

}

Delete Key

delete(m, k)
Delete the key k in m. If key doesn't exist, do nothing.
package main

import "fmt"

func main() {

	var xx = map[string]int{"a": 1, "b": 2}

	// delete a key
	delete(xx, "a")
	fmt.Println(xx["a"] == 0)
	// non exist key return a value of zero value of the value type

	// if key doesn't exist, do nothing. no error out
	delete(xx, "999")

}

Loop Thru Map

for key, val := range map { body }
Iterate thru map, each time with key key and value val in body. If you don't need key or val, name it _, else compiler will complain. The _ is called blank identifier.
package main

import "fmt"

func main() {

	var xx = map[string]int{"a": 1, "b": 2}

	for kk, vv := range xx {
		fmt.Printf("%v, %v\n", kk, vv)
	}
	// prints
	// a, 1
	// b, 2

}

Reference

Golang Spec #Map_types

Golang Spec #Deletion_of_map_elements

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